spiral.imperial.ac.uk · 2 abstract both the beginning and end of this history are ostensibly well...
TRANSCRIPT
The Early History of Radioactivity (1896-1904)
Thesis presented for the degree of Doctor of Philosophy
in the Field of History of Science
by
Stephen Brian Sinclair
Department of History of Science and Technology
Imperial College of Science and Technology University of London
May 1976
2
ABSTRACT
Both the beginning and end of this history are
ostensibly well defined. Becquerel's quiet discovery of
uranium rays came to fruition in the appearance of the
first standard textbooks on radioactivity, with their
claims for an independent subject area. One may see in
the intervening period the progressive construction of a
new bridge between physics and chemistry founded on a
coherent theory of atomic transmutation and disintegration.
My examination of the scene from the viewpoints of several
interested parties reveals an alternative picture comprising
complex linked series of discoveries, experiments and
hypotheses of various levels. At the moving boundaries of
research the results and conclusions of individuals were
always subject to reinterpretation in their adoption by
others. This study thus proceeds in the light of three
main considerations. These are, firstly, parallel
investigations in radioactivity by different workers;
secondly, contemporary related areas of physical science
such as X-rays, cathode rays, corpuscular theory; and
thirdly, the relevant concepts developed earlier, during
the nineteenth century. The introductory chapter concentrates upon the last
of these, considering some long-standing hopes and
unanswered questions concerning, for example, the
unification of matter, ether, and electricity, and the
relations between the chemical elements. This forms an
essential part of the background to radioactivity.
Chapter two describes the opening of the radiochemical
field by the Curies, following Becquerel's original
discovery, and discusses the blending of these results
with Rutherford's earliest radiation studies. The third
chapter deals with a confused phase where new observations
3
outran theory as 'emanations', 'induced' activities,
spontaneous cathode-ray emissions, and dubious
radiochemical claims combined with the ever-growing
energy problem. The fourth and fifth chapters trace
the emergence of rival theories, and the singular success
of one of these at the expense of all others.
4
ACKNOWLEDGMENTS
I wish to thank for their assistance those who
made manuscript materials available to me at the
following institutions: Cambridge University Library,
Bibliotheque Nationale, Academie des Sciences,
Royal Institution of Great Britain, Wellcome Institute
for the History of Medicine Library, Library of
University College London, Library of the Royal Society,
Bodleian Library, Science Museum Library, Imperial College
Archives.
To my research supervisor Dr. M.B. Hall of the
Department of History of Science and Technology, Imperial
College, I am greatly indebted for her patient advice
throughout the project and for her careful reading of
this thesis during its construction.
TABLE OF CONTENTS
ABSTRACT 2
ACKNOWLEDGEMENTS 4
CHAPTER 1. NINETEENTH-CENTURY THREADS 8
1. Introduction 8 Scientific revolution - summary of threads.
2. Chemical atomic theories and the unity and complex ty of the chemical elements 12
Dalton's theory - evolution and compound nature of elements - Crookes - Stokes - Lockyer - J.J.Thomson.
3. Physical theories of matter, electricity, ether 24 Maxwell - various ethers - Larmor - Lorentz -Zeeman - Mme.Curie, Rutherford - Hertzian molecule.
4. Chemical physics. Physical chemistry 32 Hertzian atom - Stoney's electron theory -electrochemistry - vortex atom - J.J.Thomson -electricity and gases - corpuscular atom.
CHAPTER 2. THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY 48
1. Becquerel's discovery of uranium rays (1896-7) 48 X-rays and phosphorescence.- Becquerel and uranium rays - nature of the rays - S.P.Thompson - other radiations - vapours and W.J.Russell's photographic work -electrical studies.
2. Rutherford, and the Cavendish Laboratory (1894-8) 70 Hertzian radiation and magnetism - X-rays and conductivity of gases - ionic theory - ultraviolet radiation - uranium rays.
3. Pierre Curie, Marie Curie and the new radioactive elements (1890.17)-- 92
P.Curie's researches - Marie Curie - thorium rays - G.C.Schmidt - polonium - radium - atomic property -energy source, speculations.
4. Theories and trends (1896-9) 110 Source of the energy - interest in uranium rays -development of theories - atomic change.
6
CHAPTER 3. EMANATIONS AND RADIATIONS 119
1. The ma netic deflection of the Becquerel rays 1 9-1900) 119
The rays from active substances and their magnetic deflection - magnetism and radio-activity - Becquerel's 'material rays'.
2. The discovery of induced radioactivity (1899) 129 The Curies' discovery, 'la radioactivite induite' - Rutherford's idea of a thorium emanation - properties of the emanation - the production of a radioactive deposit.
3. The source of radioactivity (1900) 147
The effect of temperature change and its implications - phosphorescence, Behrendsen, and the views of Becquerel - Marie Curie's speculations on atomic change and disintegration - Rutherford and the energy of radioactivity.
4. Emanations and the X-substances (1900-1) 162
Fitzgerald, and transmutations - A.Debierne and actinium - induced radioactivity: Giesel, Hofmann, and radiolead; P.Villard - Crookes and UrX - the Curies' views - E.Dorn and an emanation from radium - Rutherford's problems with the emanations - atmospheric emanation of Elster and Geitel.
CHAPTER 4. DISINTEGRATION, INDUCTION, TRANSFORMATION 179
1. The emergence of induction and disintegration gorfes (1901-1.) 179
Ionic and emanation hypotheses of Elster and Geitel - comparison of theories - induced radioactivity; the function of the gaseous medium - radioactive water; the induction theory of Curie and Debierne - Becquerel, uranium and auto-induction - Curies' criticism - Crookes and ultra-atomic diffusion - Martin and total disintegration - J.Stark and the genesis of atoms .
2. A quantitative theory of atomic transmutation T1902)
Introduction - F.Soddy - the first joint publication: an inert gas from thorium, a possible transmutation; ThX as the source of the emanation - Rutherford and the transmission of excited radioactivity - interpretations of Becquerel's views - the second publication: thorium and ThX, transmutation quantitatively observed - the new accompaniment version of the disintegration theory, and the question of induction.
196
7
CHAPTER 5. RECEPTIONS, GENERALISATIONS, SPECULATIONS 226
1. Reception of the disintegration theory (1902-3) 226 Introduction - changing views of the Caries; the heat from radium - F.Giesel - J.J.Thomson: wind, water, and atomic disintegration - Crookes and the mysteries of radium; a wider public - creation of helium; the Curies converted - the summit of fame - physicists and chemists: a campaign well fought.
2. The mechanism of radioactivity (1903-4) 252
Speculations of physical chemists: ether, energy, electrons - Soddy and the randomness of disintegration - physical models for radioactivity - Lodge's radiation-loss hypothesis - Kelvin's atomic theories - Nagaoka's saturnian atom - Thomson's corpuscular atom.
3. Conclusion 271 Rutherford and the succession of changes - cosmical, universal, and Proutian radioactivity.
NOTES FOR CHAPTERS 1 to 5 281
BIBLIOGRAPHY 331
Ri3 RE .f% 14-nonis 361e
8
CHAPTER 1
NINETEENTH-CENTURY THREADS
1. Introduction
The discovery of X-rays by ROntgen in 1895, which led
directly to Becquerel's discovery of uranium rays, has been
hailed as marking a watershed in physics, the beginning of
a revolution. This interpretation generally invokes the
further statement that from 1880 to the turn of the century
physical science had attained a satisfactory state with few internal problems.' The latter idea, with its implication
of some form of stagnation in research, seems the more
dubious of the two. The words of Maxwell's 'Introductory
Lecture on Experimental Physics' of 1871 have been used to
support this view:
the opinion seems to have got abroad, that in a few years all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry on these measurements to another place of decimals.2
That this was not 'really the state of things to which we
are approaching' is shown by his own continuation:
But we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continue to be poured...3
And although the great chemist Marcelin Berthelot in
introducing his book, Les Origines de L'Alchimie, of 1885,
wrote that the worm was then 'without mystery',4 scientists in other areas took quite the opposite view. Oliver Lodge
concluding his book Modern Views of Electricity, of 1889,. wrote:
'Conclusion' is an absurd word to write at the present time, when the whole subject is astir with life, and when every month seems to bring out some fresh aspect...5
And we will leave Lodge with a resounding word on this
particular point, in his lecture on 'The Discharge of a
9
Leyden Jar' at the Royal Institution, in the same year;
The present is an epoch of astounding activity in physical science. Progress is a thing of months and weeks, almost of days. The long line of isolated ripples of past discovery seem blending into a mighty wave, on the crest of which one begins to discern some oncoming magnificent generalisation. The suspense is becoming feverish, at times almost painful. One feels like a boy who has been long strumming on the silent key-board of a deserted organ, into the chest of which an unseen power begins to blow a vivifying breath. Astonished, he now finds that the touch of a finger elicits a responsive note, and he hesitates, half delighted, half affrighted, least(sic) he be deafened by the chords which it woara seem he can now summon forth almost at will.6
This happy view sprang from Hertz' recent effective
experimental confirmation of Maxwell's electromagnetic
theory of light. Taken at face value this opposes ideas
of a static or stagnant science at the time. And although
things were not considered to be so happy in related
fields such as spectroscopy and molecular kinetic theory,7
which were beset with problems and contradictions in the
last quarter of the century, they were always lively.
As for the question of whether a date near the turn
of the century should yet be considered as marking the
beginning of a revolution in science, this is a difficult
problem of interpretation. L.P.Williams, for example,
considers the whole lifetime of Queen Victoria (1819-1901)
as manifesting revolutionary changes in the viewpoint of
physics, biology and chemistry.8 My purpose is not to
attempt a full answer to the question, but to point out
that the understanding of the bases of, and changes in,
physical science at the time of the discovery of radio-
activity can be aided by studies of the preceding period.
These can throw light upon the science of radioactivity,
whose phenomena were interpreted in current physical and
chemical terms in its early years before it developed
some concepts of its own.
Thus an outline of chemical atomic theory and
speculations on the unity, complexity and evolution of
the chemical elements points to the Proutian and
10
evolutionary ideas which as will be seen, provided some of
the first tentative explanations of the new phenomena. So
also a thread of physical theories leads via elastic, fluid,
and electromagnetic optical ethers to Hertz' experimental
work on electric radiation. Similarly via optical electron
theories of matter to Zeeman's discovery of the magnetic
influence on sources emitting light. The former provided
the subject of Rutherford's earliest publications, which
contributed to later ideas on radioactive radiations. The
latter played a part in the development of electron or
corpuscular atomic theories. Finally to be considered is a theme in part uniting
the above threads which are mainly but not entirely
independent. This comes under the heading of chemical
physics or physical chemistry, in which a rather thin line
of physicists attempted to apply their science to chemistry;
this was complemented by a few chemists thinking in terms
of physical theory. Chemical thermodynamics does come
under this heading, but is not discussed here, since it
did not directly influence the early development of radio-
activity. But the study of the electrical phenomena and
properties exhibited by chemical substances, especially in
the gaseous state, including cathode rays, provided a vital
link between several of the areas mentioned above, as
exemplified by the work of the physicist J.J.Thomson and
the chemist William Crookes. A union of aspects of physics
and chemistry was forged strongly in the science of radio-
activity but owed much to the earlier unifying thread.
This thread itself contributed independently to experi-
mental atomic physics, for example in studies on rays of
positive electricity in the early twentieth century.
Other areas not to be discussed in this chapter will
be mentioned briefly as the story of radioactivity unfolds.
Kinetic theory of heat, and chemical thermodynamics, for
example, enter this story but briefly, although forming
part of the acceptable knowledge of most physical scientists
at the time. Some techniques and developments relevant to
radioactivity such as the work of the Becquerels and Stokes
on phosphorescence; photographic techniques for the study of radiation; electroscopes and electrometers; various
11 technical traditions of chemical analysis; discovery of the inert gases, will also be mentioned or brought out
in later discussion.
It must again be noted that the areas covered in
this chapter cannot be considered as completely independent
at any time. That the opposing processes of fragmentation
and unification appear to have occurred together during
the progress of nineteenth and early twentieth century
physical science, makes the interpretation of threads
problematical. So also does the ready transfer of ideas
among the scientists who formed something of a European
and Colonial community within which published and private
communication was abundant. Several influential scientists
lived and worked through the whole of the period of some
fifty years from the time of Faraday, the rise of chemical
spectroscopy and the creation of the periodic table of the
chemical elements, until radioactivity was established as
a school of research based on the theory of atomic dis-
integration and transmutation. These points could lead
one to place separate threads of development in the mind
of one and the same scientist; but this does not seem
impossible.
The areas discussed under the three main headings
thus point, in largely uncharted territory, towards the
scientific ideas or knowledge used by those involved with
radioactivity; the treatment is not intended to do more
than this. It may also, however, serve to provide part of
an answer to the question of how far developments of science
related to the new experimental discoveries, around 1896,
of X-rays, radioactivity, corpuscles or electrons, and the
Zeeman effect can be considered as novel.
12
2. Chemical atomic theories and the unity and complexity of the chemical elements
In reviewing7sr-Tudies on radioactivity after her
first year's work on the subject, Mme.Curie put forward
several speculations on the source of the continually
radiated energy, not yet the problem that it was to be.
One was that:
L'6nergie utilisable des substances radioactives diminue constamment. On pourrait, par exemple, rattacher la radioactivit6 a la theorie de Crookes stir 11 6volution des 616ments, en attribuant la radioactivit6 aux 616ments a gros poids atomiques, qui se seraient form6s en dernier et dont l'6volution ne serait pas encore achevee.10
Although this does not seem to be the hypothesis
she favoured (nor was it accepted by Crookes himself at
the time), that Crookes' idea of more than a decade
earlier could be brought up in this way indicates its
currency during the last years of the nineteenth century.
It turned out to be nearer the mark than either had
supposed.
In a similar way, and perhaps more important for
radioactivity, J.J.Thomson first announced his demon-
stration of subatomic corpuscles in a lecture at the
Royal Institution on 'Cathode Rays' in 1897.11 Here he
referred to the earlier suggestions by William Prout,
unnamed chemists, and Norman Lockyer on the constitution
of the chemical elements:
The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one; but a hypothesis that would involve somewhat similar consequences - viz. that the so-called elements are compounds of some primordial element has been put forward by various chemists. Thus, Prout believed that the atoms of all the elements were built up of atoms of hydrogen, and Mr. Norman Lockyer has advanced weighty arguments founded on spectroscopic consideration, in favour of the composite nature of the elements.12
Such hypotheses were thus neither dead nor forgotten
at the end of the nineteenth century, though their origin
lies at its beginning or earlier, with ideas of the unity
13
of all matter predating the atomic theory of Dalton, and
running parallel to its early development. Knight13 has shown how the alternative point-atomism of Boscovich was
entertained by Davy and Faraday, and illustrates the
debatable character of atomic hypotheses from their time
to the 1870's. Dalton's atomic theory required that there be as many different types of true atom as undecomposable
elements. Of these there were about 30 at the time,
rising steadily to about 75 by the end of the century.14
Prout's hypothesis implied that each Daltonian atom
of an element consisted of a number of combined atoms of
hydrogen. And the nearness of atomic weights to multiples
of H=1, i.e. to whole numbers, became clearer throughout
the century. But as analytical results became more certain,
so too did the discrepancies, with the atom of chlorine
obstinately weighing about halfway between 35 and 36
hydrogen atoms. Prout's hypothesis itself, and the saving
device of using 0.5, or smaller, as the basic unit provoked
strong criticism. But as Farrar, for example, has shown,15
modified Proutian ideas developed in the second half of the
century gaining much from analogies with related scientific
areas. The existence and behaviour of organic radicals
showed that compound units could behave in a quasi-atomic
manner; and homologous series had Proutian implications
which were soon taken up. Here two substances, analogous
to C and H in organic chemistry, are required, rather than
the usual single substance of Prout's hypothesis. Then,
about 1860, independent generalisations were impressed
upon the scientific public, and their combined influence
was to be of considerable importance.
The attainment of consistent atomic weights was
dependent upon the acceptance of the remarkable suggestion
by Avogadro, revived by Cannizzaro in 1860, that equal
volumes of gas contain equal numbers of particles, regard-
less of the mass or nature of these. Also essential was
the related postulate of double atoms, like 02, in gaseous
elements, which entailed the problem of the affinity of
like atoms and its implications. Discussions of chemical
philosophy, the different kinds of atomism, and the problems
14
of chemical affinity in the first half of the century
have been provided by Levere.16 Only after 1860 could
the patterns and families of chemical elements be put
together to form the great generalisation of the
periodic table. This, together with the evolutionary
ideas expressed in Darwin's Origin of Species (1859)
and the rise of spectrum analysis in the 1860's,17
provided the elements which appear in newer 'Proutian'
speculations from the 1870's to the turn of the century.
William Crookes, already well known for his work on
the radiometer, high vacua, and inorganic analysis, among
other things, combined the points just mentioned with
others deriving from geology or mineralogy. In his long-
remembered address to the Chemical Section of the British
Association18 in 1886 he remarked that: The array of elements cannot fail to remind us of the organic world. In both cases we see certain groups well filled up, even crowded, with forms having among themselves but little specific difference. On the other hand, in both, other forms stand widely isolated. Both display species that are rare; both have groups that are widely distributed - it might be said cosmopolitan - and other groups of very restricted occurrence. Among animals I may mention as instances the Monotremata of Australia, and among the elements the metals of the so-called rare earths.19
Biological evolution was considered to be an unceasing
process from the remote past and still actually occurring
in the present; but Crookes denied this implication for
the evolution of chemical elements:
The analogy here suggested between elements and organisms is indeed not the closest and must not be pushed too far ... Nor would I for a moment suggest that any one of our present elements, however rare is ... in process of extinction, that any new element is in the course of formation, or that the properties of existing elements are gradually undergoing modification. All such changes must have been confined to that period so remote as not to be grasped by the imagination ... The epoch of elemental development is decidedly over...20
This could have been written as a reply to Mme.Curie's
suggestion of the evolution of the heaviest atoms thorium
15
and uranium a decade later; it does indeed seem to express
his opinion of any such suggestions for radioactivity for
most of the period 1898-1904 during which Crookes worked
and wrote on the subject. It is true that at the beginning
of his speech he spoke favourably of Norman Lockyer's
dissociation hypothesis, set out during the previous
decade:
Mr. Norman Lockyer has shown, I think on good evidence, that, in the heavenly bodies of the highest temperature, a large number of our reputed elements are dissociated, or as it would perhaps be better to say, have never been formed. Mr. Lockyer holds that 'the temperature of the sun and the electric arc is high enough to dissociate some of the so-called chemical elements, and give us a glimpse of the spectra of their bases'.
But Crookes, apparently lining up with a majority of
chemists, spoke against this view with the question:
Is there, then,in the first place, any direct evidence of the transmutation of any supposed 'element' of our existing list into another, or of its resolution into anything simpler?
To this question I am obliged to reply in the negative ... The highest temperatures and the most powerful electric currents at our disposal have been tried, and tried in vain.21
However, he considered the mineralogical association
of like elements, and chemical periodicity, to be indirect
but undeniable evidence of a former though now frozen
Evolution. This had begun from an original 'protyle',
possibly 'helium' and developed into an oscillating
periodic table linking temperature of formation with
atomic weight, atomicity, electrical, and magnetic
properties; as temperature slowly decreased the lighter
elements were first formed, and finally thorium then
uranium.
These ideas are best not considered as purely
speculative, for they were closely linked to the experi-
mental chemistry of the time via Crookes' work on the
rare earths. With some modification these notions were
repeated over the following few years,22 although not
highly regarded by other chemists.23 The delicate
weighings required in chemical analysis had led Crookes
16
to the radiometer effect24 ten years earlier. This in
turn led to the study of rarefied gases, cathode rays,
and the 'fourth state of matter', and thence back to
'radiant matter spectroscopy' as a novel aid to chemical
analysis. The experimental thread is here easier to
follow than his ideas on the molecular or atomic structure
of matter; by 1886 Crookes had formulated an unusual
interpretation of the new spectroscopic properties of
some rare earth elements.25 Whereas radiant matter
spectroscopy indicated five fractionated components of
yttria for example, the ordinary spark spectrum and the
chemical properties of the five were identical, indicating
but a single element. He speculated:
that the structure of a chemical element is more complicated than has hitherto been supposed. Between the molecules we are accustomed to deal with in chemical reactions and ultimate atoms as first created, come smaller molecules or aggregates of physical atoms; these sub-molecules differ one from the other, according to the position they occupied in the yttrium edifice.26
The alternative required an element for each spectrum,
five new elements for yttria alone. The apparent identity
of the ordinary spark spectra of the new components
occurred because in 'the intense heat of the electric
spark, the little differences of molecular arrangement
vanish'. His 'compound molecule explanation' was supposed
to apply generally, for 'had we tests as delicate for the
constituent molecular groups of calcium' this too might
be resolved into simpler groupings.27 In this he seems to
stand not far from the dissociation hypothesis of Lockyer;
both have similarities to the theories of some physicists.
Crookes' query 'whether there is an absolute
uniformity in the mass of every ultimate atom of the
same element. Probably our atomic weights represent a
mean value ...'28 may or may not derive from Liveing's
earlier admission that considering the thousands of iron
spectrum lines he was 'almost driven to ascribe them to
a mixture of differing molecules, though we have as yet no
independent evidence of this'.29 The above statements
indicate some of the difficulties of the terms used at that
17
time, with Crookes or his reporters employing 'ultimate
atoms' in two different ways, also 'physical atoms' as
synonymous with 'sub-molecules' and 'smaller molecules',
which are different from chemical 'molecules'. Liveing,
however, may have meant aggregates of what had been
defined as chemical atoms.
The proposition that the chemical elements or atoms
might be complex seems to have been far less controversial
than its extension to an actual dissociation of the
elements; some evidence pointed only to the first statement,
but spectroscopic observations were taken as pointing to both. For example, G.G.Stokes the Cambridge physicist
early considered dissociation. His publications span the
years 1840-1902, and he communicated with scientists
ranging from Faraday at the beginning of this period to
Crookes, S.P.Thompson and Henri Becquerel on radioactivity
at its end; his rising reputation had been aided by work
on mathematical optics and fluorescence.30 In 1854 he
wrote to William Thomson (later Kelvin)31 concerning the
'enormous length' of the line-spectrum obtained from
electric discharge between metal points, compared to the
spectrum then of greatest range - that of the sun:
I cannot help thinking that decompositions of a very high order may be going on in such an arc (the voltaic arc I mean) and that a careful examination of these lines may lead to remarkable inferences respecting the bodies at present regarded as elementary. There is nothing extravagant in this supposition: few chemists I imagine believe that the so-called elements are all really such.
Now it is quite conceivable that chemically pure metals should agree with compounds of sodium in giving the bright line D. If this were made out I should say that perhaps these metals were compounds of sodium, but more probably they and sodium were compounds of some substance yet more elementary.32
Stokes was most cautious on such matters in publications;
he later found fault with Lockyer, who according to his
most recent biographer, was not.33 The inferences of Lockyer, first made public in 1873, and based on
comparisons between stellar, solar, and laboratory34
spectra, developed into a comprehensive scheme of
18 dissociation of the elements. Stokes wrote in 1876
concerning Lockyer's Preliminary Note to the Royal
Society on the 'Compound Nature of the Line-Spectra of
Elementary Bodies' saying that the simplification of the
calcium spectrum observed in the sun might well be due
to variations with temperature of the relative intensities
of the various molecular vibrations existing and observed
in undissociated calcium:
Hence, while I regard the facts you mention as evidence of the high temperature of the sun, I do not regard them as conclusive evidence of the dissociation of the molecule of calcium.35
The term 'molecule of calcium' used in this context
indicates the continued acceptance of some kind of
complexity of the element, but is otherwise not very clear.
This is perhaps an intentional reserve: Stokes was familiar
with chemical practice and nomenclature.36 The usage is
comparable with the similar expression of a structured
'molecule of uranium' used twenty years later in Stokes'
comments on the origin of the Becquerel rays.37
Writing to Lockyer in 1879, and again criticising his
inferences, Stokes made it clear that he did accept the
compound nature of the elements and believed the view to
be generally favoured:
the question observe is not, Are the elements compound bodies? But, has any satisfactory evidence been now obtained that they are compound bodies? You would, I imagine, find plenty of chemists, from Prout downwards, who would regard it as most probable that they were compounded. I may say that, in common I suppose with multitudes of others, I have long supposed for my part that they were.38
As for Lockyer's evidence the effect of impurities had
not been ruled out; and Dumas' arguments on atomic weights,
on which Lockyer had asked Stokes' opinion, were not
strong. The importance of impurities here is that Lockyer's
thesis of increasing dissociation from laboratory flame,
arc, and spark to stellar temperatures relied upon the
existence of common lines exhibited by different elements
- these indicated common simpler components of the so-called
elements. The actual presence of remaining traces of each
element in a chemically separated pair would give such
19
common lines. Lockyer's development of new techniques
for determining which lines were caused by impurities and
which were truly common to different elements, and his
controversies with some chemists over this, have recently
been described by Brock,39 McGucken,40 and Meadows.41
Besides the difficulty with impurities which always proved
a danger to interpretation there was the question whether
common lines were not accidental i.e. meaningless, or
simply very close. The latter was less likely to occur
with greater dispersion of the spectra. Liveing and Dewar
in collaboration conducted detailed comparisons of some of
the relevant metallic laboratory spectra and having caused
the majority of coincidences to vanish they published
conclusions highly critical of Lockyer's hypothesis.42
By 1885 the opinions of chemists, physicists and astronomers
tended towards the view that the dissociation hypothesis
did not hold water; but Lockyer considered that it fitted
astronomical observations such as those on the heights of
element lines in the sun and on the differing metallic
lines present in the spectra of stars of different
temperatures. He continued to publish books and articles
setting out or invoking the hypothesis, publicly and
privately put up spirited defences, and in so doing perhaps
incited others to technical advance. In later publications
such as The Chemistry of the Sun, 1887; 'On the Chemistry
of the Hottest Stars', 1897,47—;nd Inorganic Evolution,1900,
he was able to drum up supporting opinions for his work
from several chemists. But even some of these, for example
Berthelot, Brodie and Crookes were, or had been, critics
of the dissociation hypothesis.
This seems to fit in both with a general rejection
of his evidence and with Stokes' analysis of 187944 that
'multitudes' of chemists and others considered elements
to be most probably compounded. One could say that a
continuing belief that elements were complex or compounded
and occasionally even dissociated in some way, remained
in the last decades of the nineteenth century, despite
Lockyer.
The literature of this period shows a continuing
20
interest in periodic tables; discussion of Prout's hypo-
thesis with regard to atomic weights was still alive and
had always implied more than mere numerical juggling.
Rayleigh (J.W.Strutt) in his Presidential Address to the
Physics Section of the British Association in 1882,
introducing his plans for the redetermination of gas
densities, said:
The other subject on which, though with diffidence, I should like to make a remark or two is Prout's Law according to which the atomic weightsEir the elements, or at least many of them, stand in simple relation to that of hydrogen.
Some chemists think this speculative, but:
Others, impressed more by the argument that the close approximation to simple numbers cannot be fortuitous, and more alive to the obvious imperfection of our measurements, consider that the experimental evidence against the simple numbers is of a very slender character.45
The gas density determinations which Rayleigh thought
would settle the question led instead to the discovery by
Ramsay and himself of an entire new group of elements, the
inert gases. Those discoveries of 1895-8 perhaps owed
something to the periodic table,46 caused in return several
further modifications of periodic tables by Crookes, Stoney
and others, and were vital for Rutherford's interpretation
of radioactive emanations three or four years later. In
1901 R.J.Strutt (Rayleigh) then working on cathode and
radioactive rays published 'On the tendency of the atomic
weights to approximately whole numbers'47 restating his
father's views of two decades earlier, and estimating a
thousand to one probability against the randomness of the
current atomic weights. His conclusion that there must be
some law behind this indicates the continuing belief.
More remarkable than this however are views expressed
by the chemists who considered that they had effectively
demolished Lockyer's dissociation hypothesis. Liveing
believed that the simpler spectral patterns might occur . 'like the overtones of a string',48 we have seen that he
attributed more complicated spectra to the complexity of
the chemical elements, which might be of an aggregate type.
21
This was shortly after his and Dewar's first hostile and
effective criticisms of 1880-1 against Lockyer's ideas on
dissociation. In spite of this, Liveing in his Address
of 1882, providing further speculations on Prout's
hypothesis at this meeting, asked 'Why may not the
chemical elements be further broken up by still higher
temperatures? A priori and from analogy such a supposition
is extremely probable'.49 Thus both the complexity and
dissociation of elements were considered feasible by
Liveing. Dewar's views expressed six years later in a
lecture on 'Phosphorescence and Ozone150 agree with this:
In this experiment ozone is formed by the action of a high temperature owing to the dissociation of oxygen molecules and their partial recombination into the more complex molecules of ozone. We may conceive it not improbable that some of the elementary bodies might be formed somewhat like the ozone in the whole experiment, but at very high temperatures, by the collocation of certain dissociated constituents and with the simultaneous absorption of heat.
This seems to mean that some elements had been formed by
the dissociation then reassociation of others; it may be
an exception -to the view51 that the only reasonable opinion
at the time took the result of intense heat to be
dissociation only.
Some clarification of the ramifications of chemical
atomism seems to have occurred by the end of the next
decade as relevant studies on cathode rays and radio-
activity were beginning to develop. However, Liveing's
conclusions 'On the Flame-Spectrum of Mercury and its
Bearing on the Distribution of Energy in Gases' of 189852
displays a stronger debt to spectroscopy and kinetic
theory. The significance of mercury here stemmed from
its status as an element whose vapour was definitely
monatomic and not aggregated; this was well established
by chemical experiments on combining weights and vapour
density, and by gas kinetic theory. A difficult deter-
mination53 of the velocity of sound in this vapour
indicated the high ratio of specific heats expected for
a monatomic gas which could store none of the absorbed
heat energy internally. These corroborating results
22
were difficult to reconcile with spectroscopic observation
and theory, which took the complex spark spectrum of
mercury vapour to be caused by a multitude of internal
vibrations. Liveing however regarded:
the production of spectra by an electric discharge as essentially a different process from the production by heat ... a great many rays are given out by various elements in an electric discharge which have never been observed to result from mere heating.54
This point seems to distinguish Liveing's view from
Stoney's electron theory of 189555 and from other
electrical theories of spectra, to be discussed.56
Liveing's studies of the non-electrical excitation of
spectrum lines in a high temperature flame, where chemical
combination also could not occur, gave him a means of
easing the problem of mercury. He considered that:
heat ... is, in part, transformed into vibratory motion which affects the ether; and the true inference from the ratio of the specific heats appears to be, that, at the temperature at which this ratio was measured, the amount of heat converted into vibratory motion is very small...57
He identified the gaseous mercury 'molecules' of physical
kinetic theory with 'chemical atoms', and remarked:
It is possible that a chemically monatomic molecule may have, though it is not probable that it really has, a simpler constitution than a chemically complex molecule, and so may have not so many degrees of freedom as the latter, but still a plurality of degrees.58
Thus Liveing shows a clear usage of the terms of kinetic,
chemical, and spectroscopic theories, and illustrates his
current picture of gaseous molecules composed of one or
more chemical atoms, which themselves have constituents
capable of complex vibration. But evidently the principle
of equipartition of energy among all degrees of freedom
has been tacitly modified or sacrificed. This principle
was one of Kelvin's 'clouds' over the dynamical theory of
heat of 1901.59 In discussing the problems of 'practically
monatomic' gases he attempted in a confused way to disperse
the cloud by postulating 'satellites' of the atoms, with
far smaller mass, which could be the 'ions' of J.J.Thomson.
It is possible that Liveing had been similarly influenced
23
by the chemical atomic theory which Thomson founded in
1897 upon his new discovery of subatomic material
particles. If so, then according to Thomson's later
account60 he was one of very few at this time.
Thus we see that a continuing theme involving the
notions of unity and complexity of the chemical elements,
.based on observational and experimental evidence, runs
into the era of cathode rays and radioactivity. But
questions concerning J.J.Thomson's choice of Lockyer for
support in 1897 are interesting and difficult. If Thomson
took note of Liveing's views, and they did meet at the
Cambridge Philosophical Society, and from 1893 at the less
formal Cavendish Physical Society,61 one might have
expected him to be aware of the past and present low
reputation of the relevant parts of Lockyer's work.
Thomson could indeed have quoted the words of Liveing
himself on the complexity of the elements, as considered
above. Stokes too, who had discussed cathode rays and
X-rays with Thomson62 in 1896 or 1897, shared Liveing's
opinion. If only the most recent work of Lockyer63 were seized upon, ignoring as many did64 his earlier studies,
there were again strong criticisms by the reputable
Schuster.65 The latter suggested at the Royal Society
discussion meeting of March 25th 1897 that the difference
between stars emitting hydrogen and metal spectra for
example was as well explained by differences in density
and convection of layers of ordinary elements as by
dissociation. For proof of dissociation, he stressed,
traces of other elements from the electric discharge
between iron poles must be found. But Lockyer had already
blundered, along these lines,66 and surely no one seriously
expected the challenge to be met.
Thomson's first brief announcement of 'a state of
matter more finely subdivided than the atom of an element'
was made at a Royal Institution Lecture at the end of the
next month. He later recalled in his autobiography67 that
its reception was poor, and that it was perhaps not taken
seriously. Such a response is understandable when one
considers that he claimed quite clearly to have produced
24
by electric discharge not just the traces of other
elements from iron, as recently demanded of Lockyer by
Schuster, but a common material component of all chemical
elements.
The corpuscular atomic theory of Thomson was
developed during the time of the young Rutherford's work
and collaboration at the Cavendish Laboratory on closely
related areas. This was to be essential to both men's
understanding first of uranium rays and then of radio-
activity. We shall see that Thomson's ideas of the
complexity of the chemical elements were of long
standing, and founded on physical theories of ether,
electricity and matter. He was able to take up part of
a long thread of chemical Proutian ideas and to weave it
into his own physicists' theory of atomic structure.
Others too who worked and thought upon radioactivity saw
the significance of these ideas.
3. Physical theories of matter, electricity, ether
When Hertz began his magnificent experiments on electric oscillations, there were many theories of electrical action. When he had finished them there was only one, Clerk Maxwell's.
So said J.J.Thomson68 in 1894, shortly after the untimely
death of Hertz. This was the year of Rutherford's first
publication on magnetism and electric oscillations.
In the 1860's Maxwell had developed a unified theory
of optics and electromagnetism involving the explicit
provision of a single ether through which both kinds of
effect were considered to be transmitted. But this was
slow to be accepted. As Schaffner has recently commented69
at the time there were not only competing theories of
electromagnetic action but a number of non-electrical
optical theories. While optical and electrical theories
25
came closer together in the minds of many scientists
after the experiments of Hertz interactions with a third
theme of electrical chemistry had already been increasing
during the 1880's. The physical side will be discussed
in this section in so far as it can be considered
separately; both physicists and some chemists during
this period were prone to look back to Faraday as an
earlier master of chemistry, electrochemistry, and
electromagnetism.70 Aspects of all these areas became
more strongly united in the 1890's as spectroscopy,
physical theories of the chemical atom and studies on
radioactivity progressed.
Maxwell in his Treatise on Electricity and Magnetism
of 1873 went so far as to write 'before I began the study
of electricity I resolved to read no mathematics on the
subject till I had first read through Faraday's
Experimental Researches on Electricity'.71 Maxwell
considered that he had combined the methods of the German
school of 'electricians and mathematicians', which
involved 'action at a distance impressed on the electric
fluids', with the more pictorial ideas of Faraday which
involved 'real actions going on in the medium'. Faraday
however had earlier expressed doubts as to the existence
of such a medium72 and it was found to be a problematical
concept, if a valuable one, even after Maxwell's work.
The problems are indicated by the variety of ethers73
which in the nineteenth century were used in conjunction
with physical principles to explain optical and
electrical phenomena.
R.T.Glazebrook, senior demonstrator at the
Cavendish Laboratory, in his major 'Report on Optical
Theories' to the British Association in 188574 discussed
the major optical ethers and their relations with matter
with regard to their explanations of reflexion, refraction,
dispersion, diffraction, polarisation and other phenomena.
None was without its flaws, but:
The electro-magnetic theory, if we accept its fundamental hypothesis, is thus seen to be capable of explaining in a fairly satisfactory manner most of the known phenomena of optics.
26
The great difficulty is, as we have said, to account for the properties which the medium must have in order to sustain electrical stresses.75
Illustrative of the requirements of what may have been
a mainly British viewpoint, Glazebrook saw a similar
difficulty:
of realising mechanically what electric displacement is, of forming for oneself a physical idea of a change of structure in some medium of unknown properties which shall obey the laws implied by the various equations satisfied by the components of electric displacement. 76
The earlier conflict between the rigidity required for
transmission of light waves, and the fluidity necessary
for free passage of the planets, had been eased by Stokes'
view that these properties could be compatible in a
medium both of small density and low rigidity. But to conceive of such a rigidity existing in an ether which
was like an elastic solid, would not be sufficient to
account for its capability of sustaining known electrical
stresses. The solution might be to have a non-rigid,
fluid ether, with a quasi-rigidity conferred upon it by
filling it with vortices, in the form of filaments or
rings. In Glazebrooke's opinion this could explain
transmission of transverse waves and maintenance of
electric stress, whilst electric and magnetic polarisation
would then consist of definite arrangements of the fila-
ments or rings.
Similar views were held by some others at the time.
Earlier in 1885 G.F.Fitzgerald had written77 from Dublin to
J.J.Thomson with detailed suggestions of the 'infinite
possibilities in a vortex-sponge' either of 'ring vortices
i.e. molecular' or of filaments78 for explanations of electrostatic and electromagnetic phenomena. J.J.Thomson
too in the previous few years had been developing an ether
theory involving vortices, attempting to explain not only
these physical phenomena, but also chemical matter and its
properties, as will be seen in the following section. But
the adherents to electromagnetic-optical ether theories
were not great in number and were probably mostly British,
until the 1890's.
27
Included in this school, Joseph Larmor, at Cambridge,
developed a theory of optics and electrodynamics starting
from a single ether and using a form of the Principle of
Least Action expressed in terms of potential and kinetic
energy. By 1893 he had developed 'A Dynamical Theory of the Electrical and Luminiferous Medium',79 and 'a method of evolving the dynamical properties of the aether from a
single analytical basis':
We shall show that an energy-function can be assigned for the aether which will give a complete account of what the aether has to do in order to satisfy the ordinary demands of Physical Optics; and it will then be our aim to examine how far the phenomena of electricity can be explained as non-vibrational manifestations of the activity of the same medium.80
Apparently for reasons concerning an explanation of
the forces between permanent magnets, Larmor modified his
view in a later addition entitled 'Introduction of Free Electrons'.81 He considered these electrons as 'electric
centres' or 'nuclei of radial rotational strain', having
adopted Stoney's expression for the electrolytic unit
charge, and had a few words to say on the electrical
nature of chemical energy and spectra.
In another of the relatively few references to
recent experimental evidence Larmor stated that
J.J.Thomson had informed him of his determination of the
velocity of the negative rays in vacuum tubes. This
phenomenon Larmor saw as the projection of free electrons
of purely electrical inertia. Our brief mention of studies
of cathode rays is reserved for the next Section; these
cannot however be entirely separated from mathematical
physical theories, as Larmor's interesting interpretation shows.
The ether electron theory of Larmor was shortly
preceded by a differing and now much better known
electromagnetic-optical theory of electrons - that of
Lorentz, published in 1892.82 Hirosige83 and McCormmach84
have described the structure and development of this theory
in some detail. Lorentz here seems not to have unified
the entities of matter, ether and electricity as strongly. as Larmor and others, but sought explanations in terms
28
of ponderable matter and a static ether connected via
electrically charged particles, to which the usual
principles of Newtonian dynamics and of energy are
applied. The positive or negative charge was considered
to be fundamental and was not explained further in terms
of ether in the manner of some British physicists. The
fundamental charged particles were supposed to be spherical,
to possess mass and weight, to be contained within the
ponderable molecules of which all matter consists, and to
form the sole connection with the co-existing ether.85
One cannot say that the possibilities for chemical or
electrochemical explanations were explored, though Lorentz
acknowledged his debt to Helmholtz and Weber who had had
some such interests in previous years.
Lorentz' theory of 1892 and 189586 did however
contribute indirectly to J.J.Thomson's first exposition
of a corpuscular chemical atomic structure in 1897 via
the discovery of Zeeman announced a few months earlier.
Zeeman87 stated that he had used Lorentz' theory to give
an actual estimate of the charge to mass ratio of the
charged particles whose oscillations were taken to be
the cause of the etherial vibrations constituting bright
spectral lines. The discovery of the widening of the
yellow sodium doublet under magnetic influence - the
'Zeeman effect' - and the quantitative use of Lorentz'
theory gave a charge to mass ratio which Thomson noted88
as agreeing with that of his cathode ray corpuscles.
Electronic or corpuscular atomic spectroscopy was developed
rapidly after this time, but not to any great extent by
Thomson nor by the workers on radioactivity, though its
conclusions remained relevant.
Lorentz saw his electron theory as deriving in part
from Maxwell, Weber and Helmholtz. Larmor too gave credit
to these scientists and to an Irish school of mathematical
physicists including MacCullagh and Fitzgerald. Both of
these later proponents of an electromagnetic electron
theory shared the general view of the importance for their
work of the striking experiments of Hertz a few years
earlier. Hertz informed English readers in a collection
29
of his papers, Electric Waves being Researches on the
Propagation of Electric Action with Finite VelocitTg9
that his experiments had been guided by Helmholtz' theory.
This normally invoked Newtonian action-at-a-distance, but
in a limiting case gave some results similar to Maxwell's
with regard to the speed of propagation of electrical and
magnetic quantities. Hertz devised simple apparatus for
production and detection of electrical effects in the air,
and demonstrated standing waves, reflection, refraction,
polarisation, and a variety of measureable wavelengths,
of the order of metres. His work of the late 1880's was
accepted by physicists as an impressive demonstration of
the validity of a unified optical-electromagnetic ether.
This led to the fairly rapid publication not only of
learned articles on these lines but of textbooks of
various kinds.
The use of these books by young scientists shortly
to become involved in investigations of the radiation from
uranium is of interest, and may be significant for inter-
pretations of radioactivity. Paul Drude's Physik des
Aethers auf elektromagnetischer Grundldge, published in
1894,90 was used by Marie Curie91 perhaps for her earliest
researches on magnetic properties of tempered steeis.92
Its brief explanation of fluorescence and phosphorescence93
appears to be essentially a reinterpretation and development
in electromagnetic terms of Stokes' ideas of some forty
earlier.94 Drude represented the molecule of a body as a
closed wire circuit whose natural electric vibrations are
doubled in wavelength on changing to a linear form under intense excitation. This could also account for thermo-
luminescence, so-called by E.Wiedemann, where heating
alone causes characteristic luminosity in some substances.
Radioactivity was at first vaguely interpreted in terms of
phosphorescence by Becquerel, then by Mme.Curie in terms
of fluorescence, a rapid re-emission of received rays.
Drude's book shows the electromagnetic view of optical
phenomena generally accepted at the time; and he refers95
to recently published textbooks by Boltzmann96 and
Poincare,97 which professed some differences in their
30
approach, and which were also available to the student.
E.Wiedemann, who aided the work of G.C.Schmidt on
fluorescence and the new uranium and thorium rays in
1895-9, shared Drude's electromagnetic view of
fluorescence. In a letter98 to Stokes early in 1896
concerning his own and Dr.Schmidt's work on fluorescence
of metal salt solutions and metal vapours he indicated
his view that even with monatomic gases the illuminating
mechanism was more complex than commonly supposed, that
the emission from mercury vapour contradicted the kinetic
theory of gases, and that the explanation might lie in
supposing the molecule comparable to a type of oscillatory electrical circuit.
Thus we see that the view of the 'molecule' behaving
as an electrical system was developing soon after the
work of Hertz of 1888. The identification of this
electrical molecule with the chemical atom may be inherent
in Wiedemann's letter; so also may the not uncommon idea •
of complexity within the chemical atom; but these ideas
were not explicitly discussed, nor are they suggested in
Drude's book. Whether or not the idea of an electrically
composed chemical atom occurred to Schmidt or Marie Curie
before or after J.J.Thomson's corpuscular atomic theory,
neither appears to have clearly used the idea for the
early interpretation of radioactivity.
The conception of an electrically constituted
chemical atom appears more definitely in the textbooks
of 0.Lodge and J.J.Thomson cited by the young Rutherford
in his first published researches, of 1894.99 Thomson,
having edited the third edition of Maxwell's Treatise on
Electricity and Magnetism,100 published a considerable
supplementary volume.101 Besides detailed treatments of
electrical oscillations and Hertzian electromagnetic
radiation to which Rutherford referred specifically102
there is a description of Thomson's own development of
Faraday tubes of force for the understanding of chemical
combination103 and the passage of electricity through
gases.104 This approach dates from the early 1880's and
contains clear allusions to the etherial-electrical
31
construction of chemical atoms.105
The Modern Views of Electricity of Oliver Lodge of
1889106 was one of the many Maxwellian books published
soon after Hertz' experiments. Here Lodge gave non-
mathematical accounts of electricity, magnetism and
radiation. His explanations were based on a single fluid
ether 'a continuous incompressible perfect fluid filling
all space' possibly consisting of 'interlaced vortex
filaments like a sponge' and were illustrated with one
of the most thoroughly mechanical or machine-like
depictions of its motion yet produced. Air and other
dielectrics had interlocked wheels, with cogs upon them,
to represent their etherial molecular structure, but
'in a metallic conductor the gearing is imperfect; it is
a case of friction-gearing with more or less lubrication
and slip, so that turning one wheel only starts the next
gradually'.107 The oscillatory charge and discharge of a
Leyden jar, much discussed after 1888, he illustrated by a device of weights and pulleys balanced by elastic strings,
with sliding joints accounting for the residual charge
effect.
Despite the popular exposition in parts,108 Lodge
had many points to make in this book and elsewhere which
were found to be valuable. His discussion of rapidly
varying magnetic fields, and the 'skin' effect of currents
starting from the outside of a conductor, for example,
were noted by Rutherford109 who may also have noticed
some suggestive comments on phenomena explained by the
electrical nature of chemical atoms.110
These then are some of the areas developed by
physicists in the last decades of the nineteenth century,
which form a background to the understanding of the
younger physicists beginning their researches in the
1890's.
32
4. Chemical physics. Physical chemistry Aspects of chemistry and of physics in the 1880's
and 1890's have so far been discussed. Perhaps even more
important than either of these from our point of view
were the attempts at applying physical theories to
chemistry, together with the study of areas considered
to be intermediate between the two, which became stronger
during this period.
Lodge's book of 1889 contains allusions to aspects
of chemistry which are clear, if brief. He speculated as
to whether the etherial 'whirls' or wheels might represent
not simply electricity but 'atoms',111 and seems generally
to have been prepared to use the word atom in its chemical
sense. For example, in a chapter on the 'Mechanism of
Electrical Radiation' the emission of light, understood
as an electromagnetic phenomenon, is attributed to
oscillation of the unit electrical charges whose existence
he took electrolysis to have demonstrated:
It can be calculated that the oscillation of an atomic charge would give rise to only ultra-violet rays. It is probably because these ultra-violet rays synchronize with the period of vibration of atomic charges that they have such extraordinarily powerful chemical effects...112
Lodge was one of the first to attribute the effect of heat
in producing definite spectra of chemical substances to
the thermal, hence mechanical, oscillation of the charged
atoms in a molecule:
Under the influence of heat the components of the molecule are set in vibration like the prongs of a tuning fork, the rate of vibration depending on and being characteristic of the constants of the particular molecule. The atoms being charged, however, their mechanical oscillation is necessarily accompanied by an electric oscillation, and so an electric radiation is emitted and propagated outwards...113
Lodge's suggestion as to the explanation of phosphorescence
involved 'atoms receiving indirectly some of the ethereal
disturbance, and so prolonging it by their inertia, instead
of leaving it to the far less inertia of the ether alone'.114
He referred115 to the recent researches of Hertz, Ebert,
Wiedemann and others on electrical effects of ultra-violet
light, and attributed these to the same cause as the
33
chemical effects of light; this was to be clarified by
J.J.Thomson and his associates C.T.R.Wilson, E.Rutherford
and others at the Cavendish Laboratory in the next few
years and by the German physicists J.Elster and H.Geitel,
who were to develop strong interests in radioactivity.
We recall that about five years after Lodge's account
Drude, whose book of 1894 was consulted by Marie Curie,
qualitatively compared the 'molecule' to an oscillatory
electric circuit. Now although Lodge seems to have
accepted that the mechanical oscillation of fixed atomic
charges was the source of the luminosity caused by heating,
he also suggested what appears to be an alternative idea
that 'those short ethereal waves which are able to affect
the retina, and which we are accustomed to call "light",
may be really excited by electrical oscillations or
surgings in circuits of atomic dimensions'.116 Rough
estimates for a single loop of wire showed that this
circuit would have to be of atomic dimensions to give
frequencies of the right order.
G.J.Stoney, however, in 1891 made a distinction
between the two ideas, and specifically criticised not
Lodge's version, but the suggestion that discharges
between molecules could be the source of spectra:
The lines of the spectrum of a gas are due to some events which occur within the molecules, and which are able to affect the ether. These events may be Hertzian discharges between molecules that are differently electrified, or they may be the moving about of those irremovable electric charges, the supposition of which offers the simplest explanation of Faraday's law of electrolysis ... Several considerations suggest that the source of the spectral lines is to be sought not in the Hertzian discharges, but in the carrying about of the fixed electric charges, which, for convenience, may be called electrons.117
This statement was reaffirmed and extended in 1894118 when
he wrote that 'the motions going on within each molecule or
chemical atom cause these electrons to be waved about in
the luminiferous aether' and that 'the only other
conceivable source of these spectra is excluded, viz.,
Hertzian undulations consequent upon electric discharges
within and between the molecules'. This exclusion was
34
owing to Fitzgerald's estimate that the frequency would
be higher than any known part of the spectra of gases.
The proportionality of the wavelength emitted to the
geometric mean of the capacity and self-inductance of a
conducting circuit was the vital relationship for Lodge's
calculation,119 but that accepted by Stoney and leading
to such different conclusions is at present not clear.
The cause of radiation adopted is of interest for its use
of the electrolytic 'electron' entity, a term which was
coined by Stoney himself at about this time.120 Considering
its importance in the physical science of the next decade
it is valuable to see briefly how this had arisen.
A mathematical physicist with strong interests in
kinetic theory, spectroscopy, and the chemical periodic
table, G.J.Stoney had in 1871 provided a quantitative
explanation of line spectra.121 The harmonics of a
vibrating string were considered as the basis of a
mathematical comparison, but the quantitative aspects
were refuted by A.Schuster by the end of the next
decade.122 During these years however, Stoney began to
produce the basis of his electron theory which provided explanations in some areas of contact between physics and
chemistry in the 1890's. A paper read to the British
Association in 1874123 and published only in 1881,
entitled 'On the Physical Units of Nature'124 combined
chemical atomic theory with Faraday's Law to give the
required Physical Unit of electricity:
And, finally, Nature presents us, in the phenomenon of electrolysis, with a single definite quantity of electricity which is independent of the particular bodies acted on. To make this clear I shall express 'Faraday's Law' in the following terms, which, as I shall show, will give it precision, viz.:- For each chemical bond which is ruptured within an electrolyte a certain quantity of electricity traverses the electrolyte which is the same in all cases.125
Stoney here defined and discussed the chemical atom, gaseous
molecule, and the 'hands or feelers which each atom has and
which by grappling with the hands or feelers of other atoms,
establish bonds between them'. He seems not to say that
the bonds are purely electrical in nature, but stresses
35
that in electrolysis 'a definite quantity of electricity
traverses the solution for each bond that in separated'.
Helmholtz' emphasis in a similar discussion in the same
year was somewhat different as will be seen. The
definite charges were employed by Stoney in a new
explanation of spectral series of doublets, and triplets,
about 1890. This involved mathematical analysis of
waves emitted by charges following orbits within a
molecule, this consisting of one or more chemical. atoms
which may possibly be vortex atoms.127 What became of
radiation produced by rotational and vibrational motion
of molecules, which they possess according to kinetic
theory, is not explained but this was a problem not only
of Stoney's theory.
By 1895 he had combined this electron theory with
the kinetic theory of gases to account qualitatively for
several phenomena of chemical physics such as phosphor-
escence, gas absorption spectra, luminosity in certain
chemical reactions, and the ratio of specific heats of
gases with complex molecules. His explanation of
phosphorescence is of interest with regard to Becquerel's
understanding of the newly discovered uranium rays in
terms of this phenomenon six months later; the latter
did not suggest details of mechanism, electrical,
electronic or molecular.
Stoney classified vibrations within molecules
according to the ease with which they exchange energy
with translational motion, upon collision. A motions
are immediately affected, C motions are unaffected by
collisions, and B motions are intermediate:
Thus, when a phosphorescent body has been exposed to suitable light, t is an electron associated with Bb motions18 that is primarily acted on by the aether.129
When an electron is associated with more rapidly
exchanging Ba motions or events, etherial vibrations
received are transferred rapidly to translational
motion, phosphorescence does not occur, the temperature
rises and etherial undulations cease 'in other words,
the gas is one that has an absorption spectrum:130
126
36
Extending this kind of explanation he noted that'the
number of electrons within an atom may be greater than
its place in Mendeleeff's table would seem to suggest'
as shown by the chemical behaviour of potassium and bonds
between molecules in crystals. The electron is now taken
to be the principal factor in any chemical bond, and
formulae such as H.CiC.H for acetylene, with one electron
per bond, are given. He suggested that 'it is when excited
by chemical reactions that electrons produce their most
conspicuous luminous effects' which included the luminosity
in electric discharge tubes.131 Though it is uncertain
whether the electron theory of Stoney of 1895 and earlier
was directly used by those studying radioactivity, the
publications were readily available, and similar ideas
were shared by others. Among these Arthur Schuster wrote
not of the 'electrons within an atom' as had Stoney, but
of electrons 'moving along the surface of an atom' about
positions of equilibrium, to account for line spectra.132
Larmor's rotational ether electrons and their applications
have been mentioned, and J.J.Thomson's ideas were akin to
these at the time. In Germany similar developments were
occurring; Ebert's paper on 'Heat of Dissociation
according to the Electrochemical Theory' of 1894133
provided quantitative evidence that the energy of chemical
bonds was purely electrostatic and not 'specially chemical'.
He credited Helmholtz with the first statement, in 1881,
of an electrolytic atomic charge. Stoney wrote134 to
claim priority over Helmholtz on this point, and over
Ebert for his auggestion135 that 'motions going on within
each molecule or chemical atom cause these electrons to
be waved about in the luminiferous ether' to produce
spectra.136 The suggestion that the luminous radiation
produced by friction, disruption of a crystal, and chemical
reactions could be understood in terms of electrons which
'are started into activity'137 is of interest; for
Rutherford struggled with this kind of idea, as did others,
in studying the properties of radioactive emanations and
their radiations some five years later.
37
It is to be noted that such a union of chemical and
electrical science had developed comparatively recently;
C.A.Russell has described the Faraday Lecture delivered
by Helmholtz to the Chemical Society of London in 1881138
as marking the beginning of 'The Renaissance of Electro-
chemistry', after the subject had suffered thirty years
of disrepute.139 The claims of Stoney are ignored by
Russell, and with justification, for although prior by
some months, his statements of 1881140 seem not to have
had the same influence nor to have gone as far as those
of Helmholtz towards a purely electrical theory of
chemical combination. Indeed at this time Stoney seems
to have gone little beyond Maxwell's comments of 1873
that electrolytic phenomena necessitate the assumption
of definite molecular charges,141 but that 'chemical
combination is a process of a higher order than any
purely electrical phenomenon'.142 On the other hand
Helmholtz in 1881 while admitting 'other molecular forces'
did stress that in all compounds 'the very mightiest among
the chemical forces are of electric origin. The atoms
cling to their electric charges, and opposite electric
charges cling to each other...'143 That there was arising
interest by the 1890's in the subject of electrochemistry,
together with the study of colligative properties such as
osmotic pressure, is shown by the publication of experi-
mental and theoretical papers, some controversial, by
many authors.144 Oliver Lodge provided a report 'On
Electrolysis' for the British Association in 1885145 at
the urgent request of H.E.Armstrong, President of the
Chemical Section, who found Helmholtz' ideas of chemical
affinity not proven. Lodge's words show a not entirely
willing interest in the subject for 'though convinced of
the immense importance' of its study, he considered it had
'the somewhat repulsive character attaching to any
borderland branch of science - in this case not wholly
physics nor wholly chemistry'.146 Other scientists saw
this subject's importance but did not take Lodge's view
of borderland branches. Some physicists, and chemists,
though few in number found the investigation of such areas
38
rewarding. And it is true that most of the older
scientists who had sufficient interest to publish on the new subject of radioactivity in the first few years
of the twentieth century, were those studying such border
areas during the last decades of the nineteenth. Among
chemists were Armstrong, Crookes and Mendeleef; physicists
include Becquerel, Kelvin, Lodge, Stoney, Schuster and
J.J.Thomson.
The work of J.J.Thomson is of particular significance
for radioactivity both with regard to his own interpretations
of its problems, and to his influence upon Rutherford,
before, during and after the latter's three years (1895-8)
at the Cavendish Laboratory.
From early in his scientific career Thomson developed
strong interests in Maxwell's electrical theory and in the
application of such physical theories to aspects of
chemistry, especially with regard to the electrical
properties of gases. By 1894, reporting to the British
Association on 'The Connection between Chemical Combination
and the Discharge of Electricity through Gases'147 he
could conclude that his experiments:
give hopes that the study of the passage of electricity through gases may be the means of throwing light on the mechanism of chemical combination. The work of chemists and physicists may be compared to that of two sets of engineers boring a tunnel from opposite ends—they have not met yet, but they have got so near together that they can hear the sounds of each other's works and appreciate the importance of each other's advances.148
These hopes, shared by others at the time, were soon to be
fulfilled. However, Thomson's more ambitious ideas of the
physical structure of chemical atoms date back at least
as far as the early 1880's. These derived from the vortex
atom theory of Sir William Thomson149 who had taken
Helmholtz' mathematical treatment of vortices in a perfect
fluid (1858) to represent chemical atoms composed of fluid
ether in motion;150 the former had been impressed by Tait's
39
smoke-ring demonstrations (1867). Permanence, indestruct-
ibility, the gas laws, and many possible modes of
vibration for spectra, were all accounted for quantitatively
or mainly qualitatively by single, linked, or knotted
vortex rings. The explanation of gravitation and the
inertia of matter was problematical as Maxwell's
resurrection of Le Sage's theory indicates.151 Maxwell
pointed out that this theory had the flaw of predicting
a temperature rise of the material bodies involved due to
the impact of the etherial corpuscles which were supposed
to cause the net gravitational force. Kelvin in 1881152
said that he would not be satisfied with the vortex atom
theory until chemical affinity, electricity, magnetism,
gravitation, and inertia could be explained by it. He
pointed to the insoluble contradiction between the isotropy
of gravity and the anisotropy of crystals; he was evidently
also unable to develop the chemical aspects of the vortex
atom theory. And by about 1890 he seems to have rejected
it in favour of Boscovichian explanations of physical and
chemical properties; Kelvin now used action-at-a-distance
force laws between atoms of matter - meaning chemical
subatoms e.g. H = (h h) - and atoms of an electric fluid.153
Attempts to find unified explanations of the properties of
matter, electricity and luminiferous ether were not so
strong in Kelvin's thought as in that of some other British
scientists, from the 1890's through the early period of
radioactivity.
J.J.Thomson was awarded the Adams prize of 1882 at
Cambridge for 'A general investigation of the action upon
each other of two closed vortices in a perfect incompressible
fluid'. His mathematical treatment was published in the
following year as A Treatise on the Motion of Vortex Rings154
with some significant additions on chemical applications.
Referring to Kelvin, as he did on many points, Thomson
introduced the study by pointing out that the vortex ring
'possesses many of the qualities essential to a molecule
that has to be the basis of a dynamical theory of gases',
it is indestructible and indivisible; 'the strength of the ring155 and the volume of liquid composing it remain for
40
ever unaltered', and rings 'will retain for ever the same
kind of be-knottedness or linking'. It possesses kinetic
energy by virtue of translational motion, and 'it can also vibrate about its circular form, and in this way
possess internal energy', which was promising for
explanations of heat and radiation. That the treatment
was almost entirely kinematical, once having accepted
Helmholtz' hydrodynamics, was taken to indicate its more
fundamental nature compared with the ordinary 'solid
particle' kinetic theory which required the assumption
of repulsive forces.156
Thomson developed Kelvin's vortex atom version of
gas kinetic theory as far as a derivation of Boyle's law
and gave some suggestions for possible experiments to
decide between this and the ordinary kinetic theory;157
it is not known whether these were tried. Applying these
results to gaseous chemical compounds Thomson assumed
that atoms combined in the manner of the association of
two vortex rings of equal strength, when one overtakes
the other. In this case, providing their dimensions are
compatible, the hinder one passes through the one in
front, they do not separate, but continue to circulate
in and out of one another; a transverse section would
show two separate circles rotating about a point midway
between them. The disturbance of neighbouring rings
would alter the radii and cause a brief separation
resulting in continual change of partners as in the
theory of Clausius and Williamson. The ratio of paired
to free time is of importance with respect to whether
combination occurs or not, and this factor might link
the chemical strength with the dielectic strength of a
gas. His attempts to extend the results for linked
columnar and, mathematically similar, ring vortices to
chemical bonding and atomic structure is of considerable
interest. Firstly, the kind of linking considered was not
in the manner of a chain, but like the strands of a
twisted rope, thus:
take a cylindrical rod and describe on its surface a screw with n threads ... bend the rod into a circle and join the ends, then each of the n threads
41
of the screw will represent the central line of the vortex core of one of the n equal linked vortices...158
The vortices could be independent twisted rings, or if
not joined end to end there could result 'an endless
thread with n loops'. He assumed that the atom of each
element was composed in either of these similar ways, of
a number of rings, or a single ring. It was demonstrated
that six rings (or columns), or less would maintain a
stable motion; the transverse section would show, for
example, six separate circles on the points of a hexagon
moving around the midpoint of the hexagon;159 this he compared with Mayer's experiments on the stable arrange-
ments of thin vertical magnets floating in water. Thomson
used the same analogy in 1897 when his corpuscular atomic
theory was first set out.160
The picture of chemical combination is difficult to
discern from Thomson's descriptions and is in need of clarification.161 The combination of two atoms each of a
single ring is described above, and it is this kind of
circulating and mutual overtaking motion which applies
both to the linked rings within a single complex atom, and
to the associated rings in a molecule of two or more atoms.
Thus when two of the complex atoms have combined to form
a molecule, the vortices form a unified system; should
there be a total of more than six rings, they must group
into 'primaries' consisting of six or fewer 'secondaries'.
A transverse section of any molecule would show single
circles, or groups of circles (primaries), arranged on
each point of a polygon. Each primary group rotates about
its own point, and the polygon itself rotates about its
centre.
An explanation of chemical valency, the main burden of
the later additions to the essay, relied on the simplifying
assumption that the strengths of all the rings composing an
atom are equal. This led to the result that for atoms to
form a stable molecule, the strengths of the primaries must
be equal. This determines valency, and Thomson gives as an
example an atom of two rings combining with an atom of one
ring:
42
since for stability of connection, the strength of all the primaries which form the components of the compound system must be equal; the atom consisting of two links must unite with molecules containing two atoms of the one with one link.162
Thus the number of linked rings per atom is taken to be
the fundamental valency. That HO is far less stable than H2O follows from the implicit supposition that the three
rings of HO could not arrange themselves symmetrically
in section. With the assumptions firstly that the linked
rings of a single complex atom can break in various ways
into two or more primary groups, and secondly that the
single rings of monovalent atoms can join closely into a
compound primary, again with a maximum of six rings per
primary, apparent variable valency was explained. Water
was probably not H-H-O, with three primaries and oxygen a
monad, but more likely H2-0 with only two primaries and
oxygen divalent. Hydrogen peroxide would then be H2-0-01 with three primaries, each of two secondaries. Thomson's
further development of the theory in the following year,
1884,163 provoked a strong reaction from the physical
chemist Ostwald,164 which was perhaps a marginally better
reception than being ignored.
During the following decade Thomson published no
further descriptions of detailed atomic structure, and
attempted to apply more general methods such as physical
dynamics and the thermodynamics of Gibbs to aspects of
chemistry.165 The most interesting aspect of his work from
the point of view of our study was that on the relation
of electricity with chemical combination in the gaseous
state, which formed a major part of his researches. In his
paper 'On a Theory of the Electric Discharge in Gases'166 published shortly before the Treatise of 1883 Thomson made
an assumption strongly unifying matter and electricity -
both were seen as manifestations of the same ether:
Let us now suppose that we have a quantity of gas in an electric field. We shall suppose, as the most general assumption we can make, that the electric field consists of a distribution of velocity in the medium whose vortex-motion constitutes the atoms of the gas.167
The attempt was then described, to relate the electric
strength, chemical stability, temperature, and pressure of
43
a gas to one another, together with some quantitative
calculations and suggestions for experiments. He stressed
the intimate connection which he saw between electrical
conduction and chemical action:
Thus, according to the view we are now discussing, chemical decomposition is not to be considered merely as an accidental attendant on the electrical discharge, but as an essential feature of the dis-charge, without which it would not occur.168
This was a view which Thomson maintained, extended, and
studied experimentally for the next decade, and although
he did not develop the vortex atom theory further during
this period, in which its original proponent Kelvin came
to reject the idea totally,169 his explanation of the
electric field in terms of fluid motion became more
detailed. Thomson's description of the field in terms of
'tubes of force' of 1891,170 the theory of electrical
oscillations, and descriptions of the experimental and
theoretical work on the discharge of electricity through
gases, were treated in his Recent Researches in Electricity
and Magnetism, of 1893;171 these were cited by Rutherford
in his first papers, on magnetism and electromagnetic
radiation, during the next two years.
Each tube of electrostatic force is of unit strength,
starting on a unit electrolytic positive charge and ending
on a negative one, or else forming a closed ring.172 They
consist of vortex columns or filaments in the ether whose
kinetic energy constitutes the potential energy of the
electrostatic field; magnetic effects are produced by their
lateral motion, which also constitutes the propagation of
light in a quasi-corpuscular fashion.173 Thomson's theory
of chemical combination of 1893, with its hints of subatomic
structure, was purely electrical - an extension of his views
of 1883 and a modification of the theory of Helmholtz of
1881.174 All unclosed tubes join pairs of atoms, which are
considered to be chemically combined if the tube is of
molecular dimensions, but chemically free if the tube is
long: 'when a tube falls on an atom it may modify the
internal motion of the atom and thus affect its energy',175
which accounted for the differing affinity of atoms for
electricity postulated by Helmholtz.
44
'Now the laws of Electrolysis show that the number of
Faraday tubes which can fall on an atom is limited; thus
only one can fall on an atom of a monad element, two on
that of a dyad and so on'.176 The atoms in chemically
saturated compounds can receive no more tubes so that
each end of an unclosed tube always falls on a free atom.
The existence of free electricity in electrolytes, gases,
and metals too, therefore always requires free atoms and
hence chemical decomposition. Thomson gave diagrams of
the arrangement and movement of the tubes between atoms in a gas,177 but did not explain how the vortex tubes of
force were linked to the 'internal motion of an atom',
nor what form he now conceived this motion to take.
However, in his paper on 'The Relation between the
Atom and the Charge of Electricity carried by it',178
published in 1895 at about the time of the arrival of
J.A.McClelland, E.Rutherford and J.S.Townsend to study at
the Cavendish, Thomson gave something of a picture of an
electro-chemical vortex atom theory. The atom seems to be
differently constituted from that of 1882-3 and appears
incidentally to suggest an explanation of the directional
nature of valency, which the earlier theory did not:
Now let us consider the atoms on which these tubes end. Let us suppose that these atoms have a structure possessing similar properties to those which the atoms would possess if they contained a number of gyrostats all spinning in one way round the outwardly drawn normals to their surface.179
Within two years Thomson had constructed the first
corpuscular atomic model. His support for this theory
in some way involved Lockyer's dissociation hypothesis,
Lorentz' electron theory and the newly discovered Zeeman
effect, as we have seen. But the attainment of such a
theory depended not only upon the discovery of the charged
material 'corpuscle' by means of improved experiments on
the cathode rays, but on remnants of the vortex theory
of the chemical atom.
This is illustrated by others who had developed
electrical and vacuum techniques sufficiently to obtain
results similar and perhaps prior to those of Thomson,
but did not make such significant use of them at the time.
45
The publications of W.Kaufmann180 and E.Wiechert181 in
1897 are notable in this respect. From his experimental
results Kaufmann drew only the limited conclusion that
if the cathode rays were particles then their charge to
mass ratio was 107, which was unexpectedly large. Wiechert
obtained a similar e/m value; he went further than Kaufmann
by assuming that the rays were in fact particles, and that
they possessed the electrolytic charge. He thus estimated
their mass to be of the order of 1/1000 of the lightest
atom. Wiechert continued to concentrate on mathematical
electrodynamics and did not develop the chemical impli-
cations.182
It is generally stated that Thomson's first conception
of a subatomic corpuscle dates from about the beginning of
1897, perhaps after knowing of Zeeman's new experimental
results.183 This was closely linked with his experiments
showing that the magnetic deflection of cathode rays was
independent of the gas in which they were produced,184 as
well as mean free path, velocity, and other considerations.
However, Thomson's continuing interest in chemical atomic
structure throughout the experimental developments has
previously not been clearly brought out. We have already
noted his theory of electrical conduction of all kinds
published in 1895, which involved vortical tubes of force
linking atoms containing gyrostats; another aspect of
Thomson's chemical ideas soon became evident. In April
1896, reviewing experimental advances on the recently
discovered Ontgen and Becquerel rays,185 he several times
slightly misstated Rontgen's result that for a series of
metals the absorption of the X-rays is in the same order
as their density.186 For example, he mentioned 'Rbntgen's
discovery of the close connection between the absorption
of these rays and the atomic weight of the absorber'.187
Either version fitted the experimental results but Thomson's
statement may be significant for he also suggested that the
absorption of these rays, considered to be of very short
wavelength, was caused by Proutian primordial atoms. The
expression 'atomic weight' was indeed correctly replaced by
'density' in the report188 of the Rede Lecture given some
weeks later, but in this we read:
46
There seems no simple relation between the density of a body and its transparency to visible radiation or electrical vibration; in the case of the Wintgen rays, however, it seems the greater the density the greater the opacity. This appears to favour Prout's idea that the different elements are compounds of some primordial element, and that the density of a substance is proportional to the number of primordial atoms; for if each of these primordial atoms did its share in stopping the Wintgen rays, we should have that intimate connection between density and opacity which is so marked a feature for these rays.189
That Prout's hypothesis could be brought up on such
evidence indicates a predilection for the idea in 1896.
The statement was made with reference only to X-rays,
seen as radiation in terms of tubes of force,190 and not
to cathode rays, seen as particles with etherial effects.
However, in his well-known paper of 1897191 Thomson
laid stress upon the distances penetrated by the cathode
ray particles without mentioning X-rays: 'Now Lenard
found that this distance depends solely upon the density
of the medium and not upon its chemical nature or physical state'.192 By this time Thomson considered the cathode ray corpuscles to consist of 'a substance from which all the
chemical elements are built up'193 and in imagining this
building194 he considered the possibilities of Mayer's
magnets more deeply than with the vortex atom of 1882-3.
In the atom the material corpuscular components, whose
charge might be much greater than the electrolytic charge
(Stoney's electron),195 were held together by tubes of
force, many for each corpuscle. Only a single stray unit
tube would bind for example a hydrogen to a chlorine atom,
giving HC1. He did not discuss the existence of vortices
within a corpuscle to which the tubes are connected nor,
later, the possible gyrostatic nature of the unit-charge
corpuscles which he soon accepted. Thomson's stress, and
defence, of the material nature of the corpuscles seems
compatible with some kind of etherial vortex explanation
of matter, electricity and light. But Thomson had said
that such a theory 'cannot be said to explain what matter
is, since it postulates the existence of a fluid possessing
inertia'.196 A purely electromagnetic explanation of matter,
47
in which studies of the radioactive rays played a vital
part, was to develop by 1901.197
As Thomson struggled with experiments and Proutian
ideas involving X-rays and cathode rays in 1896, and was
taking an interest in the Becquerel rays, Rutherford
wrote home from Cambridge:
I am working very hard in the Lab. and have got on what seems to me a very promising line - very original needless to say. I have some very big ideas which I hope to try and these, if successful, would be the making of me. Don't be surprised if you see a cable some morning that yours truly has discovered half-a-dozen new elements, for such is the direction my work is taking. The possibility is considerable but the probability rather remote.198
Rutherford's known laboratory notes199 and his published
papers on the newly adopted subject of the electric
properties of gases exposed to radiation - X-rays,
uranium rays, and ultra-violet light - give us obscure
clues as to the nature of these new ideas. We have,
however, noted the long-standing interest of his Professor
in chemical atomic theory. We shall see that Rutherford
shared this interest and how what may have been no more
than a light-hearted prediction became fulfilled and
exceeded through radioactivity. But the existence of the
first new chemical element arising from these studies was
proposed in France within two years of Rutherford's boast -
and then he did not believe it.
48
CHAPTER 2
THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY
1. Becquerel's discovery of uranium rays (1896-7) From ROntgen's famous experimental discovery of X-rays
stemmed Henri Becquerel's discovery of uranium rays.
Within days of ROntgen's paper 'On a new kind of radiation'1
many scientists were using the readily available Hittorf,
Lenard or Crookes vacuum tubes to verify, and advance
studies on the new phenomenon. The most spectacular aspect
of the rays was their capability of penetrating certain
solid materials to produce fluorescence or photographic
action on an adjacent screen or plate, giving images for
example of bones within the living body. Most of the
thousand articles and fifty books published on the subject
during the year of 1896 were concerned solely with such
medical possibilities.2 However, there were several points
for physicists to consider, the question of the source of
the rays being an important one. Their discoverer pointed
out that they appeared to come from the most brightly
fluorescing part of the glass of the vacuum tube,3 and it
was this statement, though not at first hand, which came
to set Henri Becquerel on a somewhat novel line of research.
It is my intention firstly to describe and discuss the
experiments and hypotheses which constitute Becquerel's
discovery of uranium rays and to indicate some of the
neglected background to those studies. This is followed
by an account of other varied and novel researches
emerging during the period 1896-7 which may provide an
improved interpretation of the contemporary attitude to
the Becquerel rays.
Unlike the events leading to the discovery of X-rays,
which remain obscure,4 the beginnings of the experimental
work with which we are now concerned were clearly described
by Becquerel, some seven years later,5 leaving only a few
questions in doubt:
Dans la seance de l'A.cadgmie des Sciences du 20 janvier, au moment ou M.H.Poincarg venait de montrer les premieres radiographies envoy6es par
49
M.ROntgen, je demandai a mon confrere si l'on avait determing quel etait, dans l'ampoule vide productrice des rayons X, le lieu d'emission de ces rayons. Il me fut repondu que l'origine du rayonnement etait la tache lumineuse de la paroi qui recevait le flux cathodique. Je pensai aussit8t a rechercher si l'emission nouvelle n'etait pas une manifestation du mouvement vibratoire qui donnait naissance a la phosphor-escence, et si tout corps phosphorescent n'emettait pas de semblables rayons. Je fis part de cette idee et de ce projet a M.Poincare...6
The weekly report of the Academy for 20th January in fact
contains only a brief note of the exhibition of some of
the earliest X-ray photographs; H.Poincare,Professor of
mathematical physics at the Sorbonne, first published the idea ten days later.7 The experimental connection between fluorescence and X-ray emission was eroded throughout
the year, as it was shown that the point of incidence of
the cathode rays upon solid objects was the primary source,
that rays were not emitted from other brightly glowing
parts of the glass tube, and that metals could emit X-rays
unaccompanied by visible fluorescence.8 However, such a
connection was under active consideration at the beginning
of 1896, and whilst J.J.Thomson at Cambridge reported
failure to obtain penetrating radiation from fluorescing
glass, gases, and luminous paint,9 others, in France,
announced success.
Henri Becquerel was the third of four successive
members of the family to be Professor of Physics at the
Natural History Museum in Paris; his father Edmond and
himself were well known for their work on various aspects
of physics, particularly phosphorescence.10 G.G.Stokes had coined the term 'fluorescence' for emission of light
only during the time of irradiation, and considered
'phosphorescence' which continued after irradiation had
ceased, to be of a different nature. Edmond Becquerel,
however, insisted on an experimental continuity between
the two, using his rotating 'phosphoroscope' of 1858 for
timing short durations. Whilst he always used the term
'phosphorescence',11 some maintained Stokes' distinction,
and others continued to use the terms indiscriminately.12
During the last quarter of the nineteenth century
50
the subjects of phosphorescence and fluorescence had
provided opportunities for a considerable effort in
experimentation upon a large number of inorganic and,
in the 1880's, organic substances, as solids, solutions
or gases. Studies included examinations of the relations
between the wavelengths and intensities of absorbed and
emitted light, the rise and decay of intensities with time,
and the marked effect of traces of metal compounds in
solids. Following Stokes' discovery of fluorescence, and
his explanation of the phenomenon in terms of vibrations
of molecules, in 1852,13 E.Lommel then others in Germany
developed mathematical theories of oscillations of part-
icles, natural, forced and damped, which partly agreed
with the observed phenomena.14 A clear picture of the part
played by studies of luminescence in the development of
physical science has yet to be created; this does not
concern us here, but we have noted that several scientists
who were linked with radioactivity entertained qualitative
electrical atomic-molecular views of phosphorescence in
the 1890'8.15 E.Becquerel who is said to have dominated
the experimental field16 until about 1880 put forward only
qualitative explanations of his results;17 his son Henri
continued the researches in a similar way. Some of the
latter's comments concerning uranium salts are of interest
in throwing some light on the way in which he understood
the phenomena of phosphorescence, which came to be so
closely linked with the new uranium rays in 1896. In
discussing the 'Relations entre l'absorption de la lumiere
et 1'6mission de la phosphorescence dans les compos6s
d'uranium' in a paper of 188518 he concluded that the
compounds of uranium are in 'un etat moleculaire' such
that they exert a selective absorption of harmonically
related wavelengths of light, and that some of these
compounds in the uranic series also emit harmonically
related bands of a longer wavelength; some bands were
common both to absorption and emission spectra, which
suggested that the property 'de vibrer a l'unisson' might be the actual cause of absorption. He noted that the
broad green band of the emission spectrum of incandescent
51
uranium vapour was very close to an absorption band
characteristic of uranous salts. Perhaps he considered
that the emission of light, or of certain bands by uranium
vapour might be due to its molecular state, but Becquerel
gave no detail. Six years later, in 1891, his views 'Sur
les lois de l'intensite de la lumiere 6mise par les corps
phosphorescents119 were that the laws of rise and decay
of luminous intensity could be related to an equation of
simple harmonic vibration, modified by taking into account
the loss of energy of the oscillating molecules via the
'ether intermoleculaire', which loss was taken as proportion-
al to the square of a velocity term. And in a note of the
same year 'Sur les differentes manifestations de la phos-
phorescence des mineraux sous l'influence de la lumiere ou
de la chaleur'20 Becquerel speculated on the mechanism by
which the 'conservation indefinie dans les corps' of the
energy of phosphorescence-by-heat (later termed thermo-
luminescence) might be achieved. Was the substance in a
state comparable with magnetism, or:
La deperdition d'energie est-elle continuellement compensee? Ce sont des questions que l'on ne saurait decider actuellement et sur lesquelles les etudes ulterieures apporteront peut-kre quelque lumiere.21
He did not publish further on this point himself; his rival
E.Wiedemann in conjunction with G.C.Schmidt had provided
the beginnings of a chemical explanation by 1895.22
Becquerel's studies of the connection between the new X-rays
and the phenomenon of phosphorescence led him to his great
discovery of uranium rays and to questions of energy which
were even more difficult to answer. Henri Becquerel's first publication for some three
years was again on the subject of phosphorescence. In his note of 24th February 1896 'Sur les radiations emises par
phosphorescence'23 he cited not Poincare but Charles Henry,
of the gcole Pratique des Hautes Etudes, Paris, with whom
there were to be controversies later that year.24 Henry
in his paper on 'Augmentation du rendement photographique
des rayons ROntgen par le sulfure de zinc phosphorescent'25 showed that phosphorescent zinc sulphide placed in the path
of the rays from a Crookes tube intensified the photographic
52
effect of rays penetrating aluminium. Ho quoted Poincare's
query: Ne peut-on alors se demander si tous les corps dont la fluorescence est suffisamment intense n'emettent pas, outre les rayons lumineux, des rayons .X de Röntgen, quelle que soit la cause de leur fluorescence...26
and considered that he could answer it in the affirmative.
This statement of Poincare, repeated by Henry, does contain
the point claimed as his own by Becquerel in 1903,27 but
one cannot thereby deny this claim. Becquerel also mentioned
Niewenglowski's announcement of the previous week28 that
the sulphide of calcium as well as zinc produced penetrating
rays during irradiation merely by sunlight. Becquerel extended these observations to some unnamed phosphorescent
substances and in particular to uranium salts whose brilliant
phosphorescence of short duration was well known. The
techniques which were involved, simple though fraught with
pitfalls, are illustrated by Becquerel's description of
some of his first experiments.29 He wrapped a Lumiere
bromide-in-gelatine photographic plate in two sheets of
dense black paper, light-proof for a day's exposure to sun-
light. Upon this was placed a crystalline lamella of
potassium uranium sulphate, with a glass slip interposed
to avoid the chemical effects of vapours. Exposure to
sunlight for several hours to produce the required pen-
etrating phosphorescence, followed by development of the
plate, revealed a blackened area. Becquerel reported his
success in obtaining images of metal objects by placing
these between the phosphorescent source of the penetrating
rays, and the sensitive plate. This had previously been
achieved only by means of X-rays. The fortunate experiments made by Becquerel during
the following week and announced at the next meeting of
the Academie on 2nd March,30 were immediately seen by him-
self as important, and can now be interpreted as a sig-
nificant marker in the initial stages of Becquerel's
research. He firstly described experiments showing that
photographic action occurred after excitation by reflected,
refracted, or diffused sunlight, and through aluminium and
copper foil as well as black paper, and then stressed his
53
view of the importance and the unusual nature of some new
observations.
Combinations of a lamella of the double sulphate of
uranium and potassium, K(UO)SO4.H20, aluminium sheet, and
photographic plate prepared for irradiation on Wednesday
26th and Thursday 27th February were never thus treated,
owing to the intermittent character of the sunlight during
these days. They were placed in a drawer, and then dev-
eloped on Sunday 1st March.31 Expecting weak images, instead
he found very intense ones, and immediately concluded that
the action must have been occurring in darkness, 'l'action avait da continuer a 1'obscurit6'. He found experimental confirmation that day. Three lamellae, one placed directly
on a photographic plate, the others with glass or aluminium
sheets interposed, were left for five hours in a cardboard
box inside another cardboard box, in a drawer. The whole
operation was performed in the dark-room. Becquerel found
images, weaker with the aluminium, exactly as when the
crystals were irradiated by sunlight. His interesting,
but tentative, hypothesis was that the effects might be
due to invisible radiations 'emises par phosphorescence'
but of a duration 'infiniment plus grande' than that of
visible phosphorescence, known to be but 1/100 sec. for the salt used. He considered that he had found 'un nouvel ordre de phenomenes'.
Difficulties were seen when Becquerel repeated his
conclusions at the Physical Society meeting, later that
week, adding that certain phosphorescent crystals did not
give the rays. For if the effects were due to radiations
of shorter wavelength than those from the sun as Becquerel's comparison with Lenard and Röntgen rays was taken to imply, Stokes' law of phosphorescence would be contradicted; it
was therefore suggested that the new penetrating rays might instead be of a longer wavelength.32 Experimental invest-igation and comparison of his new rays with X-rays and an examination of the relationship with visible phosphorescence
was Becquerel's way forward and away from the heavy crit-icisms levelled at the work of G.Le Bon on Ilumi6re noire'P His remarkable results, announced within days34 and standing
54
unchallenged for two years, appeared to provide a nearly
complete answer to the question of the nature, if not the
source, of the new rays, and furthermore to clarify the
understanding of X-rays. Placing the salt within the vessel of a gold-leaf
electroscope which was shielded from electric radiations35
by a metal screen, and against ultra-violet rays by yellow
glass, Becquerel showed that the new rays caused the diss-
ipation of positive and negative charges at equal rates.
Thus applying this electrical and more quantitative tech-
nique he found that the time of collapse of the leaves was
proportional to the thickness of an interposed aluminium
screen. These were important properties, which X-rays
were known to possess by this time, so that the new rays
were evidently similar to X-rays. By means of a steel plane
mirror, and a concave hemisphere of tin, diffuse photo-
graphic images were obtained. Becquerel took these results
to indicate a definite, if diffuse, reflection of the rays,
in conjunction with more striking experiments. Glass tubes
filled with a powdered phosphorescent substance were
attached perpendicular to a glass sheet which was placed
upon the photographic plate. The resulting image, which
was a clear circle with a black disc within, surrounded by
a blackened area, proved that the invisible rays were both
reflected and refracted in the same way as light. These
experiments were later found to be repeatable,36 but
Becquerel's interpretation was to be rejected in a little
over two years37 when the images were attributed to
'secondary' radiation. These results were soon to be
joined by some which are dubious or inexplicable and by
others which stand to this day. Becquerel's survey of
phosphorescent minerals showed that several uranium salts
and two calcium sulphide specimens gave rays capable of
penetrating 2mm.of aluminium but that other phosphorescent
substances failed to give penetrating rays. Perhaps, he
thought, the phenomenon could be likened to visible
'thermoluminescence'; the latter involved earlier excit-
ation, then an emission of visible radiation upon gentle
heating at a later time.
55
The calcium sulphide specimens had vanished from
Becquerel's list of emitting substances by the time of his
fourth report delivered two weeks later on 23rd March.
How they had come to be included at all is not clear. He
remarked that no form of thermal or electrical excitation
could re-excite the emission of invisible radiation from
these still highly phosphorescent substances and that
Troost, Professor of General Chemistry at the University
of Paris, in following Becquerel's first experiments, had
experienced a similar effect.38 Thus only uranium salts
were now left.39 Their emission of penetrating rays, still
continuing after fifteen days, was reported by Becquerel
to be strongly intensified after illumination by electric
spark or arc, slightly intensified by daylight, but
imperceptibly by the light of burning magnesium, as
determined photographically; this latter method did however
cause excitation as determined electroscopically. His
conclusion was that the emission of invisible rays was a
kind of phosphorescence, excited by certain radiations,
but not closely connected with ordinary visible phosphor-
escence nor fluorescence. From the modern point of view,
there should have been no change in the intensity of the
radiation by these means. Perhaps he was misled by a
belief that the new rays ought to be capable of excitation
in the manner of ordinary phosphorescence, combined with
the flexibility of interpretation which his qualitative
experimental methods allowed. It was to become apparent
during the following year that such an increase in intensity
could not be produced; this was to place the phenomenon
further beyond accepted explanations. But even as they
were announced the problems of reconciling his conclusions
with Carnot's principle, as well as Stokes' law, were
recognised and discussed for example at the meeting of the
Physical Society which Becquerel addressed at this time.40
In his fourth note to the Academy41 Becquerel was
able to confirm the distinction between the invisible and
visible phosphorescence by the remarkable procedure of
melting a crystal of uranium nitrate in its own water of
crystallisation, when it ceased to exhibit phosphorescence
56
or fluorescence, and then allowing recrystallisation by
cooling, while excluding all luminous excitation. There
could now be no emission of visible radiation, but the
photographic effect of the invisible rays was as strong
as ever.
Becquerel was now turning towards a study of the
nature of the radiations; his final attempts at finding
their source were highly, if not completely, successful,
and are described in his next two papers. On 30th March42
he noted that 'un nouvel exemple d'ind6pendance entre les
deux ph6nomenes d'6mission' was provided by the invisible
rays detectable from the non-fluorescent solution of
uranium nitrate; and further, that visibly phosphorescent
sulphides could not be induced to emit invisible penetrating
rays, upon excitation with X-rays. After a seven-week
silence, awaiting Moissan's latest preparations of pure
metallic uranium,43 the purpose of Becquerel's sixth note
to the Academy was to announce the 'Emission de radiations
nouvelles par l'uranium metallique'.44 The inorganic
chemist Henri Moissan had turned from his successful
researches on fluorine towards high temperature studies
of boron, carbon and diamond, and silicon, in the 1890's;
he had applied his newly developed electric furnace of
189245 to the problem of preparing pure samples of .
refractory metals such as zirconium, chromium, manganese,
tungsten, molybdenum, vanadium, titanium and uranium,46
whose melting points lie above about 2000 degrees C. He
achieved the first fusions of some of these, and also
examined the carbides and carbon solutions of these and
other metals, including uranium.47
Thus Becquerel found that it was not only the salts
of uranium, but the element uranium itself from which the
rays were emitted. Uranium carbide, commercial uranium
powder and crystallised and cast uranium metal gave
stronger photographic and electrical effects than potassium
uranium sulphate; the charged leaves of an electroscope
collapsed about four times faster with uranium metal, at
rates of the order of 1 to 8 degrees per minute of time. Salts stored in a double lead box shielded from all
57
exciting radiation continued to emit radiation at a very slowly decreasing intensity. The excitation of the
emission above this level upon irradiation by the sun or
more effectively by electric arc or spark was, strangely,
confirmed; the level of emission declined to the normal
within hours. These results involving decay or excitation
were not corroborated by other workers, and were soon
forgotten, but at the time they agreed with Becquerel's
conclusions concerning the nature of the new phenomenon.
Without describing a possible mechanism he stated that
this was the first example of a metal exhibiting invisible
phosphorescence. Hence 'radiations uraniques' and 'rayons
uraniques' he named the rays in his next communication
some months later.48 Their intensity, he now cautiously
admitted, seemed hardly to have changed in eight months,
which fact was completely outside ordinary phosphorescent
phenomena: 'on n'a pu reconnaitre encore ou l'uranium
emprunte l'energie quill emet avec une si longue persist-
ance'.49 Becquerel had not solved this problem by the time of his last paper of the series, delivered six months
later in April 1897;50 the problem of the energy source was to worsen during the next seven years, to reach crisis
proportions in the view of some scientists.
But by 1897 Becquerel was concentrating more upon the nature and properties of the invisible rays; these studies
formed an important part of his discovery in the opinion
of fellow physicists. We have seen that by 9th March 1896
Becquerel's experiments indicated that the new rays possess-
ed the properties of penetration, and discharge of elect-
rified bodies, in common with X-rays, but were reflected
and refracted like visible light. On 23rd March51 he
reported qualitative confirmation of refraction, using a
glass prism and a linear source of the rays consisting of
a glass tube of lmm. diameter filled with crystalline
uranium nitrate. A further comparison between X-rays and
the new rays was the proportion absorbed by the same screen.
Becquerel found that X-rays were weakened four times as
much as the new rays, and took this to indicate that the
two kinds of radiation differed in wavelength. As for the
58
cause of the dissipation of electric charges by both
radiations, Becquerel professed ignorance, but noted
that the gas appeared to become conducting. Others with
previous experience in this area were working somewhat
more successfully on this problem at the time, as will be
seen. Becquerel's next note to the Academy was specifically
'Sur les proprietes differentes des radiations invisibles
emises par les sels d'uranium, et du rayonnement de la
paroi anticathodique d'un tube de Crookes'.52 He confirmed
unequal absorption of these radiations by different sub-
stances, both photographically and electroscopically, the
new rays being generally the more penetrating. Also, that
the new rays were non-homogeneous, as others had found for
X-rays, by interposing screens between source and electro-
scopic detector. The most strikingly successful of
Becquerel's experiments, so it seemed, was his attempt to
detect polarization of the rays, using crossed and parallel
tourmalines, with which Riintgen had obtained negative results for X-rays. Using the double sulphate of uranium
and potassium as the source of the rays, Becquerel found
that plates developed after sixty hours showed clearly
stronger intensities for parallel tourmaline crystals.
Becquerel took this to show that the invisible rays
suffered double refraction and polarisation of the two
refracted rays, followed by unequal absorption of the
differently polarised rays by the second tourmaline.
Although never repeatable and still inexplicable, these
results were eagerly accepted without public question
until 1899. A reviewer of 1898 commented that Becquerel's
demonstrations of the reflection, refraction, double
refraction, and polarisation of uranium rays showed that
'there can be no reasonable doubt that they are short
transverse ether waves';53 their possesion of all the
properties of X-rays indicated that the latter were similar.
Indeed, the view that these studies of uranium rays were
valuable in throwing light upon the nature of X-rays had
been expressed early in 1896. Some three weeks after
Becquerel's announcement of the polarisation of uranium
rays, J.J.Thomson's comment appeared in print:
59 The radiation from the uranium salts is thus intermediate in properties between ordinary light and Riintgen rays; and as there can be no question but that this radiation consists of transverse vibrations, inasmuch as it can be polarised, it affords presumptive evidence that the RCintgen rays are also due to transverse vibrations.54
J. Perrin made similar points,55 so too did G.G.Stokes
whose transversal irregular ether pulse theory of X-rays
was accepted by many physicists.56 As the most crucial of
Becquerel's results on the properties of the uranium rays,
and the conclusions drawn from them were shown to be false,
X-rays too began to be seriously considered in a different
light;57 but that is another story.
Despite the unreliability which was soon shown to
exist in some of his experimental work, Becquerel's
achievement in arriving at a demonstration of the existence
of uranium rays was of a remarkable nature. This is high-
lighted by the parallel studies by some of his contempor-
aries, which were seen as being related to Becquerel's
rays. Some of these studies proved to be very short-lived,
others were more reliable and came to be distinguished
from those on uranium rays, with varying degrees of
difficulty and rapidity. We have already noted that
Becquerel and Troost had obtained photographic effects
which they interpreted as being caused by rays penetrating
black paper and aluminium after issuing from phosphorescent
sulphides; these results stood only for a few weeks, though
they led to the discovery of uranium rays. It is notable
that S.P.Thompson, in London, followed a closely similar
path; like Becquerel he cited C.Henry on the augmentation
of X-ray photographs by phosphorescent zinc sulphide58 and
found that phosphorescent substances emitted such rays on
their own account. The merits of his claim59 to an
independent discovery of the rays from uranium salts
have been discussed;60 his explanation of the effect was
the same as Becquerel's; he coined the name 'hyperphos-
phorescence' but this was little used. Like Becquerel he
60
had found that 'a phosphorescent substance such as
sulphide of Barium' during and after illumination,
emitted, besides the visible radiation of phosphorescence,
rays which were like X-rays in being invisible, penetrating
aluminium sheets and producing a photographic effect. On
communicating this to Stokes,61 the authority on phosphor-escence, he was advised first not to delay publication
then, a few days later, that he had 'already been antici-pated'62 by Becquerel in papers earlier that month. Thompson63 specified uranium salts only after he had seen
these papers and had been notified by Stokes that the French
scientist had attributed the radiation to metallic uranium64 He asked Crookes for a specimen of uranium metal; the latter
was examining the efficacy of metals as radiators of X-rays
under the impact of cathode rays, and replied:
I have my metallic uranium in a vacuum tube at present, testing it against platinum as a radiator of the unknown X. So far it is decidedly the better of the two. I have another small piece which will be disengaged tomorrow, and then you shall have it.65
Uranium in metallic form was not readily available, though
H.Moissan provided some specimens; Crookes reported that
he had to discontinue these experiments owing to lack of the metal;66 there was no difficulty in obtaining compounds.
No doubt some of Becquerel's experiments of 1896 were
repeated by Stokes or others at Cambridge, Crookes or
S.P.Thompson, G.Le Bon in France,67 or by German physicists.
But no-one published a denial of the vital proof of double
refraction and polarization during that year nor the next.
The results of S.P.Thompson, Becquerel and Troost,
who had found that penetrating rays were emitted from
phosphorescent sulphides, were obtained by the photographic
method of detection; indeed the purpose of these experi-
ments, and those of C.Henry, was to obtain photographic
images. Becquerel from the first realised the danger of
chemical action from vapours and took precautions against
this; others did not. He knew that substances opaque to
visible light could be transparent to Hertzian waves,
infra-red and ultra-violet light, which might produce
photographic or electrical effects, and took measures to
61
avoid the possibility of confusion with uranium rays.
Others, in the rush to publish on X-rays and similar
phenomena during 1896-7, may have neglected any of these
points, and produced photographic results with conclusions which were soon proved false.
A few scientists thus took up Becquerel's work with
misguided enthusiasm finding Becquerel rays where we see
none now. H.Muraoka of the Physical Institute, Kyoto,
Japan, in his paper on the light from glow-worms,68
published towards the end of 1896, claimed that in addition
to visible light these worms emitted rays similar to those
of Becquerel in being capable of penetrating metals. The
photographic plate was blackened only in proximity to the
cardboard of the container, and the author explained this
by a concentration effect of the cardboard upon the glow-
worm rays. A partial denial of these results, attributing
some of the effects to vapours, in the following year is to his credit.69 In similar fashion, W.Arnold70 attributed
the action of certain substances such as metallic sulphides,
uranium salts, and retene on photographic plates to
'Becquerelstrahlen'. And A.F.McKissick reported from the
Alabama Polytechnic Institute, U.S.A., a similar success
in his search for phosphorescent substances which emitted
the Becquerel rays. He examined uranium salts but found
sugar to be the best emitter.71 P.Spies72 and F.Maack73
reported intensification of the photographic effect of
the rays from uranium by the interposition of certain sub-
stances; we may suppose that secondary radiations or
chemical vapours from these substances could have produced
such an effect. These results, essentially extensions of
Becquerel's work, were not developed further. But Gustave
Le Bon considered that his own researches on radiation
constituted a branch of study conceived independently of
and prior to that of Becquerel and that Becquerel's work
was encompassed within his own.74 He was supported in this
by P.de Heen, Professor of Physics at the University, and
Director of the Institute of Physics, Li6ge, Belgium.75 Le Bon had previously published on psychology,76 and he is considered by social scientists as a serious contributor
62
to that field.77 However, from the time of his first
announcement concerning an unknown type of 'lumiere noire'78
he was heavily criticised by G.H.Niewenglowski,79
A. and L.Lumiere,80 Perrigot81 and Becquerel82 at the
Academy; these scientists were either unable to repeat the
experiments or attributed the photographic effects of the
new penetrating rays, which Le Bon supposed were emitted
by solid substances after irradiation, to red or infra-red
rays. Le Bon did not accept most of the criticisms and
continued to publish.83 He extended his studies from the
photographic to the electric actions, and concluded that
since all bodies when acted upon by light produce effects
similar to but smaller than those of uranium, this was but
one instance of a general phenomenon of penetrating radi-
ation.84 After reopening and extending discussions of 'lumiere noire' when radioactivity had become more important
in 1900 he was again criticised, this time by P.Curie,85
who pointed out that all the characteristics of 'lumiere
noire' could be accounted for by the well-known properties
of 'rayons calorifiques infra-rouges'. In his writings
on the universal dissociation of matter and emission of
material particles86 Le Bon seems to have expressed views
similar to the idea of a general radioactivity of all
matter which, as will be seen, some workers on radio-
activity entertained during and after 1903. Although
Le Bon's books were popular from 1905, no reputable student
of radioactivity regarded his work as significant. However,
the criticisms of Le Bon's early photographic work were
not quite clear-cut. For in 1905 Rutherford stated not
that the effects were due to infra-red or red radiations87
but 'that there seems to be little doubt that the effects
are due to short ultra-violet light waves'.88
Other possible causes of photographic effects were
made clear during 1896, and should perhaps have been seen
by Arnold, Muraoka, McKissick and others previously
mentioned, as a warning. For R.Colson of the Conservatoire
des Arts et Metiers, in 1896-7 pointed to a decomposition
of the salts in the plate by such causes as mechanical
pressure, chemical actions especially when damp, warmth .
when damp, intense infra-red radiation, very feeble light
63
acting for a long period, as well as visible and ultra-
violet light, and X-rays.89 Freshly cleaned zinc surfaces
blackened the sensitive plate in air or in vacuo, as did
magnesium, but aluminium did not. After attempting
experimentally to determine whether effects were due to
'une radiation ou une emanation' Colson pointed to
E.Demar?ay's detection of the vapour of metallic zinc at
temperatures as low as 184 degrees C. and attributed the
effect to metal vapours which could penetrate some materials.90 These findings may explain some of the
dubious experimental results of 1896-7; they parallel to
some extent the work of Dr. W.J.Russell, F.R.S., Lecturer
in Chemistry at St.Bartholomew's Hospital, 'On the Action
exerted by certain Metals and other Substances on a
Photographic Plate'.91 This line of research is partic-
ularly significant since it was thought to be related to
uranium radiation both before and after the discovery of
the radioactive 'emanation' from the metal thorium.
Rutherford took pains to make the distinction clear92 but
was not immediately successful in persuading all scientists
of this. Russell began his paper by explaining that being
in possession of uranium compounds used for spectroscopic
examination some years before, he had repeated 'some of
the very important experiments which Becquerel has made
with these compounds%93 Russell referred only to the photo-
graphic work, ascertained that no luminous excitation was
necessary over seven months, and noted that specimens kept
in the dark seemed if anything slightly more effective in
their action. In addition to this he found that a perforated
zinc screen, intended to show up the effect of a card painted
with yellow oxide of uranium, gave instead an image which
was the reverse of that expected: the greatest action occurr-
ed beneath the zinc. He was able easily to repeat this,
with variations:
so that the only explanation of the action was that the zinc itself must be able to effect a change of the same kind as the uranium, at all events to act on a photographic plate.94
Russell went further than Colson, in showing that a variety
of substances, though not all, produced an effect through
64
many bodies, but not through glass, even the thinnest.
As for metals and alloys, which had to have bright
surfaces: 'The following is a rough list of active
metallic bodies approximately in the order of their
activity: mercury, magnesium, cadmium, zinc, nickel,
aluminium, pewter, fusible metal, lead, bismuth, tin, cobalt, antimony'.95 Zinc salts, and some metals were not active'; a vague correlation with the 'electrical
series' was suggested. Russell's findings appeared to
have extended Becquerel's results; although he did not
make the point, these may have tended to cast some doubt
upon the latter's original experiments. For Becquerel's
descriptions of the arrangements which produced photo-
graphic effects through metal screens, had not explicitly
excluded the effects which the screens themselves were now
shown to have. Furthermore, the strawboard pill-boxes,
used as containers for the uranium salts being examined
photographically, were found to be more active than the
contents; woods and varnishes were also more active than
uranium. Photographic plates laid face upwards in a
cardboard box for a week were 'very appreciably affected',
but were protected by a glass screen. Perhaps Russell
had in mind the fact that Becquerel placed his arrangement
of salt-screen-plate in a cardboard box within a wooden
drawer for long periods. For copal varnish the cause was
definitely attributable to a vapour, but this seemed unlikely for strawboard:
Still more interest attaches to the action of the metals; do they emit a vapour so delicate in constitution and in such a quantity that it can readily permeate celluloid, gelatine &c., and produce a picture of the surface from whence it came, or is it a form of energy (possibly what has been called dark light) that these bodies emit? Zinc kept and polished in the dark loses none of its activity.96
However, Russell did state that the action through glass. 'proves that there is at least a marked difference between
the action exerted by metallic uranium and that by zinc
and other metals'.97
The close link between Russell's and Becquerel's
findings was evidently still felt by W.Crookes over a year
65
later, in September 1898:
It now appears that some bodies, even without special stimulation, are capable of giving out rays closely allied, if not in some cases identical, with those of Professor RUntgen. Uranium and thorium compounds are of this character, and it would almost seem from the important researches of Dr.Russell, that this ray-emitting power may be a general property of matter, for he has shown that nearly every substance is capable of affecting the photo-graphic plate if exposed in darkness for a sufficient time.98
But by this time the connection had loosened considerably.
C.T.R.Wilson, pursuing his experiments at Cambridge on
condensation nuclei produced in gases by radiation, showed
that a uranium salt strongly influenced the condensation
of water vapour in a glass chamber, from within its
stoppered glass container wrapped for hours in tinfoil.
This proved that tinfoil was transparent to the agent
influencing condensation, and that the uranium salt
'continues to be active when kept in the dark'. Thus, a
few months after the publication of Russell's paper, Wilson
supposed 'There can be little doubt therefore that the
effects on the condensation are really due to the radiation
studied by Becquerel'.99
A further clarification from the Cavendish Laboratory
came with J.J.Thomson's note 'On the effect of zinc and
other metals on a photographic plate'.100 In this he credited
Stokes with the suggestion, at an earlier meeting of the
Cavendish Physical Society on Russell's paper, that a blast
of air between photographic plate and source of action might
distinguish clearly between radiation and vapour. The
distorted images obtained in this way showed that vapours
were the cause. Russell was invited to deliver the Royal
Society's Bakerian Lecture of 1898, which he did, on the
subject of the photographic actions,101 now attributing
the actions to vapour of some kind; at the time of Crookes'
Presidential Address of 1898, Russell considered that the
effects of metals were due to the surface formation of
hydrogen peroxide vapour.102 He continued these studies,
which became largely separated from radioactivity,
producing interesting pictures of substances in the dark.103
66
A later chemical author agrees with Russell's explanation
of the effects of metals, and describes his research as a
'classic work on the subject'.104 Perhaps Crookes was not quite up to date in his linking of Becquerel rays with
Russell's findings, towards the end of 1898, but his was
not the last statement of this kind as will be seen.
The clearest distinction was pointed out by G.C.Schmidt
who had concluded that thorium emitted a radiation with some
properties in common with uranium rays.105 Equally signifi-
cant was his statement that the other substances, mentioned
by Arnold, Pellat, Colson, Russell, Muraoka and Henry with
regard to their photographic effects, were not analogous.
Schmidt considered the most important property of the rays
to be the electrical effect rather than the photographic,
and supposed that 'Diese beiden Eigenschaften gehen also nicht Hand in Hand'.106
The electrical properties of emitting substances were
henceforth always to occupy a more important place in
investigations than the photographic. This was due not
only to Schmidt - others too showed an interest in this
aspect of the new rays, in 1897. Becquerel himself had
discovered the effect and moved very much in the direction
of electrical studies in his last three Notes, before
temporarily leaving the subject.107 C.T.R.Wilson's work
on uranium rays was related to studies of the electrical
properties of gases at Cambridge, and his fellow research
student E.Rutherford was also interested in the electrical
properties of uranium rays in 1897.108 Certainly the dis-
charge of electricity produced by the rays gave quantitative
measures which could be correlated with their intensity.
The photometric estimation of relative blackening of
photographic plates was less reliable, and far more time-
consuming, though used for several years by W.Crookes,
for example.
Once Becquerel began to use the electroscope as a
means of studying the rays, he obtained clearer indications
of their properties, if not their cause. Towards the end
of 1896 Becquerel in employing the electroscope found
that the rays were not homogeneous and were absorbed to
67
different extents by different materials. He was able to
extend to uranium rays J.J.Thomson's demonstration that
temporarily conducting gas could be drawn off after
irradiation with X-rays.109 And in 1897 Becquerel reported
that the speed of collapse of the electroscope leaves under
the influence of uranium rays was proportional to the
square root of the density of the surrounding gas whose
pressure was varied:110 J.J.Thomson had explained the
conductivity of gases in electrolytic-ionic terms but
Becquerel gave no theoretical account. Confirmation of Becquerel's results on the penetration,
duration, photographic and electrical properties of the
rays was provided by J.Elster and H.Geitel in their paper
'Veber Byperphosphorescenz'111. They, too, noted that the
source of the energy was still completely obscure,'noch
vollsthndig dunkeln'. The main aim of this paper was,
however, to investigate whether the charge loss of the
photoelectric effect, studied for several years by the
authors, might be attributed to conductivity produced in
the surrounding air by a hyperphosphorescent emission of
invisible rays. But photoelectrically sensitive substances
were found to emit no electrically detectable invisible
rays and furthermore highly hyperphosphorescent uranium
salts were not photoelectrically active. Hence they con-
cluded that the photoelectric phenomenon could not be
explained by an emission of rays of the kind exhibited
by metallic uranium and its salts. Similarly, without
providing enlightenment as to the processes involved,
E.Villari112 confirmed some of Becquerel's results
including the diminution of electrical conductivity of
gases at lower pressures, as well as Kelvin's work on
the electrical equilibrium found to exist between uranium
and any metal in an adjacent position.
Kelvin's researches on uranium formed a part of the
Glasgow experimental studies on the electrification of
air which began in 1889 and were themselves a revival of
earlier investigations of atmospheric electricity.113
Descriptions of experiments on the electrification of air
and other gases by flames, by bubbling through liquids,
68
and by electrified needles, and on the diselectrification
of air by metal gauze 'filters', using the quadrant
electrometer, were published under the names of Kelvin,
M.Maclean, A.Galt and others during 1894-5. These
Glasgow physicists took up studies of the temporary
conductivity produced in air by X-rays as found by a
number of authors early in 1896,114 extended these to the
effects of ultra-violet light and of uranium, and published
their results in the first half of 1897. The experiments
specifically on uranium 'Electric Equilibrium between
Uranium and an Insulated Metal in its Neighbourhood'1115
'Experiments on Electric Properties of Uranium',116 and
'On, the Electrification of Air by Uranium and its
Compounds'117 were performed with a disc of the metal 5 cm.
in diameter and fr cm. thick obtained from Moissan by about
February 1897. The authors firstly confirmed Becquerel's
results on the diselectrification of electrified bodies
and showed that the affected air acted like water in
allowing a pair of dissimilar metals to develop an e.m.f. between themselves. They showed empirically, with even
less theoretical discussion than in Becquerel's public-
ations during the same few months, that aluminium was
'transparent to the uranium influence'. This 'influence'
produced saturation currents i.e., not increasing with
increased voltage, in a variety of gases at different
pressures. The leakage or current at higher pressures
was approximately proportional to the pressure and at the
lower ones to the square root of the pressure. This
latter result, together with the existence of saturation
currents, had been important factors in the development
of J.J.Thomson's electrolytic theory of conduction in
gases during 1896. Kelvin and his associates had followed
the literature118 but were very reserved in their attitude
to this theory. This may be due in part to their being
on the opposite side of the contact electricity debate
from J.J.Thomson; much of Kelvin's work on conduction in
gases seems to have been aimed at resolving the disagree-
ment.119 Kelvin had three years earlier indicated his
acceptance of the idea that a molecule of a gas could be
69
charged with electricity,120 which implied a criticism of
Thomson's electrolytic-ionic theory of 1893,121 and it is not clear if Kelvin was still thinking in terms of charged
molecules in April 1897. But by May he does seem to have
inclined towards an electrolytic gas-conduction theory.
This, however, was not linked to the considerable
quantitative results obtained; neither he nor his associ-
ates developed this theory beyond the brief speculations
of a Royal Institution Lecture.122 Kelvin closely linked
his comments on conduction in gases with the electrical
phenomena exhibited by uranium. The source of the energy
for the 'quasi electrolytic phenomena, induced by uranium
in air' was a problem:
We may conjecture evaporation of metals; we have but little confidence in the probability of the idea. Or does it depend on metallic carbides mixed among the metallic uranium? I venture on no hypothesis.123
For by the time of this lecture he had accepted both
Becquerel's proof of the emission from uranium of a
radiation 'of the same species as light' and his comparison
of the phenomenon with phosphorescence. Kelvin's newly
adopted view of normal electrolysis was published within
days of the lecture; it involved a one-fluid electrical
modification of his Boscovichian chemical atomic theory.
A single chemical atom was equivalent to one of Kelvin's
'ponderable atoms', unlike the theory adopted in 1896.124.
In the new theory each atom is of a definite radius, and
contains a few detachable point atoms of pure electricity
called 'electrions'; force laws between atoms and electrions
apply. J.C.Beattie continued experiments on 'Leakage of
Electricity from Charged Bodies at Moderate Temperaturesli25
investigating 'what becomes of the electricity which leaks
away from an insulated body in certain conditions',126
without clearly distinguishing the effect of uranium salts
from that of white phosphorus and various heated salts;
he made no explicit use of the electrolytic theory of
conduction through gases which was becoming accepted by
1899. Kelvin seems to have renewed his interest in
uranium rays only from 1903 when the subject of radio-activity was exciting widespread interest; his explanations
70
were expressed in terms of the electrion-atom theory of
1897. Ernest Rutherford and the Curies, leaders in that
field by 1903, had in 1897 each begun its study with
quantitative electrical investigations of the problematical
phenomenon of uranium radiation. An examination of
Rutherford's earlier researches may help us to understand
the background to his particular success.
2. Rutherford, and the Cavendish Laboratory (1894-8)
That Ernest Rutherford successfully pursued experi-
mental researches in three branches of physics during the
period 1893-8 is well known. 127 Although these three branches required somewhat different experimental techniques,
it should be pointed out that there were underlying connec-
tions between the physicist's understanding of magnetism
and Hertzian radiation, the behaviour of gaseous matter
under the influence of various radiations, and uranium rays.
We have seen how relevant aspects of chemical and physical
theories developed in the preceding period of the nineteenth
century128 and we shall now illustrate some of the problems
and ideas which the young Rutherford considered as he moved
towards his major studies in radioactivity.
His first publication, of research performed at
Canterbury College, Christchurch, of the University of
New Zealand, was on the 'Magnetisation of Iron by High-
frequency Discharges,129 on which subject there were but
a few conflicting comments in the literature, on the
effects of introducing iron components into electrical
circuits. Rutherford's highly competent, and sometimes
ingenious experimental techniques enabled him to show that
71
iron was indeed strongly magnetic for frequencies greater
than 100 million per second. He found the magnetic effect
of the leyden jar discharge upon steel wires to be propor-
tional to their diameters, not their areas, which confirmed
that their magnetism was confined to a thin skin. The
chemical means used to examine the depth and nature of
the magnetised skin appears to be quite original. The
method developed was one of magnetometer measurement during
controlled dissolution of the surface by nitric acid, after
calibration by dissolving a uniformly magnetised wire.
Rutherford's results showed that on moving inwards from
the surface, the magnetometer deflection decreased to zero,
changed direction, rose to a maximum then returned to zero.
The depth of penetration, of order 1/100 in. was proportion-
al to the maximum current passed, and the magnetisation
always consisted of an outer layer, and an inner thicker
layer magnetised in the opposite direction. Rutherford
considered that these layers represented the first two
half-oscillations of the exponentially decaying sine curve
of the leyden jar discharge.
The effect of the leyden jar and Hertz' dumb-bell
discharge in lowering the saturated magnetisation of iron
whatever the direction of the discharge, which Rutherford
discovered in the course of these early studies, was soon
to serve as the basis of one of the earliest magnetic
detectors of Hertzian waves. This effect also gave a clear
experimental demonstration that the discharge was oscill-
atory in nature as theory predicted; further experiments
indicated a very rapid decay of intensity. He considered
that:
The subject of the decay of the amplitude of the vibrations of a leyden-jar discharge is of considerable interest, especially in connection with the resistance of spark gaps and the radiation of energy into space.130
For terms representing each of these entered into the
complete discharge equation. We recall Oliver Lodge's
idea of a Hertzian-oscillator chemical atom131 in a work
to which Rutherford makes reference in this paper132 of
1894. It is uncertain whether Rutherford had this partic-
ular idea in mind during the course of these studies, but
72
he does seem to have been thinking in chemical and mole-
cular terms. His method of chemical removal of the sur-
face magnetisation is itself remarkable; it also bears a
striking similarity to some of his crucial experiments on
radioactivity, about five years later.133 The experiments
described above led him to the conclusion that:
iron may be shown to be strongly magnetic for the highest frequencies yet obtained. If the molecules of iron can follow the changes of magnetic force, which is reversed 1,000,000,000 times per second, there can be very little magnetic viscosity, and the molecules must move as freely as when under the influence of an alternating current of 100 per second.l34
'Magnetic Viscosity! was the subject of Rutherford's
second publication;135 for this he devised an ingenious
falling-weight timing apparatus for obtaining series of
definite small time intervals of less than 1/100,000
second. Measurements of the rise and decay of induced
magnetic forces in iron and steel were compared for rapid
and slow cycles. The considerably differing curves
obtained for rapid and for slow cycles indicated 'quite
appreciable magnetic viscosity' for iron and steel at
frequencies of 1,000 per second. This does seem to have
contradicted the conclusions of his previous paper where
the use of frequencies of 100,000,000 had shown that
'the molecule of iron can swing completely round in less
than a hundred-millionth part of a second'. He found
'the interpretation of the results very difficult' and
attributed the discrepancy to a possible variation with
frequency of the force required to cause this rapid
swing.136 That this was not the only way of understanding
magnetism at this time is shown by the considerable and
perhaps better known researches of P.Curie, who viewed
molecular magnetic theories with disfavour, preferring a
kind of 'phase' or state-of-matter explanation, as will
be seen.137 But Rutherford, as well as those whom he cited
in these first two papers, mainly 0.Lodge and J.J.Thomson,138
show no signs of this. The work of the latter contains
interesting depictions139 of the way in which the rotations
73
of a molecule, composed of atoms arranged in a particular
fashion, could produce permanent magnetism by continually
disturbing the tubes of force surrounding the molecule;
a 'shearing' of positive and negative tubes past one
another would give no electrical effects, only magnetic.
The former, Lodge, as shown above,140 explained magnetism
in terms of etherial cogged wheels. Thomson also gave
explanations of radiation, chemical combination, electrical
conduction through gases, and other phenomena in terms of
chemical atoms, molecules, and the etherial vortical tubes
of force where possible. Rutherford may well have read Thomson's earlier works,
which were much involved with unified physical and chemical
explanations, as has been seen. Indeed, Rutherford later
told Rayleigh that while still in New Zealand he had read
everything that J.J. had written.141 This was a considerable
amount; by 1895 Thomson had published some fifty papers and
four books. It was perhaps because he was familiar with
and impressed by this quantity of material that Rutherford
apparently expected to find him somewhat 'fossilized'142
at their first meeting; Rutherford was aged twenty-five
and Thomson only thirty-eight at the time. Perhaps he had
classed Thomson with Kelvin (1824-1907) and Rayleigh (1842-
1919) who were older, and whose works he had also con-
sulted.143
On the basis of his experimental work on magnetism
. Rutherford was awarded an 1851 Exhibition Science Scholar-
ship as a graduate of the University of New Zealand. A
change in the Cambridge University regulations144 enabled
him to become the first non-Cambridge graduate to start
work for a postgraduate degree at the Cavendish Laboratory.
It is not clear, however, whether the new two-year degree
played any part in his decision to work with Thomson.145
During his first term at Cambridge, beginning in October
1895, Rutherford continued his work on magnetism, extending
it particularly in developing a sensitive magnetic detector
of electromagnetic radiation. Marconi was developing the
more successful 'coherer' type of detector for long-distance
signalling at about this time. By means of its demagnetising
74
effect on a bundle of magnetically saturated iron needles,
Rutherford was able to detect the radiation, through brick
walls, at distances up to half a mile,146 and succeeded in
impressing the scientific and wider circles at Cambridge
with his demonstrations. This work is summarised in his
paper on 'A Magnetic Detector of Electrical Waves and
some of its Applications' published in 1897147 by the Royal
Society with the usual delay. This included an account of
the studies on surface magnetism performed at Canterbury
College which provided the basis of the detector; but the
studies on magnetic viscosity of difficult interpretation,
as well as speculations on the nature of the molecular
motions involved in magnetism, were omitted. By the time
this paper was presented to the Royal Society, in June 1896,
Rutherford's attentions were already concentrated upon
phenomena connected with the newly discovered X-rays;
their discovery was some four months old when he wrote
in April:
I am working with the Professor this term on Rantgen Rays. I am a little full up of my old subject and am glad of a change. I expect it will be a good thing for me to work with the Professor for a time.148
He thus exchanged the study of one form of penetrating
radiation for another, and there was a further interesting
connection between Rutherford's 'old subject' and his new
one. For in mid-1896 J.J.Thomson saw the conductivity
produced in gases by X-rays in terms of a magnetic molecular
analogy. His work with Rutherford in the next few months
was to change this; to understand the significance of this
change one must look to Thomson's views of the nature of
the electrical conductivity of gases and the light thrown
upon the subject by X-rays during 1896. In doing so we
shall see that the term 'ion' was commonly used by Thomson
in his earlier electrolytic theory of electrical conduction
through gases, and that some quantitative aspects of this
theory were under active consideration before 1896, although
the 'ionisation' theory of Thomson and Rutherford developed
during this year was an advance on all previous work. In
the Recent Researches of 1893, which contains a lengthy
75
chapter on the 'Passage of Electricity through Gases'149
Thomson repeated his view of 1883 that:
chemical decomposition is not to be considered as an accidental attendant on the electrical discharge, but as an essential feature of the discharge without which it could not occur.150
And he was able on this basis to give a 'working hypothesis'
of the 'very complex and very extensive phenomena' of the
discharge tube,151 upon which we can only touch here. At
this time his view of the cathode rays, identified as
bluish lines causing phosphorescence of glass, was that
owing to their magnetic deviability and other properties
they must be charged particles and not the purely etherial
phenomena as suggested by some German physicists. However,
these were not 'molecules' as W.Crookes had called them,
but the free, and necessarily charged atoms from dissoc-
iated molecules; negative atoms in the neighbourhood of
the cathode were strongly repelled.152 However, these
negative rays or cathode rays 'play but a small part in
carrying the current through the gas', deduced partly from
the fact determined by Thomson using a rotating mirror
method that the luminosity in the tube travels in the
opposite direction and 'with an enormously greater velocity
than we can assign to these particles'.153 Instead, he
took the bulk of the current passing through a gas to be
carried electrolytically, somewhat in the manner of the
theory of Grotthus for conducting solutions, which involved
complete chains of associated molecules bridging the two
electrodes.154 Rough calculations155 of the electrostatic
force between the hydrogen atoms in a molecule showed that
'the separation of the atoms cannot be effected by the
direct action of the electric field upon them'.156 But
the existence of chains of polarised molecules, broken up
into short lengths by collisions, would ease the separation
of an atom from the molecule at the end of a chain. The
high velocity of the luminous discharge was explicable by
the jumping of the ends of the successive unit tubes of
force along a chain of molecules, at a far greater velocity
than a moving charged atom.157 Also explained in a similar
way was the stratification, whose non-luminous portions
76
were seen as parallel Grotthus chains; the luminous areas
were at their ends, where atoms were being detached.
This view of electric discharge through gases, within
the fairly narrow limits of pressure involved here, was
repeated at the British Association meeting of 1894158 and
extended as far as a quantitative estimate of the 'very
small number of charged ions' necessary to make rarefied
gases the 'exceedingly good conductors of electricity'
which they were observed to be. By the conductivity 'we
could easily detect the presence of free ions though they
only amount to one part in 7000 of the total gas'.159 The
effect of water in facilitating chemical combination and,
as Thomson found, the electrical discharge, suggested that
its presence might facilitate the formation of the aggregates
of molecules thought necessary for the discharge 'by
supplying nuclei round which they may condense'.160
C.T.R.Wilson took up a related experimental study, touched
upon by Thomson,161 of the effect of various nuclei on the
condensation of water vapour, as the latter's last comments
on electrical conduction by gases, before the discovery of
X-rays, were published. In an article 'On the Electrolysis
of Gases'162 Thomson described his use of the spectroscope
to detect qualitatively 'the movement of the ions in
opposite directions along the discharge tube', and the
resulting decomposition of hydrogen chloride and other
gases; he drew the interesting conclusion that positively
and negatively charged hydrogen atoms exhibited different
spectra. At the end of 1895, shortly before the discovery
of X-rays, Thomson's discussion of 'The Relation between
the Atom and the Charge of Electricity carried by it'163
shows the strength of his continuing interest in the
electrical properties of gases. He supposed that the ions
of gaseous electrolysis do not have the same persistency
of sign as in the electrolysis of solutions; evidently the
two kinds of electrolysis were different and such an
assumption helped to explain this; E.Wiedemann and
G.C.Schmidt performed gaseous electrolyses, and discussed
these differences in atomic-molecular terms.164 Thomson
alone invoked vortex explanations: he supposed that the
77
attraction of atoms for electricity was related to the
arrangement of the etherial vortices within atoms, and
the emergent unit tubes of force.165 His speculation as
to the mechanism of chemical combination of hydrogen and
chlorine, considering the necessity of a third substance
for this and many reactions, again involved his idea of
the association of molecules, which always remained
valuable and flexible. In this case, for example, assoc-
iation occurred to facilitate interchange of electrical
charges, but not to 'free' an atom; the gas did not become
conducting during the course of the chemical combination,
showing that ions, easily detectable by this property, were
absent.
During the first months of 1896, the new X-ray photo-
graphy was being tried at the Cavendish Laboratory, and
probably at every other physical laboratory in Europe.
By the end of February Rutherford wrote home that he was
already tired of it;166 but there was more in X-ray studies
than this, for by then Thomson and J.A.McClelland were
engaged in the investigation of one of the few properties
of the rays to which Rdntgen had not at first laid claim167
By 29th January Thomson168 had found that the rays caused
a dissipation of electrostatic charges of either sign.
Conductivity produced in the surrounding air, or in any
solid dielectric, was pronounced as the cause within days169 This was different from the effect of ultra-violet light,
which caused the dissipation of negative charges on clean
metal surfaces, as Elster and Geitel had shown.170 Thomson
stressed his conclusion that 'all substances when trans-
mitting these rays are conductors of electricity' and
repeated his view that such conductivity in any substance
occurred 'by a splitting up of its molecules'. This was
a novel link between radiation and the electrical conduct-
ivity of gases which led to advances in experiment and
theory. It was not necessary for the gas to be rarefied,
heated or electrically stressed, and investigations could
now be made over far wider ranges of conditions than in the
discharge tube. Continuing studies during 1896-7 provided
a more quantitative understanding both of radiation and of
78
the molecular electrical structure of gases. And the views
of Thomson were to undergo some modification as experimental
studies developed.
By March 1896 he was able to give an account of con-
siderable progress in a joint paper with McClelland 'On the
Leakage of Electricity through Dielectrics traversed by
Rontgen Rays'.171 An important experimental result was the
proportionality of the leak or current to the square root
of the pressure of an irradiated gas. Now from the standard
kinetic theory of the dissociation of gases 'the number of
ions is proportional to the square root of the pressure';
hence the conductivity of the gas was itself proportional
to the number of ions present but independent of their
mean free path and velocity. The most significant results
emerged from investigations of the dependence of the current
upon the voltage applied: unlike electrolytic solutions,and
irradiated solid dielectrics, Ohm's law was not obeyed for
irradiated conducting gases. Instead a maximum current
was always obtained at a low voltage; this current could
not be exceeded even for large increases of the voltage,
'a very remarkable and characteristic property of the
conductivity produced by these rays in a gas'.172 This
agreed with the pressure-current relationship in showing
that the conductivity depended only upon the number of
ions present and not upon their velocity when the maximum
current flowed. Although the explanation of the mechanism
involved was not expressed very clearly by the authors,173
if one takes into account Thomson's ideas on conduction
through gases during previous years, discussed above,174
an interpretation is possible. The authors supposed that
the effect of the X-rays was continually to produce 'chains
of molecules or aggregations of some kind'. As we have
seen, the assumption was that these could readily release
the free atoms essential for conduction. The aggregates
produced by X-rays were supposed to be of such a kind that
'the component atoms with their electrical charges could
rearrange themselves with facility; the time T required
for this rearrangement being independent of the intensity
of the electric field'. This, Thomson seems to imply,
79
would happen spontaneously to each chain, once formed,
in the absence or presence of any electrie field. Each
rearrangement would effectively transfer a definite
quantity of electricity from one end of the chain to the
other; but in the absence of an electric field the net
effect of many randomly orientated transfers would be zero.
When a weak field is applied a proportion of the aggregates
become 'polarised' in a manner 'analogous to that of the
molecular magnets in a piece of soft iron under an external
magnetic field'. With some chains similarly orientated,
the spontaneous transfers of electricity would produce a
current. As the voltage increases, more chains become
aligned and the current rises, but 'as soon as all the
chains get pulled into one direction the current will
reach a maximum value and be independent of the electro-
motive force'.175 This electrolytic mechanism is seen to
differ from that for ordinary electrolysis generally
accepted now, and probably by many then; in electrolysis
of solutions, conductivity depends largely upon the
(measurable) velocities176 of the aggregates of ion plus
neutral molecules.
Thomson's next major paper on the subject, published
after a period of more than six months and now in con-
junction with Rutherford, contained modified views of
conduction through gases, which were more closely allied
to the accepted mechanism of the electrolysis of dilute
solutions. And the continuation of this research by
Rutherford enabled Thomson by the end of 1896 to publish
for the first time an account of a mechanism by which
X-rays could produce the particles assumed to be res-
ponsible for electrical conductivity in the normally
insulating gases. It is to be noted that the use of the
electrical conductivity produced in a gas by radiations as
'a very sensitive and convenient measure of the intensity'177
did not at first depend on any particular ionisation theory.
However, all later explanations, both scientific and
historical, from 1898 to the present, of this means of
determining the intensity of X-rays and radioactive
radiations are given in terms of the theory of Thomson
80
and Rutherford of September 1896.178
There are some interesting clues as to the path of
the development of this theory from March to September.179
In April 1896, as Rutherford left his study of Hertzian
radiation, and magnetism, and began the experimental work
on X-rays and electrical conductivity, Thomson wrote to
Nature on 'The RUntgen Rays'.180 His brief comment on the
'saturation' conductivity produced by X-rays in gases, was
that:
The relation between the rate of leak and the potential difference thus exhibits the same general features as that between the magnet-isation of a piece of soft iron and the magnetising force.181
In his Rede lecture in June Thomson indicated that he still
supposed that the conductivity was caused by the trans-
mission (not absorption) of the X-rays;182 he again used
the magnetic analogy for the more detailed voltage-current
curves obtained by Rutherford and himself: 'When the rays
are strong, the curve is like that of soft iron; when the
rays are weak, it is like steel'.183 A major objective of
the work was clearly the attainment of an understanding of
the detailed mechanism of the conduction process; it may
or may not have been a deliberate attempt to ascertain
the dimensions of the aggregates or chains of molecules
assumed to be involved, which led to the observation,
again not agreeing with Ohm's law, that:
In some experiments recently made by Mr. Rutherford and myself, we found that using a constant potential difference the rate of leak was smaller across a very thin plate of air than across a thicker one; it thus appears that the process of conduction through a gas is one that requires a considerable amount of room.l84
Perhaps the researchers entertained the idea that the
dimensions of the aggregates or chains of molecules
involved might be those found in the very different
conditions of the striations of the discharge tube. These
were of the order of one millimetre in width and were thus
composed of millions, or many thousands, of molecules.
It was not made clear how these are produced by radiation,
nor how a confined space restricts the transfer of elec-
tricity. A week after Thomson's Rede lecture of June 1896
81 Rutherford wrote of his struggle with this research:
'My scientific work is progressing fairly well but it is
rather a difficult subject I am on at present'.185 But
within a further three months many of the problems of
this area had been eased into a modified theory.
The explanation expounded in Thomson's and Rutherford's
joint paper 'On the Passage of Electricity through Gases
Exposed to Rontgen Rays'186 remained one of electrolytic
conduction occurring by means of aggregates produced in a
gas by the radiation; but series of experiments had now
clarified the nature of these aggregates. A progressive
step in the experimental work had been the piping of the
gas from the point of irradiation into a separate vessel
for examination of its conductivity. Removal of the
conductivity by certain filters187 showed the 'coarse
character' of the conducting entity within the gas,
which we can see fitted with Thomson's thinking over the
previous few years. Most important in understanding the
nature of this entity was the 'very suggestive result'
that the conductivity produced by irradiation could be
greatly diminished or entirely destroyed upon application
of an electric field, of a few volts potential difference,
across the gas as it passed along the tube before
reaching the vessel in which the leakage was tested.
The electric field was applied by inserting a central wire
within a metal tube inside the tube along which the gas
passed. On placing a glass tube over the central wire,
thus maintaining the field but preventing the current, they
were able to conclude that:
the peculiar state into which a gas is thrown by the ROntgen rays is destroyed when a current of electricity passes through it. It is the current which destroys this state, not the electric field.188
This gave a simple explanation of 'saturation':
the maximum current will be the current which destroys the conductivity at the same rate as this property is produced by the RUntgen rays.l89
This could still have agreed with the earlier theory in
which the polarization and orientation of aggregates or
chains was assumed. What was new, however, was the
82
assumption that what they called 'conducting particles'
were actually electrically charged', and that the velocity
of translation played a vital part in conductivity. This
implies a different mechanism for conduction from that
envisaged previously for the chains or aggregates had
themselves been supposed to form the electrolytically
conducting path. But this was not mentioned in the paper
published in November; the authors now explicitly placed
their explanation of conductivity in line with that accepted
for the electrolysis of dilute solutions:
We shall find that the analogy between a dilute solution of an electrolyte and gas exposed to the ROntgen rays holds through a wide range of phenomena, and we have found it of great use in explaining many of the characteristic properties of conduction through gases.190
Although the ability to impart a charge to a gas,191 and
the absence of polarization192 seem to be properties
outside the analogy, the qualitative and quantitative
explanations it provided were clearly 'of great use'.
The authors were able to derive an important quantitative
relationship equating the rate of increase of the number
of charged particles with the difference between the rate
of production by the X-rays, and the rate of destruction
both by recombination (proportional to n2) and by the
passage of current. As in ordinary electrolytic theory,
this current was expressed in terms of the 'sum of the
velocities of the positively and negatively electrified
particles'.193 These equations accounted for the higher
resistance of thinner layers of gas, but without quantit-
ative agreement. Close correlations between the equations
and experimental results were however very successfully
attained for the voltage(E)/current(i) curves for various
differently irradiated gases, which rose to a limiting
value of i according to an equation of the form A - i = B.i2/E2 . The value of the limiting current
gave easily an estimate of the proportion of the gas
electrolysed as 1/(3x1012):194 compare Thomson's similar
calculations on gaseous 'free ions' in discharge tubes,
of 1894.195 Using the curves Thomson and Rutherford were
83
able roughly to estimate the time of spontaneous diminution
of the number of particles to one half after the rays had
ceased, at about 1/10 sec., a precursor of half-life
estimates for radioactive gases. And using this they
arrived at a first estimate of 0.33 cm./sec. per volt/cm.
for the sum of the velocities of the oppositely charged
particles, upon which the current depended. This was
'very large compared with the velocity of ions through
an electrolyte' but small compared to the 50 cm./sec. for
'an atom of a gas carrying an atomic charge' which implied,
as had been assumed, 'that the charged particles in the
gas exposed to the ROntgen rays are the centres of aggreg-
ation of a considerable number of molecules'.196 It seems
possible that a distinction was already being made between
'charged particle' and 'aggregation' implying a certain
kind of mechanism, but this was made more explicit as
Rutherford continued the research. It was soon after the
reading of this paper to the British Association that he
wrote home of the possibility of discovering new chemical
elements.197
Thomson in a note appended to Rutherford's next
publication, dated December 1896, 'On the Electrification
of Gases exposed to ROntgen Rays, and the Absorption of
Rontgen Radiation by Gases and Vapours1198 gave the first
description of the way in which the radiation might cause
the conductivity of a gas. Thomson supposed that the
moving tubes of force comprising the radiation produced
charged particles by 'dissociatiOn of one molecule, or
production of one positive and one negative ion'; this
implies that aggregation is subsequent to dissociation.
Rutherford's delicate experiments on the absorption of the
rays had shown that gases which were good conductors of
electricity under irradiation were also good absorbers of
the radiation. This suggested for the first time, although
it was still not stated explicitly other than in Thomson's
note, that it was not the transmission but the absorption
of the rays which resulted in conductivity. Rutherford's
bare statement that 'Experimentally it was found that the
rate of leak of a gas is proportional to the intensity of
84
radiation at any point'199 is enigmatic. For until this
time the expressions he italicised had been taken as .
practically synonymous; the rate of leak was the only
measure of intensity. It may be that his experiments
showing that the rays 'appeared to emanate in all directions
from the anode'200 led him to suppose that the intensity
diminished according to an inverse square law in the
manner of light from a point source. Once this was assumed
- and the tentative opinion was that X-rays were pulsations
or vibrations of a similar kind to light - an experimental
demonstration that the rate of leak was proportional to
the inverse square of the distance from the source could
have given rise to the conclusion as stated. Whether or
not this is so the statement itself marks an important .
clarification in his understanding of the phenomenon.
The process was now seen as an absorption of the radiated
energy by gas or vapour, with the resulting production of
a small number of pairs of oppositely charged ions, around
which aggregation occurred. J.J.Thomson added that one
Faraday tube would be removed from the radiation for each
molecule dissociated; but Rutherford never expressed
himself in terms of these tubes in publications.
Thus outlined, the theory was developed experimentally
during the next few years by several research students at
Cambridge. Rutherford concentrated on this field for his
remaining two years here, at first working on the separation
and examination of the oppositely charged ions existing in
conducting gases. Preliminary experiments201 showed that
a gas in the conducting state could be made to acquire a.
net charge, and this was attributed to an excess of ions
of one sign over the other. At this time Kelvin and his
associates were interested in such points and may have
attributed this to the acquisition of electrical charge
by the molecules; J.Perrin in Paris accepted an ionisation
hypothesis,202 but the Cavendish school appears to have
been far ahead in experiment and theory.
In Rutherford's next publication on 'The Velocity and
Rate of Recombination of the Ions of Gases exposed to
Rlintgen Radiationc203 dated July 1897, he described an
85
ingenious method of timing the passage of ions of one
sign: only half of the gas, between plates 16 cm. apart,
was irradiated the rest was screened so that ions of one
sign would have to travel through 8 cm. of non-conducting gas before arriving at the oppositely charged plate to
produce a rapid deflection of the connected electrometer.
What was perhaps at first unexpected was the result that
the velocity of positive and negative ion always appeared
to be equal, not only in elementary gases such as hydrogen,
but in compounds with asymmetric molecules such as hydrogen
chloride. Furthermore the hydrogen ion velocity was
different in different gaseous hydrogen compounds.
John Zeleny, however, in his discussion 'On the ratio of
the velocities of the two ions produced in gases by ROntgen
radiation; etc.'204 soon afterwards demonstrated experi-
mentally that negative ions generally possessed a slightly
higher velocity; these researches show the ingenuity with
which the ions were manipulated and the reality with which
these researchers endowed them.
The equality assumed by Rutherford of the velocities
of gaseous ions certainly did not apply in the electrolysis
of solutions where each ion possessed an individual mobility.
That the observed velocities depended more upon the gas
used than the nature of the dissociated ion gave rise to
the hypothesis that the size of the cluster of molecules,
formed around the central charged particle, was determined
only by an equilibrium between intensity of bombardment by
surrounding gas molecules and the magnitude of attraction
provided by the central charge of the ion. A comparison
of the observed velocity of the ion, for example 10.4
cm./sec. for hydrogen gas, with that of 340 cm./sec.
calculated for a molecule of hydrogen carrying an atomic
charge, gave estimates of the sizes of the carriers
involved. Rutherford found more moderate sizes than
formerly may have been supposed; in the present example
5.5 molecular diameters for the hydrogen ion in hydrogen gas.205 The understanding of the electrical structure of
gases was to be an essential feature of Rutherford's
future studies of radioactivity, both experimentally and
86
theoretically. For example, his experimental investigations
on the 'decay' of after-conductivity, by 'blowing' and
static methods, confirmed the relationship dn/dt = a.n2
for the disappearance of the ions. His observations
agreed with the theoretical curve relating the declining
number of ions to the time, and with the theoretically
calculated time T taken 'for the number of conducting
particles to fall to half their number', given by the
equation T = 1/N.a ; N is the maximum number which
depends on the intensity of the radiation, a is a different
constant for each gas, T was found to be of the order i sec.
The superficial similarities with studies of the decay of
radioactivity on which Rutherford was to work some two
years later in 1899 are striking. But the deeper and more
complex links between his studies of radiations, the mag-
netic and electrical properties of matter, and uranium
rays, will become more evident. Rutherford indicated in
his paper on ions in gases exposed to X-rays, dated July
1897 and now under discussion, that experiments on uranium
radiation had already begun, and that the ions produced
in gases by this means were the same as with X-rays.206
We can see that the subject of uranium rays was considered
to be of note at the Cavendish Laboratory; for in 1896
its Professor stated that he found Becquerel's discovery
'exceedingly interesting'207 and that he had obtained
photographic effects by means of uranium salts.208
G.G.Stokes also showed an interest in this new form of
phosphorescence during 1896-7.209 In July 1897 Rutherford
stated his interest in uranium rays and announced prelim-
inary results, as has been stated; in October C.T.R.Wilson
announced his confirmation of these by the cloud-
condensation method; then Stokes at the Cavendish Physical
Society, and J.J.Thomson at the Cambridge Philosophical
Society in November discussed the implications of
W.J.Russell's article.210 But it was early in 1899 before.
Rutherford's promise of further results made in 1897211
was fulfilled.
In the intervening period he published one paper on
87
the problematical subject of 'The Discharge of Electri-
fication by Ultra-violet Light'.212 This was previously
understood, and confirmed in this paper, as an effect
produced mainly at metallic surfaces and not within the
volume of the gas; the effect of X-rays was the opposite
of this. Ultra-violet light discharged negatively
electrified metals and caused zinc and some other metals
to acquire a positive charge. Rutherford cited213 some
of the literature on the subject from the ten years of
its history into which we cannot go deeply here. He
mentioned only the theory of surface disintegration,214
and not Lodge's supposition that the effects probably
'depend on some synchronised disturbance set up in the
air ... in contact with the substance, a disturbance
resulting in some kind of chemical action'.215 Rutherford's
main concern in this paper was to investigate the nature
of the carrier of the current. Using the kinds of experi-
mental technique developed at the Cavendish Laboratory
during the previous two years, he found that the current
was carried by free gaseous ions, of negative charge only,
and not by particles of metal. The use of a variety of
different metals, from lead to sodium amalgam, showed that
'the velocity of the carrier is independent of the metal
on which the light falls%216 This indicated that the
carrier was produced not from the metal itself but from
the gas near its surface. To provide a mechanism for the
phenomenon was no doubt one desired object of this research,
but none was published at this time.
For the origin of uranium rays, an equally difficult
subject, Rutherford did suggest the outline of a mechanism.
A study of this radiation was the subject of his most
substantial paper then published: 'Uranium Radiation and
the Electrical Conduction Produced by It'217 was dated
September 1898, the month in which the young scientist left
Cambridge to replace Callendar as Macdonald Professor of
Physics at McGill University, Montreal. The suggestion
adopted by Rutherford218 was not his own, though it may
have relied to some extent on his results. It had been
put forward by J.J.Thomson at the beginning of 1898 in a
88
note 'On the Diffuse Reflection of ROntgen Rays'.219
Thomson pointed out that these diffusely reflected rays
were, like uranium rays, similar to X-rays but less
penetrating. He mentioned the experiments of Sagnac on
secondary rays emitted by metallic surfaces, as did
Marie Curie a few months later in relation to her own
speculation as to the origin of uranium rays.220 Thomson
supposed that secondary rays were produced during the
ionisation of the molecules of the material, solid, liquid
or possibly gas, by the incident X-rays. He provided a
diagram showing how the tube of force joining the atoms
in a molecule could be broken by the influence of an
incident radiated tube, and concluded that owing to the
rapid movement of tubes during the course of dissociation:
Ionization (if sudden) may thus be expected to give rise to rays having properties similar to those of the secondary Ontgen rays. 221
As for uranium rays:
It seems not impossible that in the case of a complicated structure like the uranium atom regrouping of the constituents of the atom may give rise to electrical effects similar to those which occur in ionization and might possibly be the origin of the uranium radiation.222
We recall that Thomson had made his first announcement of
the subatomic cathode particle in April 1897 and had fully
set out his theory of the corpuscular chemical atom in a
publication of October 1897. The suggestion concerning
regrouping of the corpuscles constituting the uranium
atom can be considered as the first published statement
that the emission of uranium radiation is a property of
the atom; it precedes those of Marie Curie and G.C.Schmidt
made later in 1898.223
But this kind of idea was not new: as the previous
Chapter shows, some physicists and chemists attributed
the emission of characteristic radiations, but of longer
wavelengths or different wave forms, to internal vibrations
of chemical atoms. However, the energy required for the
emission of atomic spectra was known to be positively
provided, in obvious ways. Becquerel had clearly stated
the problem of the source of the energy of the uranium rays
89
and Rutherford, who cited eight of Becquerel's nine papers
as well as later French authors on the subject, saw this
as a question to which there were now some answers. An
interesting point which emerged from Rutherford's
investigations of 1898 was that the rays from uranium
consisted of two distinct portions. He considered that
these beta and alpha rays were comparable with X-rays and
secondary X-rays respectively, and speculated that the
alpha rays might thus be produced at the surface of the
active substance by the supposed primary beta radiation.224
Rutherford quoted Thomson's idea that a rearrangement of
the constituents of the uranium atom could give rays
similar to those produced by the sudden ionisation of a
gas.225 But he tacitly modified this as required by his
experimental results by insinuating that such rays from
gases were similar to soft primary X-rays rather than to
the less penetrating secondary X-rays which Thomson had
suggested. The existence of two kinds of uranium X-ray
could thus be understood, but only in part. For in his
opinion 'The cause and origin of the radiation continuously
emitted by uranium and its salts still remain a mystery'.226
Rutherford was able to ease this with the comment that on
account of the smallness of its energy 'the radiation
could continue for long intervals of time without much
diminution of internal energy of the uranium',227 but such
relief was only temporary. The question of the possible
diminution of this radiation proved to be an important
point in the struggle to understand the subject . as the
mystery deepened and widened during the next few years.
Rutherford's stated aim of 1898 was a study of
'Uranium Radiation and the Electrical Conduction Produced
by It'.228 His repetition of Becquerel's experiments, but
with entirely negative results, led him to the. firm con-
clusion that this radiation was neither refracted nor
polarised;229 Becquerel had to agree.230 And by success-
ive interposition of metal foils Rutherford came to his
conclusion that the radiation was 'complex' consisting of
at least two distinct types each approximately homogeneous,
one readily absorbed (alpha) and one more penetrative
90
(beta),231 each of about the same coefficient of absorption
in gases as X-rays.232 He may have owed something both to
Becquerel,233 who had indicated the complexity of the
uranium rays, and to J.J.Thomson and J.A.McClelland234 who
had used metal foils in demonstrating the considerable
variety and complexity of X-rays from different bulbs.
Significantly, however, Rutherford found that different
compounds of uranium gave rays of the same composition,
as indicated by the foil method and by his own more sen-
sitive method of absorption in gases. Despite the spec-
ulation that the alpha rays were secondary to the beta,
which Thomson may never have accepted,235 this result seems
to be equivalent to a demonstration of the emission of a
definite spectrum consisting of two main components. It
may possibly have been taken by the Cavendish researchers
as a further indication that the property of emission
belonged to the uranium atom. This latter view, also
suggested by Marie Curie and G.C.Schmidt in 1898, had
been put forward by each in connection with their indep-
endent pronouncements that thorium was the only other
element giving similar spontaneous radiation. Rutherford's
brief examination236 of the thorium rays showed, in spite
of some capricious but interesting variability in readings,
that they were of a different penetrative composition from
uranium rays; these were the problems he took to Canada.
With regard to the discovery of thorium rays he mentioned237
only Schmidt, who had in fact been slightly earlier than
Marie Curie in publishing the discovery. One paper of
Marie and Pierre Curie was cited by Rutherford238 but only
to criticise seriously their conclusions. Owing to the
ready absorption of the alpha rays by any material,
including that of the emitting substance itself, Rutherford
noted that 'the rate of leak due to any uranium compound
depends largely on its amount of surface'. Thus the state
of division of the layers of powdered salts used made it
'difficult to compare the quantity of radiation given out
by equal amounts of different salts'.239 Now such a com-
parison had been the very means by which the Curies had ,
firstly come to suspect, and secondly to begin to isolate
91
chemically a new element; its characteristic, they said,
was its great power of radiation compared with uranium.
Rutherford dissolved a crystal of uranium nitrate
in water and allowed it to evaporate so as to deposit a
very thin layer of the salt.. This simple exercise gave
a higher than normal leakage due mainly to alpha radiation,
which had the greatest electrical effect. 'It is possible'
he wrote: that the apparently very powerful radiation obtained from pitchblende by Curie may be partly due to the very fine state of division of the substance rather than to the presence of a new and powerful radiating substance.240
Although the Curies had noted241 the effect of the thick-
ness of the layer of salt for uranium and thorium rays,
they had published no analysis of the composition of the
rays, nor had they developed studies on their electrical
effects in gases. With his deep experience in these areas
at the Cavendish Laboratory, surely Rutherford could not
have erred on this important point: but wrong he was.
His success came with thorium; the Curies found that
studies of their new elements polonium and radium went
from strength to strength.
92
3. Pierre Curie, Marie Curie and the new radioactive elements (1890-8)
Towards the end of 1898 Rutherford disagreed with the
Curies over the existence of a new radiating element; he
had lost this point even before his paper appeared in print
early in 1899. But these parties were to disagree on more
complex issues concerning radioactivity from 1901 to 1904
and their approaches appear to have differed well before
this time. One could say that they were near to being
adherents of different schools of scientific thought.
The main point of distinction was the attitude towards the
various mechanical, molecular and etherial models. These
as we have seen were applied in considerable variety for
explanations in physical science. But a tradition which
can be called 'positivist'242 cast doubt upon the validity
and even the utility of such models, in the last decades
of the nineteenth century. While all scientists saw the
attainment of general laws as a vital part of scientific
progress some were sceptical of the models which others
made their goal and considered the seeking of general
relationships to be the sole objective of science. There
was however a variety of opinion between these views;
Maxwell for example in considering his attempt 'to imagine
a working model' explaining the rotational character of
magnetic and optical phenomena wrote that:
The problem of determining the mechanism required to establish a given species of connections between the motions of the parts of a system always admits of an infinite number of solutions. Of these, some may be more clumsy or more complex than others, but all must satisfy the conditions of mechanism in genera1.243
And W.Ostwald appears to have changed from an atomistic
understanding of chemistry and physical chemistry to a
completely anti-atomistic and purely energetic or thermo-
dynamical approach, in about 1890.244 A fuller discussion
would lead us into areas of the philosophies of nineteenth
century scientists but I wish only to indicate that
P. Curie tended to be critical of atomic or molecular
models and sought explanations rather in terms of general
laws; he was able in this way to provide a lasting
93
contribution to aspects of experimental science in
providing such laws apparently without recourse to models.
In his earliest series of researches, published
jointly with his older brother Jacques during 1880-2 on
piezo-electricity of crystals,245 it is difficult to
discern any leaning towards either of the two approaches;
both were clearly valuable in this case. The research
was performed at the Mineralogy Laboratory of the Sorbonne
whilst both brothers were 'preparateurs'; Friedel was
director here,246 and had himself worked on pyroelectricity
- the production of electrical polarity in crystals upon
change of temperature.247 Crystals possessing axes with
dissimilar extremities, i.e. hemihedral with inclined faces,
exhibited this polarity at these extremities. The Curie
brothers claimed the discovery of a new, but related, way
of producing electrical polarity - by varying the mechanical
pressure, applied along these axes; this was later named
piezo-electricity. Some indications of the production of
electrical effects by mechanical treatment of crystals had
been known for many years248 but the experimental work of
the Curie brothers was a considerable advance. They
clarified the quantitative nature, symmetry and reversi-
bility of the effect using tourmaline and quartz crystals.
When the collaboration of the brothers ended in 1883,
with Jacques taking up the post of 'Maitre de Conf6rences,
at the University of Montpellier249 and Pierre becoming
'Chef des travaux de Physique' at the new Ecole municipale
de Physique et de Chimie industrielles in Paris, each
retained an interest in the physical and geometric properties
of crystals. During the decade 1883-93 Pierre Curie per-
formed little experimental research but continued studies
of aspects of piezo-electricity.250 These led to the
development of the kind of electrometric apparatus by
means of which Pierre and Marie Curie were able to measure
the currents of order 10-11 amps produced by uranium,
thorium, and the new radioactive elements which they were
later to discover.
It was during the period 1883-93 that Pierre Curie
seems to have moved away from molecular explanations. In
94
1881, the discussion 'Sur les phgnomenes electriques
de la tourmaline et des cristaux h6miedres a faces
inclinees1251 was conducted in molecular terms. One
cannot say whether the views expressed belonged to
Jacques or Pierre Curie or to C.Friedel; perhaps they
were common to all. The authors attributed pyroelectricity
and piezo-electricity to the same cause - a contraction or
dilation along a particular axis. Discussing a deeper
structural origin they expressed disagreement with a view
which likened the rows of molecules in a pyroelectric
crystal to a thermo-electric pile. Here, a set of success-
ive cones of copper and bismuth, for example, exhibited the
required momentary polarity on change of temperature. They
considered that a better explanation of the separation of
charge by pressure, and of the particular symmetry of the
phenomenon, was provided by the hypothesis of the permanent
polarisation of the molecules in a crystal, with the end
faces normally maintained in the neutral state by a layer
of electricity 'condensee sur la surface': '1'idee que
les molecules sont polarisees est en parfait accord avec
ce fait que l'electricite ne se montre libre sur les
bases'.252 They concluded the discussion by commenting
that the 'extremite aigue' of each molecule in these
crystals was permanently negatively charged with respect
to its base, and that 'la forme de la molecule paralt
avoir l'influence preponderante'. Pierre Curie seems never again to have published
favourable comments on any molecular mechanism put forward
to explain physical phenomena. His interest in piezo-
electricity remained,253 but as others entered the field
in the 1880's and 1890's he inclined more towards a study
of the geometrical symmetry involved.254 Other authors
had considered symmetry in physical science in a vague
manner255 but Curie developed these studies in a systematic
and original way in applying the criterion of symmetry, as
used for the classification of crystals in mineralogy, to
the phenomena of physical science.256 In his paper 'Sur
la symetrie dans les phenomenes physiques. Symetrie d'un
champ electrique et d'un champ magnetique'257 Curie applied
95
considerations of symmetry to a variety of electrical,
magnetic, optical and thermal phenomena without the use
of mechanical or etherial models. He classified phenomena
by their symmetry, and used the proposition that the
existence of a characteristic dissymmetry was a necessary
requirement for the production of an effect by a cause.
This indicated which phenomena could and which could not
exist. As Curie himself pointed out, thermodynamics
provided a different and more quantitative indication of •
possibility.258 And we note that other scientists on
occasion used dynamical principles, without mechanical
hypotheses, to develop equations whose terms they supposed
might correspond to some phenomenon existing or un-
discovered.259 It seems that the consideration of symmetry
was probably the least important of these three general
methods of physical science in the nineteenth century;
but this is perhaps not so later, in the twentieth.26;)
While developing his theoretical ideas on symmetry,
Pierre Curie also performed experimental research. His
paper on 'Propriet6s magnetiques des corps a diverses temperatures'261 appeared in 1895, the year after his
publication on symmetry considerations in physics.
Although there was no specific application of the principle
to magnetism, the generality of Curie's approach to the
interpretation of his results contrasted with the usual
molecular view of magnetism. We have seen how Rutherford
understood hysteresis in terms of molecular magnets and
how he sought to follow the rapid motion of these in an
oscillating field. Curie, on the contrary, looked to
functions of physical state as an analogy by which to
explain his experimental results and touched but briefly
upon molecular theories.
Faraday had found that all bodies exhibited magnetism,
distinguished the three varieties of this property, and
had noted that iron, strongly magnetic (ferromagnetic)
at normal temperatures, became weakly magnetic (para-
magnetic) at high temperatures. Curie now showed that
diamagnetism, which was possessed by most or perhaps all
substances, did not vary with temperature. This result
96
pointed to the independence of diamagnetism from the other
kinds of magnetism; on the other hand he was able to
demonstrate by several series of measurements at different
temperatures that ferromagnetism and paramagnetism were
closely related. All ferromagnetic bodies were progress-
ively transformed, on heating, into paramagnetic bodies.
Both the inverse relationship between absolute temperature
and intensity of magnetisation of paramagnetic substances
which Curie demonstrated experimentally,262 and the curves
of transformation of ferromagnetic substances, were com-
pared in some detail with gas-liquid phase phenomena.
For paramagnetism, likened to the gaseous state, the
corresponding equations I = A.H/T and
D = (l/R).P/T possessed interesting similarities.263
The analogy was further supported by the I/T curves for
paramagnetic-ferromagnetic transformations, which were
very similar to the continuous D/T curves for gas-liquid
transformations near the critical temperature, as deter-
mined for carbon dioxide by Amagat.264 In short, the
functions f(T,H,T) = 0 and f(D,P,T) = 0
possessed strong though not complete similarities.265
This comparison led Curie to his single comment on a
molecular analogy between magnetism and the condensation
of fluids; the rapid augmentation of magnetic intensity
in weaker fields as the temperature falls may occur
'quand l'intensite d'aimantation des particules magngtiques
est asset forte pour qu'elles puissent rgagir les unes sur
lee autres'.266
His analyses of the results on magnetism of 1895 show
just the kind of minimal mechanical or structural explan-
ation which Curie was to give for radioactivity. Marie
and Pierre Curie were in 1898 among the first to describe
uranium radiation as an atomic property yet they avoided
any deeper public discussion of the mechanism of this
atomic radiation; nor did they describe, as did others,
the kind of structure an atom might have in order to
possess such a capability. As will be seen Pierre Curie
considered radioactivity in terms of a general analogy
with the transmission of heat in its various forms.
97
Manya Sklodowska had arrived in Paris in 1891 at
the age of 24 and became one of few women science students
at the Sorbonne; she followed her elder sister Bronya who
had come to Paris from Poland to study medicine, about
five years earlier.267 Manya, or Marie, was awarded the
degree of Licence es Sciences Physiques in 1893 and the
same in mathematics in 1894.268 Then, after her marriage
to Pierre Curie in 1895, she passed the examination
requirements to become 'Agregee de 1'Enseignement
Secondaires des Jeunes Filles' in mathematics in 1896.269
Her first research was of a partly industrial nature,
performed for the Societe d'Encouragement pour l'Industrie
Nationale de France, on the magnetic properties of tempered
steels of different types from various steelworks in
France.270 The magnetic measurements involved were somewhat
similar to those of her husband's thesis on magnetic
properties at different temperatures published in 1895;271
his knowledge may have been helpful for her work. Although
chemistry does not figure among her academic qualifications
she had apparently followed courses in chemical analysis
whilst in Poland.272 These may have aided her in deter-
minations of the composition of the steels as well as in
her new subject of research uranium radiation, which
quickly developed into the wider field of radioactivity.
It is largely upon her work with radioactive substances
and the discovery of the element radium that Marie Curie's
present fame rests. The beginnings of her popular renown
came in 1903 when spectacular developments in radioactivity
were very much in the public eye; public honours accumulated
considerably during 1903-4.273 The tragic accidental death
of Pierre Curie in 1906 aroused great public sympathy for
the widow; she was then appointed Assistant Professor at
the Sorbonne taking the place of her husband in the Chair
of Physics created for him in 1904. This was an unpreced-
ented appointment for a woman as was her full Professorship
in 1908.274 Her second Nobel Prize, on this occasion for
Chemistry and not shared, and her involvement in the
matrimonial separation of P.Langevin,275 both in 1911,
afforded continuing public interest. Work with the first
98
mobile medical X-ray machines in the War of 1914-18267
and the inseparable association of her name and that of
her daughter277 with the ever more important radium and
radioactive elements seal Marie Curie's lasting fame.278
Nevertheless, it must be pointed out that the theoretical
explanation of radioactivity to which the Curies adhered
during the period now under consideration, did not agree
with that developed by Rutherford and which is now accepted.
Her claim279 of priority for the transformation-
disintegration theory, made in 1906, is open to doubt:
Cette hypothese se trouve parmi celles qui ont ete indiquees par M.Curie et moi d6s le debut de nos recherches sur la radioactivite. Mais elle a ote surtout procisee et developpee par Rutherford et Soddy, auxquels elle est, pour cette raison, generalement attribuee.280
For although some aspects of the theory of 1906 were
present among the several speculations put forward in
her earlier publication of January 1899281 to which she
referred this claim, it will be shown that these were
shared by others. And she omitted to mention the Curies'
strong opposition to Rutherford's and Soddy's theory,
during the important intervening period from 1901 to 1903.
But we hope to look more deeply into such points in later
discussions.282 Our interests in the remainder of this
Section are to follow the work of the Curies into their
studies on radioactivity, to provide a much-needed dis-
cussion of their earliest theories and speculations, and
to examine their conclusion that the emission of radiation
by uranium is an 'atomic property'.
Marie Curie tells us that her, and Pierre's, first
interest in uranium rays dates from the second half of
1897, at the time when her magnetic experiments were
complete, and when she was seeking a subject on which to
begin research for a doctoral thesis.283 Becquerel's last
paper on the subject for some time was read to the Academie
in April of that year284 but there is no indication of
direct communication between Becquerel and the Curies until
later, in 1898. Besides one possibility, that Marie Curie
came upon the subject simply by reading about it285 there
99
are several possible links with contemporary scientists
interested in the subject. That some part was played by
Pierre Curie in her decision to investigate this area was
later indicated by Marie.286 The Curies, particularly
Pierre, attended meetings of the French Physical Society287
and may have heard Becquerel's reports of his researches.
J.Perrin and G.Sagnac each reviewed Becquerel's work in
1896,288 and were acquainted with the Curies, possibly as
early as 1897; there was also the long-standing friendship
between Pierre Curie and Ch.Ed.Guillaume.289 The latter's
interest in uranium rays is shown by his discussion of the
energy problem at a Physical Society meeting, after one of
Becquerel's reports in 1896.290 The researches of Becquerel
were in any case well known in scientific circles in 1896-7.
Marie Curie's experimental work on uranium rays291
began in December 1897 in accomodation at the Ecole
Municipale de Physique et de Chimie industrielles where
her husband worked. She achieved more quantitative est-
imates of the electrical intensity of the radiation than
those of Becquerel by means of an apparatus which made use
of the piezo-electric effect studied by P. and J.Curie in
the 1880's. A layer of powdered uranium-bearing material
placed on a charged metal plate caused electrical leakage
across an air gap and a rising accumulation of charge on
a parallel plate which was connected to an electrometer.
This charge was continually balanced by adding successive
weights, by hand, to a piezo-electric quartz thus main-
taining a more or less null deflection of the electrometer.
The apparatus was calibrated by means of a known. charge
and the current flowing could be calculated from the
weight/time ratio; this method of measuring small currents
had been described in the thesis of J.Curie.292 It gave
results which Marie Curie claimed to be accurate to 2%
of the values of the minute currents of order 10-11 amps
involved during the first few months of the work.293
Problems of quantitative accuracy arose later when large
weights had to be added in a short time to compensate for
currents produced by the intensely emitting substances
which the Curies were to discover.
100
If uranium radiation were a kind of short-wavelength
or X-ray phosphorescence, an Becquerel supposed, it should
diminish in time, even if slowly, and should be excited by
irradiation. If there were similarities with the storage
of light shown by thermo-luminescence then heating should
have some effect. These points which had occurred to
Becquerel and others were probably in Marie Curie's mind
when she began by seeking the effects of heating, and of
irradiation by light and X-rays, upon uranium. The
intensity of the uranium rays, on re-examination with the
sensitive apparatus after treatment as above, remained
always unchanged.294 Her lack of success in finding a
straightforward answer to the question of the origin of
the radiation was compensated by an important discovery
as her research turned towards other materials. On
surveying as complete a list as possible of other metals
or their compounds she found that thorium too emitted
rays of the same order of intensity as uranium. Her first
publication on uranium rays 'Rayons 6mis par les composes
de l'uranium et du thorium'295 shows that she had noted
that the only two elements exhibiting this property
possessed the greatest atomic weights; she had in addition
linked this point with current research on X-rays and
their secondary rays to give something of a theory of the
origin of uranium and thorium rays.
Working independently of Mme.Curie and at about the
same time, G.C.Schmidt also surveyed many materials,
discovered that thorium emitted similar electrically
detectable radiation to that from uranium, and claimed
priority.296 With E.Wiedemann at Erlangen he had earlier
studied experimentally various kinds of fluorescence,
phosphorescence and thermoluminescence; he had discussed
the theoretical basis of these phenomena in terms of
vibrating ether envelopes around molecules, vibrations of
atoms and their valency charges, and ionisation or definite
chemical separation of atoms in the molecule.297 Schmidt
sought a relationship between three phenomena: the photo-
graphic effects of various substances, including uranium,
as described by Colson, Russell, Muraoka and others; the
101
electrical conductivity produced in gases by uranium, and
lately thorium, and their compounds; and the photoelectric
effect involving a loss of negative charge or an acquisition
of positive charge by certain metals and minerals upon
irradiation with light of certain kinds.298 He stated that
he had followed the work of Elster and Geitell who had
shown experimentally both that metals exhibiting photo-
electricity did not emit radiations electrifying the air
in the manner of uranium and that uranium salts were them-
selves not photoelectrically sensitive.299 Schmidt used a
modification of the apparatus used by Elster and Geitel300
for some earlier studies on the photoelectricity of
minerals to show that thorium compounds too gave no
increase in air conductivity when irradiated with light.
We can see that this would indicate that thorium radiation
was neither of photoelectric origin nor of a kind of phosphoresence which could be excited by the light employed.
Furthermore, none of the many photographically active
substances studied in 1896-7 proved to be electrically
active. Only uranium and thorium rays possessed both
electric and photographic properties, and Schmidt noted
the similarities of these radiations in a fuller account
of the research, of 1898:301 thorium rays were reflected
and refracted but not polarised; Becquerel's conclusion
that uranium rays exhibited all three of these properties,
still stood; and it was accepted that X-rays exhibited
none of these properties. Schmidt's only hint of an
explanation of the photographic and electrical properties
of the radiation was that 'Es scheint als ob dieselben an
das hohe Atomgewicht Uran = 240, Thorium = 232 gebunden sind' .302 Although it has a place of importance in the
progress towards the modern theory, there was no reason
for Schmidt to attribute great significance to this
interesting point. If he had written that the radiation
came from within heavy atoms this would not have been a
novel conclusion at the time. For E.Wiedemann believed
that Schmidt's spectroscopic work on fluorescent vapours
indicated that a single atom of a metal could behave as
an electrically complex emitter of electromagnetic
102
radiation.303 Having provided facts and conclusions vital
to other scientists, Schmidt left the subject with
Becquerel's question of the origin of the energy unanswered
and continued his research on the complex problems of
electro- and photo-luminescence.
Marie Curie was working along the same lines as
Schmidt but had made much more of the study than he by
the time their first papers on the subject were published
in the spring of 1898. In her first paper304 she clearly
assumed, in agreement with Becquerel's conclusion for
uranium, that the property of emission of the new radiations attaches to the elements themselves. This enabled her to
dismiss phosphorus from the active class, although the
white allotrope produced a strong air conduction effect,
since compounds of this element were not active.
Phosphorus never really became involved with radioactivity
though the cause of the conductivity produced by it,
whether due to ions, fumes, or radiation, continued to be
a point of discussion into the twentieth century.305 It
is not clear whether she was aware, as Schmidt was, of
Russell's work on the photographic activity of metals
and organic substances, but Mme.Curie seems to have •
examined only inorganic specimens. She noted that the
activity of all minerals could be attributed to the presence
of an active element, namely uranium, thorium, or one of
three weakly active rare earth elements,306 and that uranium
salts were active 'd'autant plus qu'ils contiennent plus
d'uranium'; her published list of activities, however,
shows no quantitative proportionality between uranium
content and activity. What was 'tres remarquable' were
the activities of the minerals chalcolite and pitchblende,
52 and 83 respectively, compared with impure uranium's
reading of 24. The reader, having been led thus far, was
presented with the surprising deduction that 'ces mineraux
peuvent contenir un element beaucoup plus actif que
l'uranium', and it seems that 'element' here does have
the meaning of a definite chemical element. Certainly
Rutherford could not accept this conclusion; as has been
noted307 he attributed the effect to the large surface
103
area produced by finely powdering the substance. But
Marie Curie was aware of this kind of effect, had realised
that only the surface layer of uranium emitted the rays
and that thorium differed in so far as its deeper layers
contributed to the radiation; the important factor of
consistency of readings no doubt satisfied Pierre Curie,
who was always wary of premature publication. Further
evidence came from the preparation by Debray's method of
an artificial chalcolite308 which was found to possess
an activity no greater than that of an ordinary uranium
salt.
It seems that at the time of this initial publication
Mme.Curie, and perhaps P.Curie, had decided that the
emission of the penetrating radiation was a fundamental
property of an element independent of its chemical and
physical state. Becquerel had already said this, but
Marie Curie's suggestion of a new element seems to involve
the further step of assuming that the magnitude of the
conducting effect of the radiation was specific to the
emitting element. This, without further explanation, she
called the 'activite' of the element or compound. Certainly
the high readings of some minerals comprised the only
evidence given for the existence of a new element in the
Curies' paper cited by Rutherford,309 which was Marie Curie's
second publication on the subject.310 The information that
chalcolite and pitchblende emitted a radiation qualitatively
different from that of either uranium or thorium compounds,
as determined by the fraction transmitted by an aluminium
sheet, had indeed been given in the first note but without
interpretation. Such a qualitative difference may appear
to be evidence for the existence of a new element, as good
as, or better than a high activity; it seems more akin to
the emission of the visible radiation of a distinct spark
spectrum which constituted the most acceptable evidence
and was the chemist's 'court of final appeal'311 for the
proponent of a new element. Perhaps she felt that the
considerably more penetrating nature of the rays from a
thick layer of thorium oxide, compared with a thin layer,312
impaired the value of this as a criterion for identification
104
of the emitting element. But it is possible that qualit-
ative differences in the radiations may have played some
part in Marie Curie's bold deduction of the presence of
a new element.313 For the consideration of such differences
seems to be involved in her theoretical explanation of the
origin of the radiation. G.Sagnac, in the months before Mme.Curie's first
publication on the rays of uranium and thorium, had been
continuing earlier work of others and himself on X-rays by examining their effect in causing gases to become elect-
rically conducting in the presence of metal plates.314 From
several series of experiments he concluded that X-rays
entering or leaving solid materials always produced easily
absorbed secondary rays or S-rays. Thus the observed
conductivity or ionisation produced in a gas by X-rays is
the sum of the effects of the X-rays and S-rays; this
differed from Perrin's view of 1897 that the additional
conductivity resulting from the introduction of a metal
plate arose from an ionisation occurring at the gas-metal
interface struck by X-rays.315 Sagnac examined the S-rays
from a variety of substances, using fluorescent, photo-
graphic, and electroscopic means of detection. By the
beginning of 1899316 he saw the absorption and emission
of X-rays and S-rays by chemical elements in terms of
their similarity to normal spectroscopic absorption bands
though the comparison was not a clear one; and it seems
that this idea of an incipient X-ray spectroscopy was
being formed during the previous year. In her initial paper of 1898 Mme.Curie mentioned the
researches of Sagnac and pointed out that the properties
of uranium and thorium rays are 'tres analogues' to those
of the secondary X-rays; the researchers at Cambridge had
also taken up this analogy, as we have seen.317 MarieCurie
tells us that she had herself examined the secondary rays
from uranium, pitchblende and thorium oxide and had found
these to have a greater discharging effect than those from
lead.318 Her proposed explanation of uranium and thorium
rays was that:
105
Pour interpreter le rayonnement spontane de l'uranium et du thorium on pourrait imaginer que tout l'espace est constamment traverse par des rayons analogues aux rayons de Rantgen maisbeaucoup plus penetrants et ne pouvant gtre absorbes que par certains elements a gros poids atomique, tels que l'uranium et le thorium.319
It was well known that bodies of greatest density, or
atomic weight, absorbed X-rays most effectively. Marie
Curie was now postulating the existence of a highly
penetrating radiation which was absorbed and then re-
emitted in less penetrating form by the elements of
highest atomic weight only. Why lead, with an atomic
weight of about 210, should show no sign whatever of
activity whereas thorium (230) was more active than
uranium (240) was not explained; however, the relatively
high activities of some minerals may have suggested that
the atomic weight of the new element might be greater
than 210. Mme.Curie's theory of the origin of uranium
and thorium rays was disposed of by her own hand, and
those of others, by the end of the year 1898; nevertheless
her suggestion of the existence of a new element proved
to be doubly justified.
Following her original suggestion Marie Curie was
joined by Pierre Curie in the ensuing chemical search for
the new active element and the couple were advised and
aided by G.Bemont, Pierre's counterpart in chemistry at
the Municipal School. After three months they were able
to report to the Academy through Becquerel, 'Sur une
substance nouvelle radioactive contenue dans la pechblende 1320
and went so far as to give a name to the new element, though,
as they noted, E.Demar9ay was unable to confirm it spectro-
scopically. They had started with the mineral pitchblende,
two and a half times as active as uranium, and applied the
usual successive dissolution and precipitation techniques
of inorganic chemical analysis. After each operation the
more active portion, measured with the piezo-electric
electrometer, was selected for further analysis. Finally,
after separations which were effective though incomplete
the highest activity resided with the element bismuth.
Continuing fractional dissolution and reprecipitation
106
gave slowly increasing activities, but sublimation of the
active bismuth sulphide gave a substance of the highest
activity, 400 times that of uranium. As is well known,
Marie Curie in this publication named the possible new
element 'polonium' after her country of origin. Polonium
obstinately refused to exhibit a spectrum, and had a
chequered history;321 however an element of this name
survives today. Most of the products set aside in the
pitchblende analysis must themselves have been active -
a number of quite novel chemical avenues could now be
followed. By the end of the next academic term, in
December 1898, the Curies and B6mont showed how successful
a path they had trodden by announcing the discovery of a .
second new radioactive element, on this occasion with far
stronger evidence to support their claim.322
The direct inorganic analysis guided by continual
electrical measurements led to barium chloride of activity
60 times that of uranium; the assumption was that a new
active element chemically similar to barium was present.
A fractionation procedure of dissolution in water and
partial precipitation by alcohol produced progressively
more active precipitates. The authors noted that this was
evidence of the existence of a new active element whose
chloride possessed different solubility characteristics
from those of barium chloride. They were able in this way.
to attain a substance of an unprecedented activity of 900,
before the materials ran out.323 100 kg. of pitchblende
residues from Joachimsthal (lacking uranium which had been
extracted for use as a colouring material) had already
been acquired.324 The strongest evidence for a new element
which they produced at this stage was provided by the
rare-earth spectroscopist E.Demarpy325 who stated that
he had actually found a new line in the spark spectrum
of the active substance. And the Curies and Bemont noted
that 'L'intensite de cette raie augmente done en meme
temps que la radioactivite'326 which constituted 'une
raison tres serieusel for attributing the new line to
the radioactive portion of the specimen. They named the
new element 'radium'. A possibly marginally greater
107
atomic weight of the active barium, compared with ordinary
barium, was obtained by determining the chlorine in the
anhydrous chloride. The expectation of an atomic weight
greater than that of lead was indeed to be realised.
All of the evidence, radioactive, spectroscopic and
gravimetric, was fully confirmed within four years.327
In the original paper on radium of December 1898 the
Curies described radioactivity for the first time as
'une proprietb atomique, persistant dans tous les tats . chimiques et physiques',328 they henceforth persisted in
the use of this expression, throughout the controversies
in the next few years concerning the origin of the phen-
omena. Just as 'atomic weight' need mean no more than
'relative combining weight' the expression 'atomique'
may here mean no more than 'elemental'. The authors
elaborated no further, perhaps with good reason.
Marie Curie's review article on 'Les Rayons de
Becquerel et le Polonium'329 appeared shortly after the
announcement of the discovery of radium, contains no
mention of that substance, and was therefore probably
written between July and November 1898. Its contents
give us an indication of the meaning of the description
of the emission of rays as an atomic property. One of
the reasons which Marie Curie gave for excluding white
phosphorus from the class of radioactive substances,
despite large readings on the measuring apparatus, was
that it was active neither when in chemical combination
nor as the red allotrope so that 'on ne retrouve done
pas le caract6re d'activit4 atomique independante des
6tats physiques et chimiques';330 this is similar to the
statement made at the December meeting of the Academy.
But in the earlier review we are also told that since
uranium exhibits constant readings, independent of its
chemical or physical state, its radiation appears
'comme une propriete mol6culaire, inherente a la mati6re miime de l'uranium'.331 A possible origin of the rays
which could be described without contradiction as both
atomic and molecular might be the Ur-Ur bond; chemical
valency in general might also be described thus; but
108
Mme. Curie did not discuss such points. She was never to
describe the phenomenon as molecular after 1898. But
others were to do so with attendant difficulty or con-
fusion surpassing that found here.
Marie Curie's speculations of 1898 were aimed more
at the problem of the 'Degagement d'energie par lee corps . radioactifs',332 she considered several possibilities for
the source. The hypothesis that the radiation was a
phosphorescence of long duration previously excited by
light she dismissed firmly; we have seen that Becquerel
had moved in this direction in 1896. The possibility,
briefly mentioned, that the radiation 'est une emission
de matiere' accompanied by a loss of weight of the active
substances333 seems suggestive of future work and remin-
iscent of W.Crookes"radiant matter' in discharge tubes;
but she may have meant no more than a release of vapour.
This is shown by the heading 'Emission de rayonnement lie
a un etat chimique de la mati6re radiante' under which
she now discussed334 the photographic effects studied
by Colson and Russell in terms of the release of vapours.
Thirdly she wrote that the source of the energy could
come from the evolution of the elements in the manner
suggested by Crookes - the elements of greatest atomic
weight could still be in the process of formation. Much
was left unexplained concerning the original protyle and
the production of the radiation; Crookes was himself
thinking of the problems of radioactivity at this time
but in other ways. Fourthly, Mme.Curie repeated more
clearly her own earlier theory, claiming that there was
nothing improbable in supposing that 'l'espace est le
siege de transmissions d'energie, dont nous avons aucune
idee'335 and that these ultra-X-rays might be transformed
into detectable, less penetrating secondary radiation by
heavy atoms. However, the sun could not be the source
of such rays, since the interposition of the whole body
of the earth would surely absorb a proportion of these
and cause a reduction in the intensity of uranium rays,
yet the midnight and midday readings were equal. She was
more concerned to argue that although such a theory was
109
in accord with the principle of Carnot there were author-
ities who held that such a principle need not apply 'avec
un mecanisme tree petit'. Mentioning the opinion of
Helmholtz on this point, Maxwell's 'demon' of the kinetic
theory of gases, and the work on Brownian motion of
L.G.Gouy her final comment was that:
Dans cette maniere de voir, le rayonnement de Becquerel pourrait 8tre consid6re comme un reflet des mouvements non coordonn6s de molecules matgrielles.336
Thus descriptions of the phenomena as atomic or molecular
were each consistent with such an external energy supply.
Her connection of the radiation to random molecular
motion has implications which were not followed up:
G.G.Stokes, J.J.Thomson and others at this time tentatively
accepted a theory that X-rays consisted of irregular ether-
pulses; but she made no mention of these scientists nor
did she state which, if any, of the five possible theories
she preferred.
Between the times of writing and publication of Marie
Curie's review others indicated their interest in the
growing problem of the energy source, heightened indeed
by the Curies' discovery of highly active substances.
William Crookes independently put forward almost the
identical molecular explanation of the origin both of the
energy and of the attendant phenomena of radioactivity.
And J.Elster and H.Geitel had gone so far as to devise
and perform experiments which cast doubt both upon
Crookes' theory and on Marie Curie's alternative of
unknown radiations from space.337
110
4. Theories and trends (1896-9)
In his address as President of the British Association
in September 1898 William Crookes338 covered a wide variety
of topics as expected, from the beneficial effects of
chemical fertiliser on the world food problem, to the
continuing controversy over the psychic researches whose
validity he accepted. As for uranium rays his early
interest in these seems to have been increased by
Mme.Curie's announcement of a new active element. In his
account of recent development in physical science Crookes
devoted some time to a discussion of the 'radiant activity'
of uranium, thorium, and the new body which possessed this
activity in 400-fold degree, polonium: 'like uranium, it
draws its energy from some constantly regenerating and
hitherto unsuspected store, exhaustless in amount'.339
With regard to the 'haunting problem' of the nature of
this store Crookes adopted the view of proponents of the
kinetic theory of gases, such as Johnstone Stoney, who
believed that the energy of molecular motions might be
made available, as with Maxwell's imaginary demons,
contrary to 'accepted canons'. In this Crookes independ-
ently made the same suggestion as had Marie Curie. But
he went into detail, in a manner which she never adopted,
by suggesting a mechanism involving the atoms of the
elements by which the emission of rays allied to X-rays
could spontaneously and perpetually occur. Crookes
pointed out that faster moving molecules were separated
from slower ones in the case of the evaporation of a
liquid and in the separation by diffusion of a lighter
from a heavier gas: Let uranium or polonium, bodies of densest atoms, have a structure that enables them to throw off the slow moving molecules of the atmosphere, while the quick moving molecules, smashing on to the surface have their energy reduced and that of the target correspondingly increased. The energy thus gained seems to be employed partly in dissociating some of the molecules of the gas ... and partly in originating an undulation through the ether, which, as it takes its rise in phenomena so disconnected as the impacts of the molecules of the air,
111 must furnish a large contingent of light waves of short wave-length. The shortness in the case of these Becquerel rays appears to approach without attaining the extreme shortness of ordinary Rtintgen rays.340
Kinetic theory indicated a large supply of energy,
translational and vibrational, contained in the air.
Crookes' and Marie Curie's speculations were within
months subjected to experimental tests not of their own;
and each theory was found wanting.
J.Elster and H.Geitel, scientific collaborators in
Wolfenbuttel, had throughout 1896 attempted to determine
whether the electrical effects produced by uranium were
related to photoelectricity.341 In experiments, followed
by G.C.Schmidt for thorium,342 they showed that no rel-
ationship existed: uranium salts gave no photoelectric
effect, and photoelectrically sensitive metals did not
emit invisible radiations. Nearly two years later
Crookes' address of September 1898 which they read in
Nature343 induced them to compose and send in their
paper 'Versuche an Becquerelstrahlen'344 before the end of
that month. Reporting experiments begun, in part, earlier,
they noted that they too had considered the surrounding
air to be a possible supplier of the radiated energy; not
as Crookes imagined but by means of a chemical reaction
between one of its constituents and the uranium salt used.
If the air were the source then a decrease in its pressure
should reduce the intensity of the radiation. As they
remarked, Beattie and de Smolan at Glasgow had shown that
the conductivity produced by Becquerel rays in air indeed
decreased steadily to a very small value as the pressure
of the air was reduced. Unlike the Glasgow researchers,
Elster and Geitel interpreted this result in ionic terms-
fewer of the ions required for transport of electricity
would be produced in a rarefied gas.345 Thus the reduction
of the air pressure should give a lower electrical reading
whether or not the intensity of the radiation diminished.
They therefore placed the radiating substance in a vessel
connected to a vacuum pump and examined the rays emerging
through an aluminium window. No significant variation of
112
intensity was to be found using the photographic method
with a long period of exposure - not a reliable quant-
itative method. This procedure was apparently employed
prior to Marie Curie's first publication on the subject
in April 1898; after this Elster and Geitel improved it
by using the more active natural pitchblende, enabling
electrical measurements to be made. These confirmed the
conclusion that the radiating activity was unaffected by
the surrounding air. We know that Rutherford, with his
deeper studies of the conduction of electricity through
irradiated gases, had already assumed as much; this is
shown in his paper on uranium rays,346 then not yet
published, in which experiments using a variety of gases
and pressures were described. Elster and Geitel admitted
that their result did not entirely demolish Crookes'
hypothesis, for even the best vacua contained millions
of molecules which might still supply sufficient energy
for radiation and ionisation. Crookes published his views
in France soon afterwards347 and was to maintain these
even when the energy problem became much greater in the
following years.
Elster and Geitel indicated348 that by July they had
also arranged to test Marie Curie's theory of April 1898.
They believed that no material could be completely trans-
parent to any radiation *and suggested that a thickness of
more than one hundred metres of solid rock would surely
absorb to a noticeable extent the ultra-X-rays from space
imagined in Mme. Curie's hypothesis; this should affect
the 'secondary' or uranium rays. Using the same portable
electroscope and the same piece of pitchblende they
recorded no difference in readings when the apparatus
was taken 300 metres below ground into a mine. A photo-
graphic experiment carried out for them at the bottom of
the Schact Kaiser Wilhelm II, 852 metres deep, also showed
no variation of intensity. Their conclusion was much firmer
than with Crookes' hypothesis:
Nach diesen Versuchen erscheint uns die Hypothese der Erregung der Becquerelstrahlen durch andere im Raume prLexistirende Strahlen im hiSchsten Grade unwahrscheinlich.349
113
However, they were able to confirm the Curies' chemical
extraction from pitchblende of a highly active substance.
This, following the discovery of thorium rays, appears to
have been seen in 1898 as the most important experimental
result in the field since the original researches of
Becquerel.
The development of the subject of uranium rays by
Marie Curie during the early part of 1898 has been regarded
as marking the conclusion to a period in which interest
in these rays had declined to a low ebb partly because of
their submersion in a 'morass' of other radiations.350
This obscurity, as has been shown, was complemented by a
confusion of uranium rays with the Russelleffect. It is
true that in France there was a period of several months
in 1897 when Becquerel had turned to studies of the Zeeman
effect, and Marie Curie had not yet taken up her investi-
gations. But it may be pointed out that elsewhere in
Europe one can trace something of a continuing concern
with the subject during 1896-8. As we have seen, in 1896
several French physicists showed an interest in uranium
rays, which is indicated by their published reviews and
discussions. In England J.J.Thomson's351 comments on the
subject were followed by some experimental studies of E.Rutherford and C.T.R.Wilson in 1897. G.G.Stokes too
discussed the phenomenon with S.P.Thompson in 1896.
Stokes published a theoretical explanation of the radiation
in mid-1897352 and during this year he discussed at the
Cavendish Physical Society the work of W.J.Russell on the
photographic effects of metals; J.J.Thomson reported this
in October 1897.353 Stokes also discussed the subject in
communication with Crookes who had the phenomenon in mind in February 1898.354 Crookes had been attempting experi-mentally to obtain a radiometer effect from uranium
radiation in August 1897; but the definite results obtained
were attributed to temperature differences.355 Kelvin and
others at Glasgow published their researches on the
electrical effects of uranium in 1897. And one can trace
a line of minor interest, if not of continuing experimental
work, in Germany, where the publications of Elster and
114
Geitel of 1897, were followed by G.C.Schmidt's studies
on thorium rays. These, together with the distinction
he made between the effects of the new radiations, of
photographically active substances, and of photoelectricity,
were announced in February 1898.356 And J.J.Thomson in the
previous month made his important speculation as to the
origin of uranium rays.357 One can thus see an underlying
interest in uranium rays which was to be considerably
excited by Marie Curie's announcement of the discovery
of polonium in mid-1898. As studies of uranium rays and radioactivity developed
during the period 1896-9 the various theories which were
put forward can be seen as more or less related to
Becquerel's early description of the phenomenon as a metal
phosphorescence after he had isolated uranium as the source.
G.G.Stokes, the British authority on phosphorescence and
fluorescence, in advising S.P.Thompson concerning the
latter's discovery in February 1896358 of the emission of
penetrating rays from various chemical compounds, also
provided at this time a mechanical molecular explanation. Stokes took the rays to be like X-rays 'transversal
vibrations of excessive frequency' and likened their
emission to the phenomenon of 'calorescence' in which
heat radiation of high intensity could raise a body to
incandescence. Stokes regarded fluorescence 'as a
disturbance extending from more limited to more extensive
molecular groups'359 but calorescence and Thompson's new
phenomenon360 appeared to be the reverse of this:
I look on calorescence as an agitation passing from wider to more minute molecular groups. In your discovery, I think we have something of the nature of calorescence; only that whereas in Tyndall's work the disturbance was excited in the first instance in wider molecular groups, in yours the 'wider groups' are already something like the chemical molecules of the peculiar substance.361
It follows from Stokes' explanation at this early stage
that if the wider vibrating groups are something like
chemical molecules then the smaller groups should be
something like the atoms in a molecule. That Becquerel
was soon able to trace the emission to uranium metal
115
might have led Stokes subsequently to such a conclusion
concerning uranium atoms. But in his Wilde Lecture
'On the Nature of the Röntgen Rays' delivered in July
1897362 he was evasive on this point, while incorporating
most of the new evidence, and expounding a modified theory
of the nature of X-rays. In order to explain the perpetual
emission of rays by uranium without the necessity of
irradiating the metal Stokes told his audience: My conjecture is that the molecule of uranium has a structure which may be roughly compared to a flexible chain with a small weight at the end of it.363
Natural vibrations travelling from the head to the tail of
the molecule would produce ether vibrations 'not of a
regular periodic character'. Stokes still saw X-rays as
transverse disturbances of the ether; however, these were
now characterised not by an extremely short wavelength but
by consisting of completely irregular pulses. Uranium rays
would thus lie between visible light and X-rays in their
regularity. The chemical implications of imagining such
a 'uranium molecule' are not discussed.
Stokes entertained this kind of explanation of radio-
activity at least until 1900 without publishing further on
the matter. Towards the end of 1899 he sent Becquerel a
copy of his Wilde Lecture and discussed theoretical ideas
of radioactivity; Stokes maintained an explanation in
terms of a comparison with the normal visible phosphores-
cence of uranium compounds. He considered that both
phenomena pointed to 'the existence of a molecular group
which is roughly speaking isolated, in the sense that
vibrations going on in it are not very quickly communicated
to the neighbouring structure'.364 Although, as Becquerel
had replied,365 the expected effect of temperature change
on the emission failed to appear Stokes continued to
entertain what he called the 'wagtail' theory. When the
self-luminosity of radium came to his notice in 1900 he
corresponded with W.Crookes concerning experiments which
might distinguish between his wagtail theory and Crookes'
'bombardment hypothesis' 366 and continued the discussion
as the phenomena and ideas became more complex in 1901.
116
We have seen that Stokes attempted to explain uranium
rays in terms of molecular vibrations akin to those assumed
for phosphorescence, that his ideas changed little during
the period 1896-1900, and that they began independently
of Becquerel. By the end of 1898 Marie Curie had explicitly
rejected the earlier statement of Becquerel that the phen-
omenon might be a long lasting invisible phosphorescence,
together with S.P.Thompson's label of hyper-phosphorescence.
It is notable however that her own first analogy of uranium
and thorium rays with secondary X-rays seems similar to the
phenomenon of fluorescence. Apart from Kelvin's guess in
1897 that the effects of uranium might be due to carbides367
two other possible sources of the rays were seriously
considered during the first three years of their investi-
gation. Firstly there was the atomic-molecular hypothesis
independently suggested by W.Crookes and Marie Curie
involving the impact of gaseous molecules upon sensitive
atoms which then produced radiation. Secondly, following
J.J.Thomson's setting out of corpuscular atomic structures
towards the end of 1897 there was his suggestion that some
kind of rearrangement of the constituents of a 'complicated
structure' like the uranium atom might be the source of the
rays. Remarkable speculations which fitted with this latter
view were soon published in Germany.
Rutherford had tentatively adopted368 the above
explanation provided by Thomson. The former's new colleague
at McGill University, R.B.Owens, attempted in 1899 to
incorporate experimental results on the non-homogeneity of
thorium rays and similar radiations into this theory. He
wrote:
Certainly it would be difficult to formulate a theory for the production of such rays which would account for only a particular number of kinds being produced. If x-rays and the radiations from uranium, thorium, polonium, &c. are disturbances in the aether occasioned by the internal motion of certain constituent parts of the atom, as had been suggested, it might be expected that such disturbances would shade off with some degree of regularity from a more intense to a less intense kind...369
But it was left to Elster and Geitel370 to put forward in
117
1899 speculations on the chemical implications of radio-
activity; these may have influenced others, in a manner
not previously brought to light.
Following their dismissal of Mme.Curie's first
hypothesis of the source of the energy of uranium rays
and their doubts as to Crookes' molecular bombardment
hypothesis, Elster and Geitel added further experimental
evidence, all negative, some of which had been previously
obtained by others but not published. Without definite
influence upon the radiation from pitchblende were sunlight,
Lenard rays, temperature changes, and being kept in darkness
for months. They noted the remarkable luminous effect of
the Curies' radium on a fluorescent screen. Their conclusion
was that since the emission of energy from all compounds of
an element could not result from a chemical reaction the
source must be the atoms themselves of the elements concerned.
Now this was similar to what the Curies had said in 1898.
And their further speculation that the atom of a radioactive
element behaves like a kind of unstable molecule emitting
rays on returning to a stable state is no more than
J.J.Thomson and E.Rutherford had written shortly before.
Elster and Geitel may or may not have derived this idea from
Thomson's publication of January 1898 or Rutherford's of
January 1899; Elster had replied to Rutherford's request
for advice on demonstration apparatus and indicated that he
had read Rutherford's paper on 'Uranium Radiation etc.' at
the time of writing, 10th February 1899.371 Elster and
Geitel's final deduction in the paper of 1899 was that the
change of a substance from an active to an inactive state
might necessitate a change of elementary properties: Der Gedanke liegt nicht fern, dass das Atom eines radioactiven Elementes nach Art des Molecules einer instabilen Verbindung unter Energieabgabe in einen stabilen Zustand Ubergeht. Allerdings Warde diese Vorstellung zu der Annahme einer allmAhlichen Umwandlung der activen Substanz zu einer inactiven nbthigen and zwar folgerichtiger Weise unter Aenderung ihrer elementaren Eigenschaften. 372
This conclusion says, almost in so many words, that a
transmutation of one chemical element to another of diffe'rent
properties should be taking place as the radioactive
118
radiations are emitted. Rutherford had considered that the
energy of the emission from uranium was so 'extremely small'
that radiation could continue for long periods 'without
much diminution of the internal energy of the uranium'.373
This avoids Elster and Geitel's conclusions by emphasising
the smallness of the effect. Now since the existence of
the highly active radium could not safely be denied in 1899
Rutherford could possibly have entertained these speculations
of the German scientists, which indeed could have followed
from his own; it is probable that he knew of them374 and
that he came to do so whilst studying the strange properties
of thorium rays at McGill University in 1899.
We have seen that at this time Rutherford accepted
the idea that the ether vibrations emitted by radioactive
substances could be attributed to rearrangement of the
corpuscles constituting the chemical atom and that he
probably knew of the speculation that atomic transmutation
might be occurring. It has also been noted that one of the
speculations put forward by Marie Curie, published early in
1899 and upon which there appears to have been no later
comment but her own, was that the heaviest elements may be
in process of evolution. All ideas on the source of the
radiations, on their energy, and nature, were to be com-
plicated by two discoveries announced towards the end of
1899. One of these, that Becquerel rays could be magnet-
ically deflected, showed the rays possessed an unexpected
property which linked them more closely to cathode rays
than to X-rays. And cathode rays were thought by
J.J.Thomson, his disciples, and some others, to consist
of the material particles which constituted the chemical
atom. But the increasing complexity of the new phenomena
allowed no easy conclusions.
119
CHAPTER 3
EMANATIONS AND RADIATIONS
1. The ma netic deflection of the Becquerel rays 1 9 -1900)
The chemical isolation of highly active substances
by the Curies in 1898 provided new opportunities for
physicists and chemists to pursue experimental studies.
One cannot say whether G.C.Schmidt, E.Rutherford or
others interested in the subject would eventually have
hit upon the existence of the new active elements but
once the Curies had opened this field others soon followed.
In this Chapter we shall trace the complex form of the
development of radioactive studies during the period
1899-1901. It will be seen that the discovery of the
new highly active substances heralded the first magnetic
deviation of radioactive rays. This had far reaching
theoretical implications particularly, in combination
with further new discoveries discussed in Section 2, for
radioactivity. F.Giesel, chemist at the Buehler quinine manufacturing
company in Braunschweig, was one of those who had studied interest in new X-ray fluorescence and photography.
1 His nterest
in the related area of radioactivity began by January 1899
when he spoke in the discussion of Elster and Geitel's
paper at Brunswick.2 During this year he followed the
analytical procedure of the Curies and Bemont to prepare
polonium, and claimed to have independently discovered
active barium compounds which possessed the novel property
of spontaneously illuminating a fluorescent screen;3 he
was able to provide Elster and Geitel with such a highly
active sample.4 Giesel had obtained his uranium residue
starting material from De Haan, chemical manufacturers of
Hanover5 and the first commercial radium-barium samples
were soon advertised by this company6 to whom Elster
advised Rutherford to apply.7 The physicists Elster and
Geitel used an active sample provided by Giesel to deter-
mine whether the electrical conductivity known to be
120
produced in air by Becquerel rays could be altered by a
magnetic field.8 A peripheral idea involved in their
apparently successful attempt led Giesel himself to a
crucial discovery.
It had been known for some years that the electrical
conductivity produced in a gas, by glowing metals for
example, could be suppressed by the application of a
magnetic field whose direction did not coincide with that
of the current.9 The explanation, if based on the assumption
that molecules, ions, or particles of any kind carried the
charge, was that the lateral force suffered by these current-
carrying particles in the magnetic field would deflect them
out of the line of conduction. A marked reduction in con-
ductivity was indeed obtained, but more interest attaches
to the control experiment which Elster and Geitel devised
to ensure that it was not a deflection of the rays them-
selves which gave the observed effect. The Becquerel rays
were thought to be similar to secondary X-rays but there
was nevertheless the possibility that the rays might be
magnetically deviable like Lenard or cathode rays. The
property possessed by Giesel's very active barium salts
of exciting phosphorescence in a screen was valuable for
the straightforward experimental arrangement of Elster
and Geitel. They placed the active substance in an evacuated
glass vessel; the emitted rays passed both through the glass
walls and an aluminium plate upon which rested a barium
platinocyanide screen 1.5 cm. distant from the source.
The visible phosphorescence excited in the screen was un-
altered when their iron horseshoe electromagnet was switched
on. They concluded that the rays were undeflectable by a
magnetic field and hence different from cathode rays; all
of the properties of the Becquerel rays were thus comparable
with those of the X-rays. Others also considered this
point. In France the Curies had earlier, in 1898, similarly
obtained negative results, unpublished, in seeking an effect
of magnetic and electric fields on radioactive rays.10 And
Becquerel later stated11 that towards the end of 1899 he
too had been seeking independently some such influence,
and had in fact found one: on placing a fluorescent screen
121
at one pole and an active sample at the other pole of a
magnet, the application of the field produced a con-
centration of the fluorescent patch into a smaller area.
All experiments on the subject of radioactivity bore
repetition at this time, autumn 1899. Thus Giesel
repeated12 Elster and Geitel's experiments in simpler
fashion without a vacuum but with a more powerful magnet.13
Using a phosphorescent screen placed upon the poles of a
vertical horseshoe magnet and an active freshly prepared
polonium specimen placed beneath the screen he became the
first to succeed in obtaining quite definite results.
Upon switching on the magnet, the luminous spot was
displaced in a blurred fashion but in a definite
direction in relation to the field. Giesel also produced
variously shaped images by the photographic method; exposures
of up to ten minutes were all that was required for a fixed
record of the effect. Having achieved these results Giesel
at first published no interpretation, but various explan-
ations were not long in coming. Two privatdozenten at
Vienna who also followed the work of Elster and Geitel
moved towards a curious misinterpretation of their own
results and expressed an interesting if short-lived
speculation bearing on the source of Becquerel rays.
S.Meyer and E.von Schweidler extended their work on
the magnetic properties of the chemical elements, which
included correlations of atomic magnetism with the periodic
table, to a study of the magnetic properties of radium
preparations and the rays emitted by these.14 They used
both Curie and Giesel barium-radium and bismuth-polonium
preparations placed 12 cm. from the air-gap through which
the rate of electrical discharge was measured electros-
copically. On applying the magnetic field this rate was
considerably reduced; the Curie polonium however was un-
affected. The results of varying the intensity of the
field indicated that at least two different effects
operated to reduce the conductivity of the air-gap. They
provisionally accepted the statement of Elster and Geitel15
that the rays were not deviated by the magnet, without
knowing that Giesel had proved this wrong, and speculated
122
that the magnetic field might act directly upon the radium
in reducing its radiation:
so ware die Ursache diener Erscheinung nur in einer direkton Beeinflunnung der Emission der Substanz selbst zu suchen wenn eine Ablenkung der Strahlen nicht stattfande.16
In a matter of days they sent off a second paper now
mentioning Giesel's work on the magnetic deflection of
rays not yet available in print which they had confirmed
for themselves. With their powerful electromagnet they
had been able for example to bend the rays in a tight
semicircle back to the screen upon which the active
substance stood; they noted that the direction of the
effects was entirely similar to that of the negatively
charged cathode rays.17
However, their initial conjecture that the magnetic
field might influence the emitter itself, ill-founded and
temporary though it was, gives us an indication of one
way in which radioactivity could be understood towards
the end of 1899. The possibility that a magnetic field
might affect the property of radioactivity was not lightly
to be discounted. It is true that none of the various
attempts to influence radioactivity by a variety of
physical and chemical means had given any positive result
and that magnetism was to remain on this list. Yet it is
interesting to note that an effect somewhat analogous to
that at first assumed by Meyer and von Schweidler had
recently been detected. Zeeman's discovery made at the
end of 1896 was accepted as demonstrating that a magnetic
field could influence the source of atomic spectra,
altering the frequency and direction of vibration of the
radiation. J.J.Thomson had used this discovery to support
his corpuscular atomic theory of 1897.18 In 1899, when the
Continental workers were studying the effect of magnetism
on the new atomic property of radioactivity and upon
Becquerel rays themselves, he was defending this developing
theory against criticism.19 In his paper 'On the Masses
of the Ions in Gases at Low Pressures'20 he insisted on
the corpuscular structure of the chemical atom and,
probably arguing against the view that all observations
123
could be explained by means of a small number of free
electrons or valency charges, he again pointed to Zeeman's
discovery, now extended. Since many spectral lines
exhibited an effect of the same order each atom must
contain many corpuscles despite the fact, shown by e/m
estimates for ions in gases, that very few could be
removed.21 At this time Thomson considered that Becquerel
rays themselves originated in the motion of these
electrified particles. Although the hope of finding
evidence to support the theoretically possible link between
magnetism and radioactivity apparently remained with him
for several years22 the negative interpretation of the
experimental results of 1899 remained unchallenged.
Thomson believed that both chemical and electrical actions
essentially involved 'the splitting up of the atom, a part 2 of the mass of the atom getting free and becoming detached'.3
The positive discovery that radioactive substances did not
simply radiate soft X-rays but released streams of subatomic
corpuscles turned out to be of extraordinary interest to
those such as Rutherford who entertained Thomson's ideas.
However, R.J.Strutt later wrote24 that he clearly
remembered that this striking interpretation of Giesel's
results was at first acceptable neither to Thomson nor
Rutherford on account of the high penetration of these
radioactive rays. The former wrote to the latter:
I see Giesel makes out that the radiation from polonium is affected by a strong magnetic field, if this is so it might be worth while trying whether your emanation from thorium were so affected.25
Rutherford replied in January 1900:
The results of Giesel & Becquerel are very interesting and remarkable. I expect the 'emanation' in thorium is also true for polonium & radium when prepared in a special manner & that the deflection due to the magnetic field is due to the action on a charged particle cast off from the active body.26
Within a few months Rutherford indeed saw the phenomenon
as a direct emission of cathode rays and had made certain
deductions from this. He had previously adopted a tentative
comparison of the beta and alpha uranium rays with X-rays
124
and the secondary radiation produced therefrom27 but had
given no decision between Thomson's soft X-ray or Sagnac's
Lenard-ray view of the secondary radiation. The demon-
stration during this period by P.Curie and G.Sagnac28 that
secondary X-rays in fact carried a negative charge may
have provided Rutherford with indirect evidence for the
new view of radioactive rays. The phrase 'secondary
X-rays' which he used29 to describe the non-deflectable
radium rays may thus have acquired the new meaning of
actual X-rays produced as a result of the impact of cathode
rays. Yet this slightly modified causal link between the
two known types of radioactive radiation was soon to be
completely broken. Extensions made by Becquerel to his
magnetic results provided direct evidence of the nature
of the deflectable rays during the first part of 1900.
By June of that year Rutherford was able to write that
the property possessed by some active substances of
'naturally emitting a kind of cathode rays'3° did not
contradict Thomson's theory of regrouping and vibration
of the constituents of the atom. But we note that such
a mechanism had been intended to explain the production
only of electromagnetic radiation. Rutherford's assumption
that the radiations in question were 'cathode rays of low
velocity',31 which agrees neither with Becquerel's con-
temporary studies nor with Rayleigh's later comment on
penetrations, illustrates the difficulties of this novel
aspect of radioactivity.
The successful magnetic deviation excited an interest
corresponding to its initial problematical nature; it gave
a further impetus to investigations of the rays themselves.
A number of physicists concerned with the cathode rays,
such as R.J.Strutt (later Lord Rayleigh), P.Villard, E.Dorn,
W.Kaufmann, used samples of radioactive substances in their
researches from about 1900, with the attainment of sig-
nificant results pointing towards an electromagnetic theory
of matter by 1901. And the effect of the magnetic discovery
upon the French students of radioactivity was considerable
not only in exciting the great activity in investigating
these rays which arose during 1900 but also in the changes
125
of view which the rapidly developing experiments helped
to produce. One can, for example, interpret the discovery
of the emission of cathode rays as marking the quiet beg-
innings of a severe conflict of the ideas of P.Curie both
within himself and against his colleagues. In the dis-
cussion following a report to the French Physical Society32
in which Becquerel announced his successful deviation of
the rays by an electrostatic field, further demonstrating
their similarity to cathode rays, P.Curie spoke. He thought
it surprising that radium should emit radiations having the
properties of cathode rays as well as X-rays since it was
generally agreed that while X-rays were the propagation of
a disturbance cathode rays were a flux of ponderable matter.
Despite his own considerable researches on the electrical
nature of the radiation, after two years he found himself
forced to reject this interpretation of cathode rays. This
was in reaction against views, implied by the emission of
subatomic masses, which several scientists began to develop
with growing supporting evidence following the magnetic
deflection of radioactive rays.
By March 1899 Becquerel, re-entering the field, had
accepted Rutherford's point published in January that
uranium rays did not behave like ordinary light with
regard to polarisation, refraction and reflection; they
were more like X-rays.33 Becquerel continued investigations
of the nature of these rays by examining their penetration,
secondary radiation, and the possible effects of magnetic
and electric fields. It was the work of this scientist,
who was fortunate in having access to the Curies' most
active specimens, which contributed perhaps most to a
clearer understanding of the nature of the rays during
the year following their first deflection. Becquerel's
initial attempts to obtain an electrostatic deviation
during 1899 failed as he perhaps expected since the field
applied was known to be insufficient to deflect ordinary
cathode rays.34 But his magnetic studies35 soon gave highly
significant results. The Curies were first in making a
valuable correlation of three aspects of the rays namely
deviability, penetration, and conductivity. Their polonium
126
radiation was easily absorbed and remained undeflectable
and making use of radium they were able to show that
whilst the deviable portion of magnetically analysed rays
was more penetrating than the non-deviable portion, it
contributed little to the total radiation as measured
electrically.36 Becquerel immediately moved the study
to a more quantitative level. He pointed out37 that if
the radium rays consisted, like cathode rays, of electrically
charged material particles then the well-known equation38
H.R = v.m/e held true. H was known, R was found by
photographic impression; this gave a value for v.m/e of
the same order as determined for the cathode rays by
J.J.Thomson, W.Wien, and P.Lenard. Becquerel deepened the
correlation between deviability and absorption: he found
that the interposition of screens before the detecting
plate gave a kind of absorption spectrum in which the most
easily deviated rays were the most easily absorbed.39 He
expanded the point in a paper 'Sur la dispersion du
rayonnement du radium dans un champ magnetique'.40 The
sharpness of shadows was unimpaired on covering the radium
source with an aluminium screen which indicated that
aluminium was truly transparent to the rays and not a
re-emitter of secondary radiation. Becquerel noted minimum
values of HR for the radium rays (dispersed as well as
deviated by the magnetic field) to which screens of various
materials and thicknesses behaved as transparent; to rays
having HR below a certain value, all screens were opaque.
It was still debatable at the time whether or not the
aluminium screen used to allow Lenard rays to escape from
the discharge tube acted as a window; penetration of
opaque bodies by cathode rays seemed unlikely to Crookes,
for example, at the end of 1900.41 J.J.Thomson's answer
in 1896 to Lenard's claim of 1895 that ether waves, but
not material particles, could penetrate a metal window
seems to have been that X-rays together with a re-emission
of charged particles were responsible for the observed
effects.42 From Thomson's discussion of 189743 one might
deduce, considering the cm. penetration of cathode rays
into ordinary air, that aluminium might not function as a
127
window if thicker than about 0.001 mm. Becquerel's
aluminium screen was 0.2 mm. thick; if the particles
were all of the same charge and mass his experiment
would seem completely to correlate velocity with pen-
etration. A reviewer in an article on 'Becquerel rays.
Confirmation of the materialist theory of the deviable
rays of radium' seems to have been first to publish the
clear inference that 'the particles which strike the plate
furthest from the source will be those possessing the
greatest speed, and it is natural that they should also
be the most penetrating'.44 Radioactive studies in 1900
thus lent temporarily a sharp clarity to the projectile
character of the cathode rays. R.J.Strutt staked his claim
in the matter writing to Nature45 from the Cavendish
Laboratory that the magnetically deviable rays from radium
exhibited, like cathode rays, a coefficient of absorption
very approximately proportional to the density of a series
of materials ranging from solids to gases, but that this
coefficient was only about 1/500 that of the cathode rays.
Hence 'One must suppose either that the particles consti-
tuting them are much smaller, or that their velocity is
much greater'. Becquerel's results together with the
Curies' demonstration that radium exhibited a continuous
emission of negative electricity, spontaneously acquiring
a high positive charge,46 provided clear evidence that
radium emitted negatively charged particles of matter.
That the question of the size of these was not simple is
further demonstrated by the suppositions of the Curies
themselves. They thought it reasonable to infer that
'le radium est le siege d'une emission constants de
particules de matiere electrisee negativement'47 which
could traverse either conducting or dielectric screens
without becoming discharged. However, one can see that
the Curies at least were not yet thinking in terms of
subatomic corpuscles for, knowing the rate of loss of
charge, they gave in March 1900 what was probably the
first published estimate of the loss of mass involved on
the assumption that e/m was the same as in electrolysis..
They did not say how these particles could penetrate screens.
128
Such an emission of negatively charged atomic or molecular
masses gave an estimated weight loss amounting to
3 milligram-equivalents in a million years. Attempts to
detect such losses from ever more active specimens were
to occupy the Curies and some others during the next few
years. However, the Curies' view of the rays soon altered
in a way perhaps similar to that of Rutherford.
This shift in opinion followed the current rapid
experimental progress. For later in that month Becquerel
effectively consummated the study by his 'Deviation du
rayonnement du radium dans un champ electrique'.48 He was
thus able to confirm that the projected particles were by
no means atomic in mass; the variability of this mass was
not yet an experimental question. Becquerel's photographic
method provided measurable and reversible deviations of
rays passing through a narrow slit in a small lead container;
the quantity e/mv2 could thus be calculated. As we have
seen the value of v.m/e could be found by magnetic deviation
experiments so that values of v and e/m could now be
separately determined. The 'point delicat' was to ensure
that the values obtained from magnetic and electric experi-
ments both applied to the same part of these deviable rays,
which exhibited a considerable range of dispersion.
Becquerel achieved this, as far as possible, by selection
of the rays using partially absorbing screens. Thus he
obtained values for v of 1.6 x 1010 and for e/m of 107
for a portion of the deviable radium rays, similar indeed
to cathode rays. For the latter Becquerel cited values
of v up to 0.81 x 1010. He made no explicit comment on
the possible subatomic nature of these particles, nor on
their uniquely high velocities, but estimated the rate of
decrease of mass which would be caused by their loss as
far lower than that of the Curies; one would expect this
from the emission of particles lighter than atoms but
carrying the same electrolytic charge. From the value
of mv2 for the particles Becquerel estimated that the power
emitted by the rays was a few ergs per square cm. per sec.
Such a value was very soon shown to be excessively low:
this portion of the radium radiation carries but a fraction
129
of the total radiated energy. Nevertheless, the experi-
mental deflection of these rays made a permanent impression
upon all of the various views of the source of radioactivity,
as elaborated below in Section 3. Even while this radiant
success was in progress Rutherford came to attribute his
own interesting results on thorium rays to something more
than a radiation - to the release of a material gas-like
substance which he took as the cause of another new phen-
omenon, 'excited' or 'induced' radioactivity. The nature
of induced radioactivity was to become. an area of partic-
ular disagreement between the Curies, Becquerel, Rutherford,
and others, but also a point of progress.
2. The discovery of induced radioactivity (1899)
Yet another branch of radioactive research unfolded as a
series of novel experimental results began to appear.
In parallel with magnetic studies, several scientists
worked in this new field on similar lines, at first indep-
endently. In some cases the extraction of the new intensely
active substances was an essential factor, in others ordinary
compounds sufficed. The discovery of the radioactivity
induced by radium and polonium was made by the Curies
towards the end of 1899, and the title of their note 'Sur
la radioactivite provoquee par les rayons de Becquerel'49
indicates their initial view that this effect was due to
the direct incidence of radiation. They found that samples
of polonium and radium with activities 5,000 to 50,000 relative to uranium could produce, in all substances tried,
130
temporary activities ranging from 1 to 50, the higher
values being obtained with longer exposure to the rays;
the effects diminished to about one tenth in two to three
hours. The new phenomenon which they named 'radioactivite
induite' created many scientific problems for the Curies.
As they stated, the value of the activity produced seemed
not at all to depend on the nature of the surface made
active. However, they were unable to explain this fact.
Could the effect be due to vapour or to the deposition
of dust particles from the active substance? They thought
that the steadily declining induced activity could not be
attributed to non-volatile particles of the activating
radium-barium chloride for the activity of the latter did
not so decline; furthermore, washing the activated surface
with water should remove this soluble salt, but the induced
activity remained unaffected by the process. Neither could
it be attributed to a vapour, for an activity was apparently
produced by radiation which had passed through the aluminium
window of a metal box within which the activating specimen
was sealed. Rutherford at this time did in fact attribute
induced radioactivity to an active gas-like substance
which could penetrate metal screens. E.Dorn shortly after-
wards in 1900 agreed50 that some kind of active gas was
responsible, but pointed to the possible presence of pores
in thin metal screens. One must note that the problem may
also have been complicated by external traces of radium as
contemporary scientists may soon have realised. However,
the Curies in 1899 concluded that 'Le phenomene de la
radioactivite induite est une sorte de rayonnement
secondaire du aux rayons de Becquerel',51 though dist-
inguished by its longer duration from the direct emission
of secondary rays. Becquerel evidently considered that
this fitted with views he had held since 1896 and he
commented52 that the new phenomenon should be placed
alongside the production of secondary rays of low pen-
etration by thorium and uranium rays; this he considered53
had led to the earlier false deductions by himself and
Schmidt that diffuse reflection of these rays took place.
Thus induced radioactivity and the emission of secondary
131
rays were respectively similar to phosphorescence and
fluorescence. Becquerel did not say how such an analogy
could explain why different substances exhibited the same
magnitude and duration of induced activity. The Curies
themselves merely expressed surprise at this and relegated
to a footnote their suggestion that the condition of the
surrounding air might have some influence in causing
irregularities in the induced effect. Similar points
had already been noticed by Rutherford in his work on
thorium, and had played a vital part in his own sharply
differing interpretation of 'excited radioactivity'.
Whilst Rutherford was studying the electrical effects
of uranium radiation at the Cavendish Laboratory in 1898,
G.C.Schmidt and Marie Curie had announced the discovery of
a similar radiation from thorium. Rutherford in his paper
on uranium rays54 wrote of his own attempts to study the
new thorium radiation. It appeared to be complex and of
a different kind from that of uranium as shown by its
penetration of aluminium screens. Unlike uranium salts
thick layers of thorium nitrate gave a greater proportion
of penetrating rays than thin layers but 'On account of
the inconstancy of thorium nitrate as a source of radiation,
no accurate experiments have been made on this point'.55
The rate of leak varied 'very capriciously'. This was the
problem which Rutherford took to Canada towards the end
of 1898. In his first preliminary paper from McGill
University some eight months later he again noted that
thorium was far from exhibiting the constant radiation
which was such a notable property of uranium:
The inconstancy of the radiation from thorium oxide was examined in detail, as it was thought it might possibly give some clue as to the cause and origin of the radiation emitted by these substances.56
It was R.B.Owens, Macdonald Professor of Electrical
Engineering, working with Rutherford on the thorium rays
in 1899, who found the first clue as to the variability
132
of thorium radiation readings - the marked effect of
slight currents of air.57 With thorium placed in a closed
box the measurements remained constant, but on opening a
door in this box the readings were consistently lower,
and were further diminished if somebody opened or closed
the laboratory door; the readings recovered on standing
for some hours in the closed box. The authors mentioned
that they had performed a large number of experiments,
for example examining the effects of blowing air over the
surface of the material, but that they had found 'no clue'
as to why the oxide of thorium should exhibit the phen-
omenon. The explanation which they provided is fascinatingly
unclear. The effect of an air current was not due to
removal of the ions but due to its action at or near the
surface of the substance:
It appears as if in the pores of the thick layer of thorium oxide some change takes place with time, which increases the intensity of the radiation, and if the result of the action is continually removed, the intensity of the radiation is diminished.58
In his next paper on 'Thorium Radiation'59 Owens detailed
how they had overcome the variability problem to study
what appeared to be the true penetration characteristics
of thorium rays. He concluded that these were composed
of many distinct types which was consistent with the
suggestion that 'the internal motion of certain constituent
parts of the atom' produced such 'disturbances in the
aether'. We shall examine his expansions and enigmatic
explanations of the original air-current phenomenon, as
well as Rutherford's more successful alternative.
Necessarily to anticipate this discussion, here are
Rutherford's words of introduction to his description of
'A Radioactive Substance emitted from Thorium Compounds': 60
In addition to ... ordinary radiation, I have found that thorium compounds continuously emit radioactive particles of some kind which retain their radioactive powers for several minutes. This 'emanation', as it will be termed for shortness, has the power of ionizing the gas in its neighbourhood...61
Two points turned. out to be most important for the
continuation of this research towards the successful
133
theory of 1902-3, both of which became the subject of
dispute. These were firstly that the 'emanation' from
thorium compounds consisted of a material vapour which
was not thorium vapour, and secondly that this emanation
produced, upon any solid body, a radioactive deposit of
a distinct chemical nature. This deposit could be con-
centrated upon any object negatively charged.
Since the discovery of the thorium emanation, I have always taken the view that the emanation consists of matter in the radioactive state present in minute quantity in the surrounding gas.62
Thus wrote Rutherford a few years later in 1903 when his
theory was widening rapidly in scope. We note that this
statement is borne out by the earlier publications, and
is not tautological in the later context. The vital
conception of a material emanation seems to have appeared
during the period May to July 1899 and that of an active
deposit probably during that period also.63 Although
Rutherford chose to discuss each aspect separately in his
publications, he evidently studied these in close parallel.
This is indicated for example by Thomson's reply to
Rutherford's communication with Owens, who was spending
the summer of 1899 at the Cavendish Laboratory. And here
began an infrequent correspondence between Thomson and
Rutherford which seems to have developed into something
of a disagreement, lasting until 1903. The views of the
former fluctuated somewhat, as will appear; but some of
his information, theoretical ideas and suggestions for
experiments, though historically neglected, clearly
influenced Rutherford. Thomson first commented on the
points which he thought were of particular importance or
difficulty:
I have today been reading the paper on thorium oxide you sent to Owens - the results are certainly very surprising. The points that struck me most were the effect of the air currents, & the necessity of the plate which is to be made active being negatively electrified. It seems to me that it would be worth while trying blasts confined to various strata between the plates ... It seems to me that it might be urged that the effect was due to the gas close to the surface of AB being very intensely ionised & giving out a kind of radiation which produced a phosphor-escent effect on the plate...64
134
Another point which seemed to 'want settling' was what
part the negative charge on a plate played in its
activation. Thomson suggested that clarification might
be achieved by the interposition of a metal gauze,
positively charged. This would prevent a positively
charged 'emanation' drifting across to be deposited upon
the negative plate, but would not prevent irradiation of
this plate. Perhaps in response to these comments
Rutherford performed further experiments on the connection
between emanation, induced activity and electric charge
during the next few months and was able to publish some
answers. With regard to the emanations Thomson continued: It is remarkable that the emanation should stand bubbling through strong 1-4SO4 & yet this substance should destroy the activity of a plate. As the emanation moves so slowly it presumably is large. Hence it must get through the Al. foil by some chemical or quasi chemical process like the 11,01 in Russell's experiments. If this is the ease perhaps gilding or silvering the aluminium might make it opaque to the emanation. I see you tried some experiments & did not get a cloud by expansion - did you use Wilson's apparatus for getting very sudden expansions...
The use of this apparatus could indicate the size of the
particles of emanation which might be larger than ordinary
ions; Wilson had obtained a fog produced by 'something
given off from metals'. Thomson's closing question on
the subject was 'If the active plate is very highly
polished can you see any trace of an alteration of surface
under the microscope'.65
Rutherford withheld discussion of this last point
and of the nature of induced activity for his second
major paper on thorium; his first, dated September 1899,
dealt almost exclusively with the emanation. Now Owens
working with Rutherford had by mid 1899 obtained various
interesting experimental results with thorium rays. In
his paper on 'Thorium Radiation' he described his attack
on the early problem of fluctuating readings. By main-
taining a very still condition of the air he was able to
conduct an examination of the thorium rays by their
penetration of aluminium screens. His readings told him
that the radiations consisted of at least two different
135
penetration types, only the most absorbable kind being
homogeneous. He thought that such complexity fitted
with the theory of internal atomic vibrations, as has
been noted. Investigations of the air effects themselves
he treated separately in ingenious experimental style.
For example, removal of the air gave marked reductions
in conduction; on the other hand agitation of the air
within the chamber by means of fitted vanes produced small
increases in readings. His explanation of this was that
the radiation from within the compounds 'changes the nature
of their surfaces, forming in the neighbourhood a more
active material'66 which could be removed; it is easy to
read Rutherford's 'emanation' into this part of the study.
In a set of related experiments Owens found that
using sheets of ordinary writing paper as screens over
the substance gave 'very curious results'. A thin layer
of thorium oxide gave a regular absorption curve, as with
aluminium foils, indicating the homogeneity of a portion
of the rays. But with a thick layer of oxide, not only
did fifteen successive sheets of paper fail to diminish
the current below the 50% reduction produced by the single
first sheet, but 'a very considerable time was required
for the current to come to a steady value as successive
layers were added'. In his opinion:
The explanation may possibly be that the penetrating radiations from a thick layer of the oxide in passing through the paper causes it to give off a secondary radiation comparable in its ionising effects to the more absorbable kind that fails to get through.67
The account thus expressed in terms of direct radiations
only was evidently neither complete nor satisfactory.
Rutherford took over the research entirely as Owens
left for Cambridge in the summer of 1899 with many
questions unanswered. Some five months after this, having
reached a second point of publication, he wrote to his
fiancée: I sent off on Thursday another long paper for the press which is a very good one, even though I say so, and comprises 1000 new facts which have been undreamt of.68
136
It is of interest to outline these facts, and the related
experiments, and hypotheses of different levels. The
letter in July from Thomson, who communicated the public-
ations to the Philosophical Magazine, shows that Rutherford
was already using his new emanation theory to good effect.
By this means he was able coherently to explain several
lines of experiment, namely, fluctuations in readings due
to air currents, the differences between thin and thick
thorium oxide layers with regard to this effect, and the
time-dependent paper screen results. Thus Rutherford
dropped Owens' suggestion of secondary rays in connection
with paper screens together with his conclusion that the
radiation was complex. 'At first sight' he wrote:
it appears as if the thorium oxide gave out two types of radiation, one of which is readily absorbed by paper and the other to only a slight extent.69
However, the curious results:
receive a complete explanation if we suppose that, in addition to the ordinary radiation, a large number of radioactive particles are given out from the mass of the active substance. This 'emanation' can pass through considerable thicknesses of paper.7O
In a manner very similar to that of his experimental
researches at Cambridge some three years earlier on the
ionisation of gases, which extended continuously to the
period now under consideration, Rutherford effected con
siderable clarification and progress. By using a slow
current of air he removed the *conducting gas from its
source for examination in a separate vessel. His recent
studies told him that:
If the ionised gas had been produced from a uranium compound, the duration of the conductivity, for voltages such as were used, would only have been a fraction of a second.71
Yet his electrometer showed that the gas withdrawn from
thorium oxide remained conducting for up to ten minutes.
The radiating particles, or emanation, whose presence in
the gas was taken as the cause of the conductivity, passed
unchanged through cotton wool, water and acids, unlike
ordinary ionised gases.72 As with Russell's photographic
actions the emanation passed through foils of metal but not of mica. However, Rutherford stressed that hydrogen
137
peroxide vapour had 'purely a chemical' action on the
photographic plate and failed to ionise, and thus make
conducting, the gas carrying it. He stated that the
radiation from the emanation, not the emanation itself,
caused both electrical and photographic actions; though
it appears that source and radiation had not been experi-
mentally separated.
One of the most interesting points concerning this
radiation from the emanation was its exhibition of a
rapid, regular 'decay' with time. The conductivity of
the carrying gas, taken as the measure of the 'intensity
of the radiation emitted by the radioactive particles',73
declined exponentially falling to half the initial value
in about one minute. From the phrasing of the discussion,
and on consideration of his later researches74 it seems
that Rutherford assumed it to be the radiation from each
individual particle of emanation which 'diminishes in a
geometrical progression with the time'. Ionic and
emanation theory fitted certain experimental observations
well: the air surrounding an envelope of paper enclosing
a thick layer of thorium oxide was drawn continuously
into an attached vessel effectively fitted with electrodes
and electrometer. The readings, starting at zero, increased
gradually and after a few minutes reached a steady value
of electrical conduction. The electric current decay curve
obtained after stopping the airflow matched the asymptotic
growth curve perfectly. Rutherford was able to account for
the latter as a balance between the increasing current
caused by a constant supply of new radiating centres which
had diffused through the paper from the thorium, and the
decreasing current due to the geometric decay of intensity.
Rutherford boldly expressed the rate of decay in the form
dn/dt = -A.n , where n was the number of ions. The
observed growth curve was indeed of the theoretically
deduced form i/I = 1 - e-A.t , where I is the maximum
current, attained at the steady state.75 This is the first
example of the growth and decay curves later to be assoc-
iated by Rutherford and Soddy with all radioactive sub-
stances. Here also was the first recognition of a decrease
138
in the radiation from what, to Rutherford, was a definite
radioactive substance. This perhaps added strength to his
year-old suggestion regarding the energy of uranium rays76
that these should eventually die away; a view already in
conflict with the French scientists' acceptance of the
fundamental constancy of this atomic property.
One may ask what significance can be attached to his
repeated statement77 'that the curve of rise of the current
is similar in form to the rise of an electric current in a
circuit of constant inductance', which seems also to imply
a similarity in the current decay curves. Was he attempting
to say something new about the emanation, about electricity,
or making a merely algebraic comparison? The first of these
possibilities is evidently in some way true. As for the
second, it is remarkable that the newest view of electrical
conduction in metals visualised its mechanism as a kind of
diffusion process somewhat analogous to that of ionic
conduction in gases. About six months after Rutherford
wrote his paper on thorium emanation J.J.Thomson divulged
'Some speculations as to the part played by corpuscles in
physical phenomena' to the 'wider circle' of the readers
of Nature.78 He pointed out that the recent demonstrations
by Giesel, Curie and Becquerel of the magnetic deflection
and electric charge of the rays from radium demonstrated
the presence of corpuscles in this substance. Thomson'.s
major point was an explanation of the electrical conduct-
ivity of metals in terms of a gas-like diffusion along
the wire of subatomic negatively charged masses or
corpuscles, temporarily dissociated from the fixed
'molecules'. He considered that this had consequences
for the relationship between electrical and thermal
phenomena. The older theory of jumping tubes of force
was not explicitly mentioned here. Now Thomson in his
earlier letter to Rutherford of July 1899 which contained
the comments on the emanation quoted above, did not mention
the theory of conduction in metals but only the work on
conductivity in gases:
I am inclined to think that at low pressures negative electricity is always carried by the small corpuscles however the electrification
139
may be produced - while the positive charge remains on the big atom - this idea leads to very interesting views as to the structure of the atom.79
In the published note Thomson recalled his first demon-
stration of the existence of corpuscles in cathode rays
in 1897 and added that 'Ever since then I have indulged
in speculations' concerning their presence in ordinary matter.80 But it is not clear whether Rutherford knew
of the corpuscular or electron-gas theory of metallic
conduction nor whether his own researches of 1899 had
any bearing upon it. If one seeks an influence in the
opposite direction the question arises whether Rutherford's
identification of formulae involved the notion that each
radiating particle of emanation behaved like a miniature
or atomic version of the radiating electric circuits
which he had studied during 1894-6. He was still concerned
with the subject and it was very much in his mind during
the month in which he wrote up the paper on thorium eman-
ation, as shown by his letter to Mary Newton:
I am giving a course of postgraduate lectures this year on Electrical Waves and Oscillations which will give me a good deal of trouble to arrange. This is the first thing of this kind ever done here and rather surprised them when I suggested it.81
Although his explicit identification of the formulae may
be no more than a mathematical guide, our discussion
indicates connections between radioactivity, and former
and contemporary electrical studies. The same points
apply to Rutherford's publication on the excited activity
with its much slower decay; this is shortly to be discussed
along with the accounts of possible mechanisms which he
fortunately detailed. It may be noted that Pierre Curie
was soon to develop explanations of the phenomena on the
basis of a different algebraic comparison, firm to the
point of analogy, between the observed decline of radio-
activity and the fall of temperature due to loss of heat
energy from a cooling body.
Two questions posed by these studies of the emanation
concerned its origin and its nature; the need to answer
these served as a stimulus to experimental research.
140
The emanation seemed to be produced spontaneously and at
constant rates which were different for differing thorium
compounds. Rutherford's conversion of the nitrate to the
oxide by gentle heating produced a considerable increase
in the current due to the emanation, and the direct
radiation increased proportionately. On the other hand,
maintaining the temperature at a white heat caused a
steady decline in the discharge due to the emanation
down to 1/20 of the initial rate for the oxide, apparently
without so diminishing its direct rays. Rutherford provided
no explanation at this time but kept the problem in mind.
What then was the nature of the emanation, of the
radiating particles comprising it? As for the size of these
particles, the fact that they passed unaffected through
cotton wool and metals was against the possibility that
they were a fine dust, as were also the results of water
vapour condensation experiments; Thomson's suggestion
regarding Wilson's more powerful method was mentioned by
Rutherford82 as a future possibility, but he may not have
tried it.83 The results indicated that the particles
constituted 'a vapour given off from thorium compounds'.
Could it be the vapour of thorium metal? There was 'reason
to believe that all metals and substances give off vapour
to some degree'.84 As Rutherford knew, this might be the
vapour of the metal or substance itself, or of hydrogen
peroxide. We have noted the dismissal of hydrogen peroxide
vapour85 on the grounds of its direct chemical photographic
action and its lack of conductivity; other differences can
also be seen, such as the need for a clean unoxidised
surface to obtain the Russell effect with metals which
contrasted with the production of an emanation by the
oxide and compounds of thorium. That the emanation might
be the vapour of thorium itself was a possibility which
he thought did agree with its declining radioactivity.
And he again pointed hopefully to future experiments
which were in this case actually completed and published
more than a year later, though not with thorium; these
were measurements of the rate of interdiffusion of the
emanation into other gases to determine its density and
141
molecular weight.86 But even without such evidence he felt
able to make the cryptic but significant comment that 'The
emanation from thorium compounds ... has properties which
the thorium itself does not possess'.87 Rutherford's know-
ledge of such properties was a part of his discovery that
by association with the emanation any surface whatever
could be made to emit radioactive rays, different from
those of uranium or thorium in being more penetrating.
The surface activity lasted for several days; it could
be concentrated upon a negatively charged body. Now the
particles of emanation were themselves electrically neutral.
Their discharging effect was entirely uninfluenced by
application of the electrostatic field which, as Rutherford
had successfully demonstrated in 1896,88 would create a
current and thus destroy the ionic conductivity of a gas.
We can see that this should dispose of the hypothesis
that the particles of emanation were themselves simply
clusters of molecules of the surrounding gas about ions,
though Thomson later adopted the idea for a time. Hence
Rutherford, whose experiments were still in progress,
assumed in September that it was 'the positive ion produced
in a gas by the emanation' which possessed 'the power of
producing radioactivity in all substances on which it
falls' .89 But he had progressed beyond this view by the time
of completion of his next publication on the 'Radioactivity
Produced in Substances by the Action of Thorium Compounds,90
in November 1899. Rutherford appears to have retained the
opinion that the emanation was not thorium vapour without
providing further evidence. It is tempting to interpret
his experimental results in terms of the later supposition
that this new 'excited' radioactivity was itself due to a
third material substance. Even if he entertained the idea
at this early date Rutherford did not mention it in his
publications, preferring other hypotheses; yet the evidence
seems suggestive. Naturally adopting the measurable
characteristics of its emitted radiation as a means of
identification, Rutherford noted that the rays from the
excited activity were of a longer duration than those of
142
the emanation and more penetrating than thorium or uranium
rays. Another distinction was the chemical one: to repeat
Thomson's earlier comment of July, 'it is remarkable that
the emanation should stand bubbling through strong H2SO4
& yet this substance should destroy the activity of a
plate'. But Rutherford in his sections on chemical and
mechanical actions on the radioactive surface91 now stated
that the induced or excited activity was not in fact so
destroyed but had been simply dissolved from the plate
by this acid, or by hydrochloric acid, in which it was
afterwards to be found; other reagents such as nitric acid
or caustic soda had far less effect. He reported without
comment the interesting observations that a microscopic
examination (suggested by Thomson) revealed no surface
changes although the intense activity could be removed by
scraping; and that raising the temperature to white heat
had little effect.92 These chemical and mechanical
approaches were to enjoy an early and successful future
development with the aid of Frederick Soddy, a trained
chemist; they also bore a resemblance to Rutherford's
past researches on surface magnetism93 performed five years
earlier. And his demonstrations that the intensity of the
induced activity was roughly proportional to the amount of
emanation, by passing the emanation along a tube containing
a series of negative electrodes, were very similar in
techniaue to earlier studies of the duration of the conduct-
ivity of ordinary ionised gases.94 The experimental and
theoretical continuity is also illustrated by other experi-
ments, by the strong element of explanations in terms of
the mechanical interplay of ions and other particles, and
by the lasting links with the researchers of the Cavendish
Laboratory. Rutherford summarised his weighty paper by considering
three possible explanatory mechanisms of the phenomena of
induced radioactivity, one of which he adopted. The hypo-
thesis that the excited radiation was a kind of phosphores-
cence produced in direct response to thorium radiation
could explain neither the production of activity outside
the incidence of the radiation nor the concentration upon
143
a negative electrode; he therefore dismissed it. Too
abruptly perhaps, for Rutherford made no mention of the
different phosphorescence hypothesis, with possible
experimental tests, which Thomson had privately suggested
in July. The latter supposed that gas molecules close to the thorium surface might become so intensely ionised that
they emit a type of radiation capable of producing a
phosphorescent effect on the plate; Thomson had not forgotten
this by 1903, when he published a similar explanation of
induced radioactivity as one possibility.95 It is clear that the identity of the induced radiation whatever the
material of the excited surface militates against this.
A second idea was that the positive ions produced in
the surrounding gas by the rays from the emanation particles
could deposit upon any surface; certainly there was no
problem here in accounting for concentration by an electric
charge, nor an identity of deposit. This had probably been
Rutherford's favoured view in September; now he wrote that
the hypothesis 'at first sight seems to explain many of
the results'.96 In consequence of the intense rays
(presumably of X-ray type) close to a radiating particle
of emanation, ions may not only be produced but 'the charges
on the ions set in violent vibration'. The observed surface
effects would occur as the ions 'gradually dissipate the
energy of their vibration by radiation into space'. What
became of the negative ions was not stated. However,
observations at low pressures, where even a large negative
charge in fact failed to concentrate the activity, were
not explained by this theory. And testifying against it
was the fact that an increase in the distance between the
emanating thorium layer and the plate to be activated from
3 mm. to 3 cm., which gave the expected higher current
attributable to the creation of more positive ions, yielded
no increase in the plate's activity.
Rutherford made his decision: 'The theory that the
radioactivity is due to a deposition of radioactive particles
from the thorium compounds affords a general explanation
of all the results'.97 The emanation itself was thus dep-
osited. He overcame the difficulty of explaining how
144 this neutral substance acquired the charge necessary for
movement to a negative electrode by supposing that positive
ions, generally present in excess in gases due to their
lower velocity, diffuse to the surface of the emanation
particles. At low pressures a lack of ions, indicated by
the observed small currents, would allow active neutral
particles to diffuse to the sides of the vessel unaffected
by the negative electrode. The implication of these
statements seems to be that excited activity consisted
of deposited particles of the emanation possibly sometimes
attached to the positive ions of various gases. But did
larger particles such as dust or smoke which Rutherford
knew caused rapid ionic recombination,98 acquire the
positive charge that they apparently should according to
this hypothesis? And why did the rate of decay of radiation
become some 660 times slower when the gaseous particles
were deposited, though obeying the same law; why were
there marked differences in chemical behaviour, for example
with sulphuric acid; did these point to the existence of
a third radioactive substance? Rutherford did not say so.
And he avoided any description of the nature of the
particles of emanation themselves, and of the mechanism
of their production. These were some questions which
Rutherford's discoveries of '1000 new facts' appear to
have posed. Research students and collaborators were to
aid him in answering them, as new information arriving
from Europe created still more problems.
In a late footnote to his paper on induced radio-
activity, Rutherford remarked that he had .received the
note of 6th November in which the Curies had announced
their discovery of 'radioactivite provoquee par les
rayons de Becquerel',99 an effect produced by the new
substances whose activities had now reached a level
50,000 times greater than uranium. Rutherford stated
that Curie had, without electrical studies, attributed
the results to a kind of phosphorescence excited by the
radiation.I00 He pointed out that 'in the case of thorium
the author has shown that such a theory is inadmissible'
so that further comparisons were required. The magnitude
145
of the activity induced by radium of several hundred
uranium units, as reported by the Curies, appears to have
been similar to Rutherford's estimate for the maximum
electrically concentrated activity produced by thorium.101
The latter's suggested general comparison between radium
and thorium was taken up directly by E.Dorn and by his
student F.Henning, in Halle.
In December 1899 Thomson wrote to Rutherford that
he had sent on this second paper on thorium to the
Philosophical Magazine. He also told him of Giesel's
magnetic deflection of polonium rays, whose implications
we have considered. Thomson found Rutherford's results
'exceedingly interesting' but saw fit to suggest a
different mechanism for the acquisition of the emanation's
charge, which in effect reversed the positions of emanation
and ions: the idea that I got on reading the experiments was that the radio-activity was due to thorium vapour or emanation which was carried by the + ions - I mean that the emanation in the field tended rather to condense round the positive ions than the negative ones, as we might expect an electro-positive substance to do.102
Evidently Thomson was not satisfied that the emanation
was not thorium vapour - a metal should tend to attach
itself to positive ions. And others' opinions of
Rutherford's results were not quite so straightforward
as has been made out. John Zeleny had been at the
Cavendish with Rutherford under Thomson, who had written
to Rutherford in 1898 when both students had left the
Cavendish, 'I hardly think at present that we have any
in the new lot as good as Zeleny & Wilson the amount of
glass they break at present is appalling'.103 Zeleny
had been first to announce, in that year, that the negative
ion in a gas generally travelled with a slightly higher
velocity than the positive.104 Indeed, he had made an
observation, which seems unfavourable to Rutherford's
emanation-charge theory of 1899, that a metallic body
suspended in a current of ionised air generally acquired
a negative charge, the air being left positive.105 Zeleny
attributed this to the differing ionic velocities. He
146
explained that such a body could acquire the positive
charge sometimes observed only if another metal were
immersed in the same quasi-electrolytic medium. This
fits with Thomson's view. An implication of Rutherford's
hypothesis seems to be that the emanation behaved not as
suspended particles but as a part of the gas. However,
it was Thomson who was soon to argue in this way, on the
basis of fresh evidence. In March 1900, soon after reading
Rutherford's papers 'with very great interest', Zeleny
wrote 'I am about ready to believe that most anything is
possible. The facts you present are certainly strange'
but 'more light on the nature of those things would be of
still greater value'. He said he would be 'interested in
your work on the energy for producing an ion. I see that
you are in for getting ahead of J.J.'.106 Rutherford had begun experiments to this end towards the close of 1899
partly if not wholly in order to clarify the question of
the energy of radioactivity. His associated speculations
are of very great interest to us, as are contemporary
discussions by others. However, Zeleny's latter suggestion
turned out to be wide of the mark with regard to Rutherford's
attempt to quantify the energy required for producing an
ion, as the following account will show.
147
3. The source of radioactivity (1900) Attempts to quantify the energy emission from radio-
active substances, together with a more advanced under-
standing of the radiations, led to interesting speculations
and anticipations. These are worthy of examination for
the improved perspective of radioactivity which they
provide. To that end we will consider formerly neglected
discussions set out by researchers in the field as their
experimental studies developed from the end of 1899
through 1900. Generally during this period theories were
as easy to knock down as to set up; Rutherford indeed
found this to be the case with the hypotheses which he
preferred. Nevertheless, even at this early stage the
view that atomic change was the source of radioactivity
began to appear in sharper focus.
The earliest explanations of radioactivity were
perforce qualitative; they likened the phenomenon either
to fluorescence or to a long-term phosphorescence. The
respective problems of the origin of the initiating
radiation or of the nature of the required ancient store
of energy, among other difficulties, had not been solved.
The subsequent suggestion, made most clearly by Thomson
in 1898, that a subatomic rearrangement might be involved,
had implications not to be fully realised until further
chemical researches, to be discussed, had come to fruition.
However, we may recall that the physicists Elster and
Geitel had early in 1899 already expressed their willingness
to sacrifice, in theory, the chemical atom to provide the
energy of radioactivity. Their experiments demonstrated
the inadequacy of Crookes' and Marie Curie's hypotheses,
each of which required external, though differing, energy
provision. These results confirmed the atomic nature of
the phenomenon and induced their proposal that the radiated
energy might come from a transmutation of the chemical
elements.107
Such a suggestion did not escape strong criticism
put forward with the support of apparently positive
experimental evidence. Both phosphorescence and all
chemical reactions were known to be highly dependent
148
upon temperature; E.Wiedemann and G.C.Schmidt108 had
indeed supposed that these two types of phenomenon were
closely related. In turn re-linking these processes with
radioactivity, 0.Behrendsen published his 'Beitrgge zur
Kenntniss der Becquerelstrahlen',109 which exhibit both
an understanding of and a direct challenge to Elster and
Geitel's solution to the 'Energiefrage'.110 The mode of
attack on his contemporaries' views was a study of the
effect of temperature upon radioactivity. Behrendsen
used a gold-leaf electroscope to determine the intensity
of the radiations emitted from specimens placed within
the electroscope vessel and an apparatus by which the
entire electroscope could be cooled. He satisfied himself
that pitchblende and its active sublimate each gave
radiation which increased steadily with temperature from
-50 to +130 degrees C. and that Moissan's uranium metal
exhibited a decreased intensity only on cooling. However
he considered that convection of the ionised air on
warming might have influenced the result with uranium.111
Raising the temperature of pitchblende to red heat
markedly decreased the intensity. This he attributed to
a chemical decomposition, which illustrates his general
assumption, or conclusion, that radioactivity was the
manifestation of ordinary interactions between atoms and
molecules. The slow conversion of a complex compound
such as pitchblende, which contains uranium, the Curie
elements and acids, into a more stable one, should indeed
be influenced by temperature variations; radioactivity
was to be compared with thermoluminescence. The Curies
and Elster and Geitel, he wrote, are of the opinion that
Becquerel rays were in fact not the result of any chemical
change so that the emission was an 'Atomeigenschaft' of
the radiating element. They had said that the atom of a radioactive element might itself be constructed like a
kind of molecule, able to emit rays while being trans-
formed into a more stable entity.112
Gegen diese Ansicht scheint mir vor allem zu sprechen, dass die Annahme eines instabil gebauten 'Atomes' nicht mit dem Atombegriff als solchem sein diarfte.113
149
Retaining his conception of the atom, and assuming the
energy to be stored within the material, Behrendsen
supposed it possible 'dass die Atome der radioactiven
elemente die Fghigkeit besgssen, miteindander und auch
mit fremden Atomen zu instabil gebauten Molecalen
zusammen zu treten' like the allotropes of sulphur or
selenium. This interpretation fitted with his thermal
results. However, in his argument114 that the salts of
uranium gave a greater total radiation than the metal
itself, which should dismiss the Ur atom as the source,
he included the visible. Now Becquerel's original discovery
had involved the creation of a clear distinction between
uranium rays and ordinary phosphorescence. Behrendsen's
paper thus may only have served to spread wider the
suggestions of those whom he criticised. His experiments
and conclusions were put in further doubt on several counts
as scattered studies of the effect of temperature continued.
F.Himstedt's report one year later 'Uber Versuche
mit Becquerel- und mit ROntgenstrahlen'
l 5 noted the view
which attributed the energy of radium to a slowly proceeding
chemical reaction. An investigation of the influence of
cooling upon the intensity of the rays was therefore of
considerable interest. His results however gave no support
to such a theory. For whilst luminous paint was completely
extinguished at the temperature of liquid air radium
radiation, as measured with an electrometer, showed but
a small diminution which itself might be attributed to
additional absorption of the rays by the liquid air.
Himstedt's conclusions were more in accord with the
conclusions of the earliest students of radioactivity
than with Behrendsen's. Others had considered this approach to radioactivity
as worthy of investigation. Before radium became
important, Rutherford had measured the activity of
uranium at 200 degrees C. but found little difference
in the rate of discharge. His opinion was that 'The
results of such experiments are very difficult to interpret,
as the variation of ionization with temperature is not
known'.116 Marie Curie had found that the activity of
150
uranium was always unchanged on returning to room
temperature after heating or cooling.117 G.G.Stokes, who entertained what can be called a 'sub-phosphorescence'
explanation of radioactivity, had corresponded with
Becquerel in August 1899 concerning uranium rays.
Stokes thought it 'very probable that the efficiency of
a substance (suppose metallic uranium) which emits them
would depend very materially upon its temperature'.118
Becquerel replied119 with a description of experiments
using uranium which he had performed two years earlier
but had not published. These involved arrangements
similar to those described by Behrendsen. Becquerel on
the contrary cautiously remarked that the photographic
method of detection was itself sensitive to changes in
temperature, so that further study was required. He
similarly noted that an electrometric method gave only a
small difference between +100 and -20 degrees, which
might be attributable to air currents of convection, and
that smaller changes in temperature appeared to have no
effect. These attempts to find a thermal influence upon
the radiations from active substances were continued by various scientists.120 The generally negative results were more than once cited positively, as theoretical
debates intensified during the period 1899 to 1903.
But these five years saw at their beginning the
discovery of the new phenomena of the radioactive
vapour-like 'emanations' released from thorium, then
radium, then actinium, and the development of an under-
standing of these. Rutherford in his earliest publications
on thorium emanation, issued in 1900, noted that temperature
here had a definite effect: he considered that heating the
thorium oxide source greatly reduced the rate of production
of emanation.121 Behrendsen however insisted on his earlier
conclusions, now with regard to 'Das Verhalten des
"Radiums" bei tiefer Temperatur' .122 His experimental
diminution of the electrometric intensity of radium
radiation to half value at the temperature of liquid air
led him to state that this behaviour was precisely similar
151
to that of visibly phosphorescent bodies. He thought
that the spontaneous decrease in the radiation from
Giesel's polonium confirmed the comparison. Yet con-
temporaries probably saw his case with radium to be
weakened by new discoveries. The complex nature of the
composition of the rays was becoming clearer during 1900;
temperature experiments ought therefore to be more dis-
criminating in this respect. And a preliminary point in
another direction is Elster and Geitel's note of January
1900 that radium released a volatile constituent on
heating;123 this may have been seen merely as comparable
with the sublimation of polonium from bismuth. Their
paper partly anticipates Dorn's description later that
year of a gaseous radioactive emanation associated with.
radium.'24 Future attempts to clarify the source of
radioactivity by seeking an effect of temperature were
compelled to take the emanations into account.
It is a sign of the complexity of changes in the
interpretation of radioactitrity occurring during the year
1900, that developments of Becquerel's views, which we now
consider, were produced largely by factors other than those
just described. Neither his experiments giving negative
or inconclusive effects of temperature change, nor the
discovery of the emanations, appear to have been immediate
influences in his definite shift from the original phos-
phorescence analogy of 1896-9. Instead, experiments on
the radiations themselves which indicated that they
consisted not only of ether vibrations but also of an
actual emission of material particles, seem to have been
most significant. Thus in March 1899, after he had
accepted that uranium rays were not after all like light,
Becquerel in his 'Note sur quelques proprietes du
rayonnement de l'uranium et des corps radio-actifs'125
considered again the energy of radioactivity. If uranium
did not lose energy in producing its rays then this metal
might be in a special state like that of the iron in a
magnet maintaining a field around it through which it
could create the observed effects by the transformation
of some external energy source. However, he accepted the
152
Curies' view that photographic and phosphorescent actions
of the new highly active substances did in fact constitute
a spontaneous release of energy 'dont on ne voit pas la
source ailleurs que dans la substance radio-active'.126
Since this was of small magnitude:
it ne serait pas contraire a ce que nous savons sur la phosphorescence, de supposer que ces substances ont une reserve d'energie relativement consid6rable qu'elles peuvent 6mettre, par rayonnement, pendant des annees, sans affaiblissement sensible; toutefois i1 n'a pas etc possible de provoquer par des influences physiques aucune variation appreciable dans l'intensite de cette emission.127
The communication by Becquerel of his unpublished temperature
experiments and earlier papers to Stokes was sufficient to
make the latter drop his phosphorescence-radioactivity link
by September 1899.128 Yet Becquerel himself apparently
retained the idea till November. This is indicated by
his application of the terms phosphorescence and fluores-
cence respectively to the new temporary induced radio-
activity and to secondary rays.129 But the analogy was
not to be stretched beyond this point.
Becquerel's new corpuscular view of the rays from
radium led him to a calculation of the energy and the
matter carried off in this way. His respective estimates,
announced in March 1900, of 1 mg. in 109 years and a few
calories per cm.2 per year from a radium sample, were
sufficiently small to allow the continued belief that
the radiated energy 'peut titre empruntee a la matiere elle meme 130' without measurable loss of weight. In the con-
cluding comments of his paper read at the International
Congress in Paris in August Becquerel cited the same
estimates and gave something of an expansion of his views.
Since the loss of mass involved was immeasurably small
n'y avait aucune contradiction entre la spontangite
du rayonnement sans cause apparente, et le principe de la
conservation de 11 6nergie'.131 The apparent assumption of
an equivalence of mass and energy seems not to alleviate
the contradiction with Carnot's principle, but he made
no mention of this. His final word here was that 'Le
153
phenomene d'emission materielle pourrait etre du meme
ordre de grandeur que l'evaporation de eertains matieres
odorantes'.132 Becquerel's firmest expression, vague
though it was, of this approach appeared in a review paper
on 'The radio-activity of matter' published in Nature some
six months later, in February 1901.133 A 'material emission'
of the order of 'certain scented substances' had now become
'the first cause of the observed phenomena'. Becquerel
mentioned aspects of Rutherford's 'very penetrating
"emanation"' from thorium without making it clear that
the emanation behaved like a vapour: he had not yet taken
the opportunity of fitting this with his 'material emission'
hypothesis. Yet towards the end of that year, by con-
sidering both of these in combination with the 'new
horizons'134 opened up by developing studies on induced
radioactivity, Becquerel arrived at his own important
unifying theory of atomic disintegration.135
Such developments were followed by some scientists,
but not by all. F.T.Trouton writing to Nature from the
Physical Laboratory, Trinity College Dublin, in March 1900
put forward a 'Suggested Source of the Energy of the
"Becquerel Rays"'136 which seems very similar to the magnetic
analogy mentioned and rejected by Becquerel himself one
year earlier. Trouton suggested that because of the
difficulties arising from the supposition of a continuous
release of energy from the active material 'the possibility
should be kept in view of the real source of the energy
being found in the things themselves in which these effects
are manifested'. Including in the expression what others
called 'rays' he wrote that 'the emanating influence would
be looked upon rather in the light of lines of force than
as a wave propagation'. Ionisation and other effects
'would then be viewed as due to what might be called a
Becquerel field of force'. The Curies' observation that
a phosphorescent screen became 'exhausted' after a time,
but could be rejuvenated by exposure to light, he inter-
preted as an indication that some or all of the energy
originated in the screen itself. According to his magnetic
analogy 'forces should exist between the acted-on substance
154
and the source of the "Becquerel Rays"'. One cannnot say
that these ideas were influential, but they serve as an
epitome of one undercurrent of conjecture on the subject
during and beyond137 the period under consideration. It
is interesting to note that W.Crookes earlier138 and
P.Curie with A.Laborde later, each sought experimentally,
and unsuccessfully, just such a force; they, however,
were thinking in terms of the pressure of radiation truly
emitted from the radioactive source. The French scientists
came to a remarkable and quite different discovery139 in
the attempt. Like all active researchers in the field, the Curies
seem always to have regarded 'radioactivity' according to
its name, which they had coined. But it is perhaps an
indication of the appeal of the interpretation described
above that the Curies, shortly after their discovery of
induced radioactivity, still found it necessary to stress
the point that energy was being released. Such was the
conclusion of their note of 20th November 1899 on the
'Effete chimiquesproduits par lee rayons de Becquerel'.140
This may be the research which prompted Trouton's paper
for the Curies here described their observations with the
fluorescent barium platinocyanide screen. The colouration
of glass and particularly the newly observed (by the spectroscopist E.Demargay) transformation of oxygen into
ozone by radium was 'une preuve que ce rayonnement represente
un degagement continu d'6nergie'. Trouton's comment con-
cerning radioactivity that 'we have no conclusive evidence
that the effects are those of waves',141 which may be
associated with the recent demise of Becquerel's earlier
description of the rays, remained true for at least a
decade. However, the Curies as well as others accepted
the view that both particles and ether vibrations were
involved. Following the successful deflection of a portion of
the radium rays, their detection of the spontaneous
acquisition of a positive electrostatic charge by an
insulated specimen of radium told the Curies that this
element continually lost negative electricity.142 'Or,
155
jusqu'ici' they wrote 'on n'a jamais reconnu l'existence
de charges electriques non liees A la matiere ponderable',
hence radium must be the seat of a constant emission of
negatively charged material particles. They cited the
work of J.Perrin who had detected the transport of a
similar charge by the cathode rays but the Curies made
no reference to J.J.Thomson's related conclusions that
these particles were smaller than atoms. Indeed, their
estimate of the continuous loss of mass which should
result from the emission was based upon an assumed e/m
ratio 'le meme que dans l'electrolyse'; thus in March 1900
they took the particles to be atomic in size. A few weeks
after their note was read Becquerel demonstrated an almost
total similarity between the deviable radium rays and
Thomson's corpuscular cathode rays.143 Becquerel's studies
may have been a cause of their acceptance, as far as it
went, of the subatomic corpuscular view of radium rays.
For in Marie Curie's review paper on 'Les nouvelles
substances radioactives' read in June 1900,144 and in the
Curies' joint paper presented to the International Congress
in Paris two months later,145 discussions on the origin of
the energy of radioactivity centred on two possibilities
neither of which involved the emission of atomic particles.
At the Congress they squarely put the question 'Quelle est
la source de l'energie des rayons de Becquerel? Faut-il
la chercher dans les corps radioactifs eux-mgmes ou bien
a l'extorieur?'146 and answered it by suggesting that one
group of radium rays might be secondary to the other.
Support for this view may have been derived from current
researches by P.Curie and G.Sagnac, and E.Dorn which
showed that secondary X-rays, easily absorbed, were them-
selves cathode rays; 147 this implied an apparent symmetry
between the production of X-rays and cathode rays. Despite
the refutation by Elster and Geitel148 the Curies extended
Marie's earlier speculation, and suggested that the primary
Becquerel rays, whichever these were, might themselves be
secondary to some unknown exterior radiation, again with
the rider that this would be contrary to Carnot's principle.
156
Yet their ideas had developed, and discussions were
couched in slightly different terms from those of the
previous years. For the Curies now debated for the first
time an alternative hypothesis which was based upon
J.J.Thomson's view of cathode rays as subatomic particles;
it involved the discovery, to which they had themselves
contributed in respect of the electric charge, that radium
emitted such particles. Although Marie Curie, in her June
reviewl49 made the disclaimer that the origin of the
radiation from radioactive bodies was no better understood
than when uranium rays alone were known, yet she evidently
felt able to express one qualitative hypothesis fully and
clearly; from this one may surmise, despite later events,
that she perhaps even considered it favourably. Thomson,
she explained, had shown that if one took the cathode rays
or 'matiere radiante' of Crookes to be electrically
charged material particles then 'ces particules transportent
a poids egal 1000 fois plus d'electricit6' than hydrogen
in electrolysis. Thomson had concluded that 'Ce ne seraient
done meme plus les atomes libres de la chimie, mais des
sous-atomes bien plus petits encore, et anim6s de vitesses
prodigieuses'.150 Radium itself therefore seemed to behave
like a spontaneously excited cathode. Thus Marie Curie
arrived at her first speculation on the disintegration of
matter. Although the loss of mass would be too small to
detect, 'La matiere radioactive serait done de la matiere
oil regne un etat de mouvement interieur violent, de la
matiere en train de se disloquer'.151
Setting out the chemical implications more clearly than
had J.J.Thomson, for example in his 'Speculations as to
the part played by corpuscles in physical phenomena'
published the previous month,152 and with the advantage
of a year and a half's new evidence over the conjectures
of Elster and Geitel on radioactivity and atomic change,
she wrote: La theorie materialiste de la radioactivite est tree seduisante. Elle explique bien les ph6nomenes de la radioactivity. Cependant, en adoptant cette theorie, it faut nous rosoudre a admettre que la matiere radioactive n'est pas a un 6tat chimique ordinaire; les atomes n'y sont pas constitue's a
153
157
11 6tat stable, puisque des particules plus petites que l'atome sont rayonn6es. L'atome, indivisible au point de vue chimique, est divisible ici, et les sous-atomes sont en mouvement. La matiere radioactive eprouve done une transformation chimique qui est la source de l'energie rayonnee; mail ce n'est point une transformation chimique ordinaire, car lee transformations chimiques ordinaires laissent l'atome invariable. Dans la matiere radioactive, s'il y a quelque chose qui se modifie, c'est forcoment l'atome, puisque c'est a l'atome qu'est attachee la radioactivite.154
Here indeed can one see the elements of a theory involving
both atomic disintegration and transformation. It has not
generally been appreciated that Marie Curie expressed such
a view at this time. The fact that she did so illustrates
the early trend; but from this the Curies turned away.
The ideas proposed by Marie Curie were readily taken up,
or independently conceived, by others in the following year
or two;155 Rutherford's experimental researches developed
along a path different from the Curies', but he may have
entertained such thoughts. However, by this time, June
1900, Rutherford had been concerned to point out that the
emission of corpuscles or cathode rays was not a property
of all radioactive substances. Mme.Curie knew that her
polonium, though highly active, gave none of the deviable
rays;156 she did not mention this in her review paper;
it was not to be her only theoretical problem with
polonium.157
In his joint paper with the Demonstrator R.K.McClung
on the 'Energy of Rdntgen and Becquerel Rays, and the
Energy required to produce an Ion in Gases'158 Rutherford
remarked that Giesel's polonium emitted deflectable rays
but Curie's did not,159 that Becquerel had 'found no trace
of magnetic action in uranium radiation',160 and that
magnetic experiments at McGill had deflected neither
uranium nor thorium rays. The point was clearly made:
This emission of rays similar in character to cathode rays of low velocity is very remarkable, but does not seem to be a necessary accompaniment of a radio-active substance ... The rays which are deflected by a magnet seem to be present or absent according to the mode of preparation of the substance, and depend possibly on the age of the specimens.161
158
This illustrates a difference in emphasis from that of
Mme.Curie. Rutherford tells us that the aims of the
study were to determine experimentally the energy required
to produce an ion in a gas, 'and to deduce from it the
energy of the radiations emitted per second by uranium,
thorium, and other radio-active substances'.162 An inter-
mediate, quantitative study of X-rays was the basis of his
method. By January 1900 he had obtained a measure of the
small heating effect of X-rays playing upon a specially
absorptive platinum bolometer to an estimated accuracy of
2%, as he helped McClung to master the complex techniques.163
The assumptions were that X-rays manifested their energy
as a heating effect when absorbed by metals,164 but as an
ionisation-conductivity effect when absorbed in a gas.165
Comparing these, and knowing J.J.Thomson's recent value
of the charge on an ion, Rutherford calculated the energy
required to produce an ion in the gas; conductivity
measurements provided the required estimates of the
energy released by radioactive substances. Referring to
his own and others' work on ionisation Rutherford argued
that 'the same energy is required to produce an ion
whatever the gas:166 This conclusion played a part in
his speculations, the more fascinating for their confusions,
upon the structure of the matter from which ions were
produced; these speculations in turn had implications
for the source of radioactivity. He implicitly adopted the purely electrical view of
atomic and molecular bonding:167 from the experimental
value of 3.8 x 10-10 erg for the minimum energy required
to 'produce a positive and a negative ion from a neutral molecule ... against the forces of electrical attraction',
he calculated the distance between ions before separation
as r = 1.1 x 10-9 cm. Rutherford took this to be a
significant value supporting Thomson's ionisation theory.
For 'The average diameter of an atom, calculated from
various methods, is about 3 x 10-8 centim.' which was
'very much greater' than the above ionic distance 'in
a molecule'.168 Now Kelvin in one of his later lectures
on 'The Size of Atoms, had in 1883 apologised:
159
I speak somewhat vaguely, and I do so, not inadvertently, when I speak of atoms and molecules. I must ask the chemists to forgive me if I even abuse the words and apply a misnomer occasionally.169
One reason for this seems to be that none of the phenomena
of optical dispersion, contact electricity, capillarity,
gas viscosity and diffusion used for the estimates specifi-
cally involved the atom of chemical combination. Rutherford
made no such apologies for his variable usage of the terms
atom and molecule; sometimes it seems quite clear. Part
of his aim may indeed be construed as the making of a case
for Thomson's corpuscular theory of matter against 'the
atomic theory, as ordinarily understood',170 radioactivity
being the test. We know of Rutherford's open attack to
this end upon 'the chemists' in 1901.171 He considered in
1900 that his own results on radioactive energy, and those
on the emission of 'a kind of cathode rays' from active
substances, indicated 'that the present views of molecular
actions require alteration or extension'.172 However, apart
from the employment of such imprecise expressions his
explanations were acknowledged as being unable to cover
the experimental results. According to Rutherford, Thomson
considered 'that an atom is not simple, but composed of
a large number of positively and negatively charged
electrons';173 we note that the latter had not publicly
stated this exact view. The problem of the positive charge
was a difficult one for it appeared that only negative
electrons or corpuscles were detectable experimentally;
Thomson's usual supposition from the first was that the
space which the corpuscles occupied behaved somehow as
if it possessed a positive charge. Nevertheless, for the
purpose of calculating theoretically the energy within an
atom which might be available for the manifestations of
radioactivity both Thomson and Rutherford appear to have
assumed a structure consisting of pairs of oppositely
charged electrons.174
Rutherford and McClung measured the energy emitted
from uranium oxide as 0.032 calories per year per gm.
and found that from thorium oxide somewhat greater.
160
Becquerel's earliest work told them that radioactivity
persisted unaltered for years and 'appears to depend on
the uranium molecule alone, and not what it is combined
with';175 this is a phrase in which 'atom' would seem to
be more suitable than 'molecule' particularly since
chemical combination was in question. The energy
measurements induced Rutherford's clear dismissal of
normal chemical change as the source: 'It is difficult
to suppose that such a quantity of energy can be derived
from regrouping of the atoms or molecular recombinations
on the ordinary chemical theory'.176 Now his estimate of
at least 300,000 calories emitted per gramme of uranium
over ten million years involves assumptions and calcul-
ations not explicitly revealed. One can however infer
some of these, knowing his view that 'a greater concen-
tration or closeness of aggregation of the components of
such a complex molecule'177 as that of uranium 'would
possibly be sufficient' to supply the required energy,
and that these components were positive-negative electron
pairs. There should be 1000 (or 500?) such pairs, taken
to be existing independently within, and in fact constit-
uting the hydrogen atom; hence 200,000 per Ur atom. Rutherford's measured energy of ionisation being about
2 x 10-10 erg, the energy per Ur atom is thus 4 x 10-5 erg.
Using Avogadro's number178 the energy per gramme of Ur is
about 2 x 1019 ergs or 5 x 1011 calories. If we assume
that, say, 0.1% of this becomes available for radiation
during the attainment of a greater 'closeness of
aggregation', the resulting 5 x 108 calories, at the
measured emission rate of 0.032 calories per year, would
last about 1010 years. Rutherford in fact stated his
supposition that uranium had already been radiating for
107 years. It is therefore reasonable to suppose that
he went through some such procedure as the rough calcul-
ation above. One interesting point implicit in
Rutherford's assumptions is that the atomic-ionic energy
contained in unit weight is the same (5 x 1011 cals per
gramme) for all elements. But what did this imply for
the element radium?
161
The Curies' statement that they had used radium
specimens 100,000 times more active than uranium told
Rutherford that about 3,200 calories per gramme would be
released per year. We have seen how Becquerel explained
his far smaller estimate. But Rutherford, try as he
might, could find no plausible answer to his own larger
problem: It is evident that, unless energy is supplied from external sources, the substance cannot continue emitting energy at such a rate for many years, even supposing a considerable amount of energy may possibly be derived from rearrangements of the components of the molecule.l79
This statement may be no more than the rejection of an
ordinary chemical or phosphorescent supply, depending on
the meaning of 'molecule'. However, he made his view
entirely clear that an electronic source though 'many
thousand times greater' was still insufficient:
The energy that might possibly be derived from regrouping of the constituents of the atom would not, however, suffice to keep up a constant emission of energy from a strong radio-active substance, like radium, for many years.180
A direct comparison with our calculation for uranium
suggests why not, though Thomson in 1903 theoretically
extracted more than enough energy from this source. As
for the third possibility that 'the radio-active substance
in some way acts as a transformer of energy' from its surroundings, he thought that 'this does not seem probable
and leads us into many difficulties'.181 Rutherford's
last word for some time on the problem is enigmatic:
It is of importance that experiments to test the constancy of a powerful radio-active substance, like radium, should be carried out at definite intervals. If the radiation should keep constant from year to year, it would be strong evidence that the energy of the radiation was not derived at the expense of the chemical energy of the radio-active substance.l82
It seems that if the activity remained constant the source
must be external. If the activity in fact declined, this
should indicate a chemical source for the energy - but of
what kind? Direct observation was not to be the way in
162
which the apparent constancy of the radiations from
radium, uranium, and thorium was eventually broken
down. In the prelude to this much-disputed advance,
the mysterious thorium emanation with its rapidly
declining activity played a part, as did quite novel
studies on the chemical side of radioactivity which
were now growing apace.
4. Emanations and the X-substances (1900-1)
In May 1900, shortly after F.T.Trouton sent his
letter concerning radioactivity from Trinity College
Dublin to Nature, the better known physicist
G.F.Fitzgerald wrote from the same address to
Rutherford. Expressing his interest in the experiments
on thorium emanation Fitzgerald remarked: that Debierne says he has isolated a substance he calls actinium which he thinks is the active material in the thorium experiments. This actinium gives out something that is magnetically deflected, but I am not sure that this is not almost always present to a small extent in all these cases and that it is merely the very powerful ones in which it has been observed.183
Was the well-known activity of thorium then not its own?
And was the emission of corpuscles or 'disembodied
electrons'184 a general property of radioactive substances
after all? I shall in this Section discuss questions like
163
the first of these as they began to be asked with
increasing persistence during the year 1900. In exploring
the expanding web of experiments, ideas, and communications
concerning the precise chemical nature of radioactive
materials we shall see how the notion of radioactive
induction was invoked and extended by some with a view to
explaining the observations. The new chemical-radioactive
studies can be seen as a vital step towards the more
successful transmutation-disintegration theory of 1902-3,
or alternatively as providing a large amount of empirical
information which helped to broaden the scope of theoretical
explanations of radioactivity.
Outside the confines of radioactive studies, though
loosely related to them, there was no lack of discussion,
continuing from the earlier period through 1900, regarding
the possible transmutation of the chemical elements.
Lockyer repeated some of his old astronomical-chemical
arguments in a new book on Inorganic Evolution.185 On the
physical side, Fitzgerald's note on 'The Theory of Ions'
appeared in Nature in September.186 He viewed favourably
the varied evidence for an electronic structure of all
matter, and hence thought that there seemed to be 'no
impossibility in the dreams of the alchemist, and an
element of one kind may some day be transmitted into
that of another'. No mention of radioactivity was here
made by Rutherford's recent correspondent. Discussions
of purely chemical transmutations at this time, and earlier,
are exemplified by F.Fittica's claim and C.Winkler's
refutation 'On the alleged transformation of phosphorus
into arsenic',187 followed by Fittica's rejoinder 'On the
transmutation of phosphorus into arsenic and antimony'.188
W.Crookes corresponded with the American chemist Dr.Etmens
concerning the latter's claim of 1897, to have converted
silver into gold by hammering; Crookes thought it doubt-
ful.189 And at the end of 1898, before the American
Chemical Society, F.P.Venable on general grounds vigorously
attacked 'so dangerous a doctrine' as that of a changing
chemical atom, which had been suggested to explain variable
valency. His view was that:
164
It will be an unfortunate day for chemists when the unchanging atom is given up. Chaos will indeed enter into all of our theories when this, the foundation rock, is left at the mercy of every shifting tide of opinion and can be shaken by all manner of unfounded hypotheses.190
It is possible that a considerable proportion of chemists
clung to the atomic theory in this way. By 1900, radio-
activity had already begun to impinge upon this view; the
phenomena evidently required new concepts of one kind or
another to explain them. With the Curies' guidance, the chemist A.Debierne
followed another line of inorganic analysis in the
pitchblende residues. In October 1899 he reported 'Sur
une nouvelle mati6re radio-active'191 which appeared to
be chemically identical to the element titanium and
comparable in its high radioactivity with radium
(100,000 Ur). By April 1900 he had taken the step of
naming a new element, 'Sur un nouvel element radio-actif:
l'actiniumi.192 At this later date, however, Debierne
associated his perhaps questionable193 new element in its
chemical properties with thorium. He noted that his
actinium, which emitted magnetically deflectable rays,
caused very weakly the permanent induced activity dis-
covered by the Curies. Now the latter had published
descriptions only of a temporary induced radioactivity.
However, without discussing duration, they had indeed
been first to pose the vital question of whether 'la
radioactivity, en apparence spontanee, n'est pas pour
certaines substances un effet induit'.194 Debierne was
one of those who followed such a suggestion and who also
becelme enmeshed in the connected problem of whether certain
ostensible radioactivities should be attributed to active
impurities. Debierne's approach is illustrated by his
conjecture that thorium, weakly active, might owe its
activity to traces of a chemically similar foreign
substance like actinium, and by his opinion195 that
Rutherford's results with thorium also might lead to
this conclusion. Study of Rutherford's publications on
thorium emanation has revealed no grounds for the
165
statement; he himself could see none.196 Nevertheless,
Debierne announced that he intended to start with thorium
compounds themselves, rather than with mineral residues,
and to separate from these either an inactive thorium or
the strongly active foreign substance, actinium. Perhaps
Debierne expressed his intention in the alternative form
because he realised that the preparation of an inactive
thorium would be an impossibility if it owed its activity
to actinium, and if this substance really induced a
permanent radioactivity upon adjacent materials. Several
scientists struggled among the confusions between induction,
element, and impurity.
F.Giesel's experimental studies ran parallel to and
sometimes ahead of those in France. In August 1899, two
or three months before announcing the first magnetic
deflection of the rays, he published 'Einiges Uber das
Verhalten des radioactiven Baryts and Uber Polonium'197
which effectively began a new line of research; though
at first he provided no theoretical interpretation of
his results. Firstly, the activity of radium was not
constant; after crystallisation it rose from a very low
level as indicated by a fluorescent screen to a constant
maximum during the course of days or weeks.198 Secondly,
the activity of a solution of the chloride gradually
faded. And his polonium sulphide precipitates had
completely lost their radioactivity after two months.
Though making no claims, Giesel at the end of 1899 seems
to have been the first explicitly to describe a new method
of induction, the artificial activation of salts by
admixture in solution. His preparation in this way of an
artificially active bismuth199 had significant consequences.
For this seems to be the beginning of his long-held belief
that polonium was merely induced bismuth. Debierne too
made progress in experiments on this new form of induction
and cited Giesel and Rutherford in his note 'Sur du baryum
radio-actif artificiel'.200 Debierne considered that the
far stronger effects which he found with the solution
method were due to the more intimate contact of the sub-
stances concerned. Ordinary barium, precipitated as
166
sulphate from a solution containing actinium, turned out
to have an activity several hundred times that of uranium;
this could then apparently not be altered by chemical
reactions. His somewhat surprising conclusion was that:
The radioactivity of barium rendered active by contact is an atomic property in the same way as that of radiferous barium, since it persists in all chemical transformations.201
The artificially active barium could, like radium, be
concentrated to comparably high activities by crystall-
isation, but Debierne held its lack of a spectrum to be
one of the 'important' differences between the two. The
fact that its activity diminished spontaneously to one
third in three weeks provided a further distinction from
both radium and actinium, each of whose activity steadily
increased to a maximum after preparation. Its vanishing
radioactivity did not deter him from claiming to have
produced:
par induction un baryum radio-actif, qui se distingue nettement du baryum et du radium et qui se presente comme un terme interm6diare entre ces deux elements.202
He was sure that his results were not due to traces of
actinium or radium. However, perhaps partly in his own
defence, Debierne accused B.von Lengyel of committing this
very error. For the latter in his report from the
Chemical Laboratory of the University of Budapest 'On
radio-active barium',203 had stated that he found uncon-
vincing all of the evidence for new chemical elements
based on radiatiom measurements. Even in the case of
radium atomic weight and spectroscopic determinations
were not sufficient. The experimental preparation of an
artificial radium or of a radioactive barium would tend
to show that radium was not an element; in his attempt
to do this von Lengyel considered himself successful.
He claimed that the active barium compounds extracted
from a melt of uranyl nitrate and barium nitrate crystals,
exhibited in qualitative fashion the increasing radio-
activity possessed by the radium of others:
It appeared that one can transform ordinary barium into a radio-active form which apparently
167
possesses all the properties of radio-active barium observed by different experimentalists.204
This conclusion was immediately criticised by F.Giesel
in his note 'Veber radioactives Baryum and Polonium'.205 He stated that uranium nitrate owed much of its radio-
activity to highly active impurities such as radium and
that these would simply have contaminated Lengyel's
barium. It is notable that the constancy of uranium rays,
so important for radioactivity up to this time, was by
implication discarded by Giesel as casually as that of
thorium had been by Debierne. With regard to active
'impurities' the subject was interconnected in such a manner that by saving radium Giesel seems in a way to
have forfeited uranium. Whether he was aware of this it
is not clear. On the other hand, he considered that the
Curies' claims for polonium were weakened by a different
factor, by their own discovery of radioactive induction.206
As yet another new element came into contention
Giesel exhibited the same cautious tone. Both the poss-
ibility of radioactive induction and of contamination by
active traces played a part in his continuing criticisms
of the assertions of K.A.Hofmann. From the chemical
laboratory of the Academy of Science in Munich, Hofmann
and E.Strauss in November 1900 announced their discovery
of several new radioactive elements. One was close to
lead in its chemical properties, 'das radioactive Biel',
'radiolead'; the others were rare earths.207 Within weeks
Giesel pointed out that traces of radium, polonium or
actinium, chemically undetectable, were the likely causes;
similarly Debierne's artificially active barium precipitated
from actinium solutions had merely carried down a trace
of the latter element, undetectable but for its radiation208
Hofmann and Strauss concentrated their researches upon the
radiolead; their determination of a chemical equivalent
of 65, hence anatomic weight of 260, seemed highly
significant (lead 207)209 but the value was never
confirmed210 and the statement was not repeated. Then
in February 1901 they claimed, apparently in reply to
Giesel, that qualitative chemical tests had ensured the
168
absence of all three new active elements from their
radiolead; this missed the latter's point. In the face
of criticism these incipient radiochemists appear to
have sought all possible means of qualitatively identi-
fying their materials. The title of their paper, 'Veber
die Einwirkung von Kathodenstrahlen auf radioactive
Substanzen',211 indicates one such method. The idea was
not new, but their attainment of seemingly definite
results had been anticipated, in part, only by one other
scientist.
During the preceding summer, P.Villard had spoken
of the induced radioactivity of bismuth;212 he had extended
the Curies' discovery of temporary induced radioactivity
by producing a similar though weak effect in bismuth by
the action of cathode rays alone. Villard expressed the
hope that researches using the vacuum tube could lead more
readily than solution studies to a simple explanation of
radioactivity. We note Villard's view, which appears to
relate current researches directly to Prout's hypothesis,
that the cathode rays or radiant matter of the Crookes
tube consisted not of sub-atomic particles but of the
lightest element hydrogen.213 His work in 1900 on the
penetration of gaseous hydrogen immediately influenced
the ideas of Crookes himself on radioactive rays;214 while
that on induced activity was taken up in 1901 at the
Cavendish Laboratory in the form of researches 'On a kind
of Radioactivity imparted to certain Salts by Cathode
Rays'.215 These studies appeared to be of theoretical
importance for radioactivity at least until 1903.
Hofmann however made no reference to Villard's work
as he explained early in 1901 that by the influence of
cathode rays he could revive the activity of radiolead.
This, unlike radium radiation, had vanished entirely in
a few months; the renewed activity lasted for several
weeks. He considered that these observations constituted
conclusive evidence, in addition to the chemical tests,
that polonium too was absent; for the similarly declining
radiation of this substance could not be restored.
Iiofmann gave a vague theoretical explanation of such
169
effects in terms of vibrations of small wavelength excited
within the metal atome.216 This could not have impressed
Giesel, for his extensive experiments with the radiolead
kindly sent to him by Hofmann confirmed none of its
distinguishing features217 not even the effect of cathode
rays. Furthermore, he turned the idea of induction
against Hofmann by precipitating an articifially radio-
active lead sulphide of declining activity from a solution
containing radium.218 The dispute continued into 1904,
with claims,219 rejections,220 and further claims,221 to
have found a distinguishing feature. By 1903, in order
to back his assertion of 'Radio-active lead as a primarily
active substance'222 Hofmann claimed to have characterised
this material by analysing its radiation, despite his
employment of techniques unrefined compared to those of
some physicists. He still assumed that the rays could
directly induce radioactivity into other substances.
According to the theory developed at that time, and now
accepted, each radioactive element is distinguished both
by the character and by the rate of decline of its rays,
but not by any inductive property. Experimental problems
of isolation and identification certainly continued to
exist within the area covered by this theory. Nevertheless,
the disagreement concerning Hofmann's preparations was
settled more or less in his favour during the following
few years. Shortly before the radiolead dispute began William
Crookes read a paper on 'Radio-activity of Uranium'223
which was to have a more direct influence than those
discussions upon the development of the understanding of
radioactivity. It was he who in May 1900 first pointed
to the problem of entrainment of radioactive traces on
precipitates, a legacy of his own spectroscopic studies
of the rare earths: a substance present only in traces
tends to follow the analytical reactions of the bulk
material even if chemically dissimilar. Crookes did not
mention radioactive induction though he probably read of
it in Debierne's papers on active titanium and actinium,
170
which he cited.224 Confused though he admitted himself
to be during the following year,225 on account of the
results of others together with his own difficulties with
radioactive fractionations,226 Crookes seems never to
have entertained the possibility of direct induction;
perhaps entrainment and diffusion made it seem unnecessary.
His laboratory notebooks record some attempts to prepare
radium and polonium from pitchblende in October 1898,227
to activate barium using uranium solutions in January 1900,228
and the vital fractionation of uranium in the following
month.229 Crookes tells us that his original intention was
simply to purify uranium for use as a photographic standard
against radium or polonium preparations.230 But on appli-
cation of a known purification method the portion of uranyl
nitrate dissolving in ether turned out, unexpectedly, to
be entirely inactive photographically while the aqueous
layer, normally discarded, gave a strong effect. He
directed his remarks against the Curies' statement that
the activity of uranium or thorium was a constant property
of the metal independent of its state of combination.231
Crookes had shown on the contrary that the radioactivity
exhibited by uranium belonged not to itself but to quite
another substance which he designated UrX. This conclusion
was supported by marked differences in photographic
intensity between various commercial samples.232 Thorium
too had begun to separate into portions of different
activity upon fractional crystallisation; he remarked upon
Debierne's supposition that actinium was the true source
of thorium rays. It appears that this notion became
persuasive for a time. Crookes informed P.Curie of his results and suggested
on the basis of extractions from a barium-free pitchblende
that radium might not really resemble barium.233 Curie
replied that he had not seen Crookes' paper but that 'it
is absolutely certain that the substance you have extracted
from your mineral is not radium'. Curie's answer was
actinium: he noted that Debierne had detected it in
commercial uranium specimens and had already succeeded in
decreasing the activity of these; and he believed that
171 the results of von Lengyel and Giesel234 of May and June
could also be explained by the presence of this material.235
In his reply, Crookes did not dispute the point;236 indeed a statement made in 1902 shows his acceptance of the
identity of his UrX and Debierne's actinium.237 By that
time the influence of Crookes' publication had run through
Becquerel and Baskerville to Soddy and Rutherford.
The American chemist C.Baskerville, in his paper 'On
the existence of a new element associated with thorium'
delivered in August 1901238 cited Crookes on UrX and
thorium. Baskerville too, like Debierne before Crookes,
thought that thorium's activity might possibly lie with
actinium; he noted that Rutherford's results on the radio-
activity induced by thorium made photographic comparisons
difficult, but discussed this no further. Five years
earlier he had used the designations Th and ThX for the
two separated elements.239 These results and symbols
featured in the future progress of the science though
this belief in the ubiquity of actinium did not.
Becquerel too had wished to prepare a pure uranium,
in his case for (successful) attempts to deviate magnetically
its rays. In a 'Note sur le rayonnement de l'uraniume of
June 1900240 he drew from the studies of Debierne on
actinium in thorium and von Lengyel on the activation of
barium by uranium. Becquerel described an actual lowering
of uranium's activity by two successive additions of
barium chloride to its solution, each followed by precip-
itation of the barium as a now active sulphate. He under-
stood apparently independently of Crookes that entrainment
of an active material, perhaps actinium occurred here but
concluded that uranium did emit a radiation of its own.
Becquerel was not so sure of this after a series of 28
entrainments performed241 during the following weeks
yielded continual diminutions in uranium's activity,
successively smaller, with some irregularities. He
acknowledged Crookes' attainment of a completely inactive
uranium nitrate, to the Paris Congress in August.242 It
was Becquerel's recognition during the following year of
the self-recovery of this activity which produced a leap
172
forward in radioactive research.
As the Curies saw the position in August 1900243
the experiments of Millard on activation with cathode
rays showed that one could 'creer la radioactivite sans
faire intervenir une substance radioactive';244 Debierne's
'baryum active' by actinium was unaffected by chemical
change, 'son activite est donc une propriete atomique'.245
They thus considered radioactivity to be an atomic property
which could be induced upon the atoms by an external agent;
the source of the energy released, once this property of
radiating had been acquired, was another question.246 As
for Crookes' work on UrX and inactive Ur the defence
against the implied criticism was clear. It was an assoc-
iated element, probably actinium, to which the radioactivity
of uranium must be ascribed. Owing to the difficulty of
obtaining this element free from actinium uranium would
simply have 'l'apparence d'un element atomiquement radio-
actif'; there was no 'contradiction avec l'idele que la
radioactivite est une propriete atomique' .247 It may appear
however that the underlying evidence for this idea was now
in considerable disarray. Moreover, the Curies' notion of
radioactive induction appears to have been at a point of
transition. Rutherford's results on thorium emanation and
its electrically sensitive inducing effect, which they were
unable to repeat in regular fashion with radium,248 together
with solution studies, had probably caused the Curies to
discard direct radiation as a simple cause of induced radio-
activity. Their readers waited until the spring of 1901
for the appearance of a coherent alternative.
The physicist E.Dorn at Halle, having recently
published on X-rays and radium rays, had already attained
some experimental success with radium induction before the
time of the Paris Congress. To radium and polonium he
applied the techniques used by Rutherford upon thorium.
With French and de Ha?n German samples he succeeded, where
Rutherford could not, in demonstrating that radium and to
a small extent polonium released a gas-like emanation with
the inducing property. His discussion 'Veber die von den
173
radioactiven Substanzen ausgesandte Emanation'249 indicates his use of Rutherford's term. Dorn also accepted the view
that the cause of induced activity was the deposition of
emanation.250 Yet he may not have grasped or adopted this
completely for in his reported experiments, some of the
earliest, on the electrolysis of radioactive solutions,
he applied the Curies' expression 'secondary activity' to
activated electrodes without visible deposit.251 And he could not understand why a wire sealed in a glass tube
after activation by radium should lose its activity in a
day, unless the glass were in fact permeable to the
emanations in an undetectable degree.252 The different
rates of decay showed him that there were qualitative
differences between the emanations from thorium and active
barium, and between the secondary activities. His discovery
of large, if irregular, increases in electrometer readings
upon moistening thorium oxide or radium specimens seemed
to him of particular interest; radioactivity was thereby
placed in close connection with a 'physikalisch-chemischen
Prozess'.253 Dorn's student F.Henning continued electrical
researches upon the emanations and upon aqueous solutions
during the following year, 1901,254 but drew no significant conclusions; he seems to have thought that the particles
of emanation could disappear spontaneously in the air.255
We recall Rutherford's assumption that the intensity of
radiation from each particle slowly declined.
On the basis of research upon the emanations, only '
Rutherford and his collaborators were to make progress.
His study of the energy of radioactivity, completed by
mid-1900, was problematical in various ways. Dorn had
accorded due acknowledgement to Rutherford regarding
emanation studies and now demanded256 the same from the
latter regarding the energy of X-rays; priorities were in
fact given, in the full published paper.257 Happily, Dorn
in his 'Bemerkungen au der Mitteilung von Rutherford und
Mc. Clung Tiber die Energie der Becquerel- und
ROntgenstrahlen etc.'258 agreed with the values therein.
But J.S.Townsend, as we have seen, disagreed with estimates
used at a succeeding stage of Rutherford's reasoning towards
174
the energy of radioactivity. The question of its source
seems not to have had an answer, nor was there any clear
direction of research which might lead to one. The same
was not quite true of the emanations: to the standing
questions of their nature and means of production
Rutherford redirected his attention. His examination of
the 'Einfluss der Temperatur auf die "Emanationen"
radioaktiver Substanzen',259 dated March 1901, constitutes
an extension of his researches of 1899 upon thorium.260 He reported that the rate of production of emanation from
both thorium and radium increased steadily with temperature
up to a red heat, above which it was almost destroyed, not
to be restored. This latter statement was to be very
significantly revised within the following months. Somewhat
like the Curies, Rutherford noted that there were irregul-
arities in the production of induced radioactivity by
radium emanation; it could be confined less readily to a
cathode than thorium emanation especially when provided,
in large amounts, by heating the radium. Rutherford con-
cluded, adopting an expression similar to that of Dorn whom
he cited, that the two emanations were probably produced by
a chemical process 'einem chemischen Vorgang im Material'261 J.J.Thomson was prepared to say much more than this
concerning the emanations, though not as yet in print. His
reply of 12th April to Rutherford's enquiry262 concerning
Tait's Chair at Edinburgh contains a comment which may seem
familiar in the context of the contemporary chemical-
radioactive researches:
I suppose you have seen Debierne's work on actinium, a substance which is closely associated with thorium, and which has extraordinary powers of producing induced activity; do you think there could have been any of this in your experiment on the thorium radiation?263
This was Rutherford's second private warning concerning
actinium; one of the main tasks of the chemist whose help
he enlisted in the following months was to see whether
the emanation really came from thorium, or not. As for the
emanation itself Thomson now thought he knew 'pretty clearly'
its nature and the mechanism which produced its observed
175
electrical properties. We have seen264 that he had
reversed Rutherford's explanation both of the concentration
of induced activity or emanation upon a cathode, and of
the failure of this at a low pressure. Rutherford had
postulated an excess of positive ions clustered around
the emanation particle, Thomson the attachment of electro-
positive emanation particles around positive ions. The
failure of electrical concentration in rarefied gases was
attributed by Rutherford to a scarcity of air molecules
and hence ions; this outweighed the increased mobility of
a charged particle. Thomson needed far fewer ions than
Rutherford both in his old account of 1899 and in his new
one of 1901: I was much interested in your paper on the effect of temperature on the emanation which I was reading last night. I think your experiments show pretty clearly what the emanation is - does not the following view explain most of the effects - suppose that thorium or radium gives out a gas (the emanation) & that this gas is radio-active in the same way as radium i.e. by giving out negatively electrified corpuscles - the effect of this emission of corpuscles will be that the particles of the emanation will behave as if they had a slight + charge. The equivalent charge will be only slight because though the emission of the corpuscle will momentarily leave the emanation with a + charge this charge will soon be neutralised by a negative ion from the surrounding ionised gas The equivalent charge ought to be less at a high pressure than at a low (or rather at a very low pressure) for the smallness of the charge is measured by the quickness with which it is neutralised & at a very low pressure there would be few ions to do this. I should not expect the effect of pressure to be great until the pressure got very low as until then the diminution of the number of ions would be compensated for by their increased mobility.265
Thomson was perhaps first thus to combine the eighteen-
month old knowledge of the corpuscular nature of the rays
with emanation studies. His account entered strongly into
Rutherford's publications266 after initially suffering
setbacks. One of these appeared in conjunction with the
justification of Thomson's description of each emanation as
a 'gas', which Rutherford announced within weeks, in ful-
fillment of a promise of 1899.267 Rutherford's experimental success as he began to answer the second major question
regarding the emanations - their precise nature - owed much
176
to radium emanation, effectively unknown in 1899; its
radiation lasted for days, rather than the minutes of
thorium emanation. His paper with Miss H.T.Brooks268
on 'The New Gas from Radium'269 and his note to Nature
on 'Emanations from Radio-active Substances', of May 1901,270 describe the diffusion experiments on radium
emanation which now gave a value for its molecular weight.
This, being between 40 and 100, excluded the vapour of
radium. 'We must therefore conclude that the emanation
is in reality a heavy radioactive vapour or gas'.271
The correct modern value of 222272 certainly would not have excluded the vapour of radium; nevertheless, the
conclusion held firm. Of great interest are two points
which Rutherford briefly made in the closing statement of
both papers, each of which weakened previous hypotheses.
Firstly, the emanation emitted a radiation 'apparently
similar in character to easily absorbed ROntgen rays' and
presumably not the charged deviable rays required by
Thomson; Rutherford was later273 to make sure of this, with
important consequences. Secondly, this emanation 'in some
way manufactures from itself a positively charged aabstance,
which travels to the negative electrode and becomes a
source of secondary activity'.274 This statement is
apparently not entirely consistent with Rutherford's and
Thomson's previous notion that it was a deposited layer
of the emanation itself, positively charged by association
with ions of any surrounding gas, which produced the
excited radiation. Had the differences between emanation
and deposit in their chemical behaviour, and the reality
of the new gas, convinced Rutherford of the occurrence of
a second strange chemical process? It was months before
radiochemical studies on thorium began in earnest at
McGill, and nearly a year before his first and barest
hint of an observed transmutation appeared in print. But,
considering the background of speculation in the field,
we are perhaps entitled to ponder privately upon Rutherford's
ideas of May 1901; though at this time he would only say
that 'Space is too short to enter into the interesting
177
question of the possible explanation of these complicated
phenomena'.275
The researches made by Elster and Geitel during
1900-1 came to stand between Rutherford and Thomson in
1902; praised and used as support by the former, they
led the latter into criticisms and doubts.276 J.Elster and H.Geitel maintained their early interests in radio-
activity; they were pleased to incorporate Rutherford's
ideas on emanations and active deposits into another of
their areas of study, the electrical phenomena and
conductivity of the atmosphere. A slow spontaneous
increase in the natural conductivity of an enclosed
portion of air over several days led them, towards the
end of 1900, to suggest that natural atmospheric conduct-
ivity might not be produced by solar radiation as formerly
supposed.277 Instead, the rays from traces of emanation
in the air, and from the resulting induced activity on
the walls of a containing vessel might be the cause.
They found observations to support this view, again going
underground. Here they found abnormally high conductivities
in the air from caves and cellars278 where there was no
possibility of contamination from their laboratory. And
the final comparisons, made by October 1901, with
Rutherford's results on the emanations were the concen-
tration of induced activity upon a negatively charged wire
simply placed in the open air,279 and the removal of this
activity from the wire by mechanical or chemical means,280
where it then continued to decay. They did not explicitly
discuss the important question of whether the active layer
was a deposited material, or the surface itself put in a
radioactive state; but the answer implied by their exper-
ments and discussions would seem to be that it was both.
For they used specific methods directed at metals to
178
remove the activity from their surfaces,281 and yet stated that the production of surface activity was
the same for a variety of substances.282 The problem-atical theory which they devised to encompass these
results was one of creation and disintegration of
radioactive matter: though unusual in certain respects,
it was not the only one of its kind.
179
CHAPTER 4
DISINTEGRATION, INDUCTION, TRANSFORMATION
1. The emergence of induction and disintegration THiiories (1901=7)
Dispersed among the various works on radioactivity
published during 1901-2 all of the pieces which were
shortly to coalesce into a coherent theory may be dis-
cerned; later priority claims testify to this. Also
present were other persuasive concepts which were to
handicap their employers greatly; but this only became clear after the event.
At the end of 1901 the physicists Elster and Geitel
provided discussions which seem typical of the period in
being both suggestive and incomplete. Their comments
were contained in a paper whose main purpose was to des-
cribe 'Recherches our la radioactivite induite par l'air
atmospherique1.1 During the past year they had linked
their discovery of atmospheric-induced radioactivity with
Rutherford's conclusion that the emanations and induced
activities of thorium and radium were distinct materials.
But now the German workers were much attracted by an
alternative view which did not require the existence of
special substances. Ironically, this occurred just at
the time when Rutherford and Soddy were making great
progress on that basis. Though it had obvious weak areas
Elster and Geitel's thesis was persuasive. For it pointed
towards a fundamental explanation not only of the universal
induced atmospheric activity but of natural radioactivity
also. All gases, they thought, possessed an ionic
constitution and could thus provide positive ions which
's'unissent aux electrons nogatifs du conducteur blectrise'.
The result was 'une sorte de combinaison instable qui se
de-bruit par l'6mission des electrons, c'est-6.-dire par la
production de rayons de Becquerel' as required.2 Accordingly,
induced activity would be due to a temporary compound
derived both from the surrounding gas and the metallic
anode. Their approach towards a theoretical advance was
180
to ask 'd'une maniere generale si l'on peut distinguer
l'une de l'autre les radioactivites primaires et induites'.3 In order to assimilate these two phenomena they argued
from the 'lois d'energie', as they had in 1899, that the
apparently permanent activity of the elements uranium,
radium and thorium in fact suffered an imperceptibly slow
decline. The particularly rapid decay of induced radio-
activity was simply attributed to the 'tres petite quantite
de matiere recueillie'. Of great interest is their idea
that this material was actually in the course of creation
- that one might be witnessing 'la veritable elaboration
d'une substance active'. The implications of this for
uranium and the other active elements are fascinating
indeed, but were not discussed. On the other hand they
ascribed the observed decay of induced activity to an
emission of electrons by which the material 'se detruit'
or 'est tres vite ramenee a l'etat indifferent'. Whilst it may be described as a theory of disintegration this
account skips over the intermediate chemical stages so
crucial for other workers. Even with the limited dis-
cussions which Elster and Geitel provided there were certain problems some of which were mentioned and some not.
The theoretically implied but unobserved effect of dilution
or quantity upon the rates of decay falls into the latter
category. In addition their statement that the Curies'
radium-induced activity, excited 'par le contact immediat'
or via fluid media, 'n'est point une propriete durable'4
seems to admit two distinct types of induction. And in
the open admission that their theory could not account
for the extraordinary high atmospheric activities observed
in certain caves may be seen their reason for not completely
rejecting Rutherford's special emanation hypothesis.
Having failed to produce the predicted induced activity
by attracting the negative ions of the air on to a pos-
itively charged wire5 their studies of the different
natural conductivities of air from caves and from the
laboratory finally convinced Elster and Geitel of the
existence of a primarily radioactive gas in the air.6
181
But by then in mid-1903 such phenomena had already been
absorbed into a disintegration theory more successful
than theirs.
That others were moving in that direction towards
the end of 1901 is indicated by Elster and Geitel's
remark that their 'idee se rapproche beaucoup' with the
analogous considerations presented recently and independ-
ently by W.Nernst and H.Becquerel. The comparisons are
interesting but not simple. Nernst gave only the briefest
comments on radioactivity to conclude his discussion
'Veber die Bedeutung elektrischer Methoden and Theorien
fUr die Chemie'.7 He supposed that electrons escaping from
their dynamic equilibrium with metallic elements constituted
the Becquerel rays. And whilst Becquerel himself would
certainly have agreed with Nernst that uranium rays consisted
in part of electrons yet on the basis of his own experiments
he postulated a much more severe dissociation of the active
material. In advance of my examination of his and other
theories of radioactivity it may be helpful roughly to
classify these according both to the kind of disintegration
envisaged and to the use made of the notion of induction
which can now be seen as a completely false trail. Elster
and Geitel's indifferent attitude towards induction has
already been noted. They shared with G.Martin and
H.Becquerel the idea of a complete or destructive atomic
disintegration. The latter was the only one of these to
employ the conception of induction by contact in his
explanations. W.Nernst, J.Perrin, W.Crookes and
J.J.Thomson all seem to have believed or implied that an
atom or molecule lost sub-atomic electrons or corpuscles
only to pick up others from the surrounding material, so
restoring the original situation. On the other hand
Becquerel, J.Stark, and Rutherford and Soddy viewed the
dissociation of radioactive atoms or molecules as passing
through a series of irreversible steps. The latter two
scientists in collaboration argued most forcefully con-
cerning the chemical consequences of such a process.
The case of the Curies is an interesting one. At
the time of her lecture of mid-1900, which contained
182
lively discussions on the relationship between atomic
change and corpuscular emission, Marie Curie would appear
to fit moderately well into the last group of the above
scheme. Within a year however all such considerations
had been effectively shelved. Reasons for this are
uncertain, but one can point again to the unresolved
contradictions regarding the non-corpuscular rays of
polonium. It is also just possible that one or both of
the Curies came to realise the chemical implications of
Marie Curie's speculations only gradually. In any case,
P.Curie and A.Debierne were able to work experimentally
towards an apparently superior theory uniting both
inductive and radiative phenomena. Their first step,
however, involved the partial disconnection of a prematurely
formed link between these two. In a paper of March 1901
'Sur la radioactivite induite provoquee par les eels de
radium'8 they described experiments performed with thorium,
radium and actinium in sealed vessels, which proved con-
clusively that radioactivity could be induced without
direct irradiation. Significantly, the hint of a remaining
bond can be seen in their comment that polonium was an
exception in producing neither induced activity nor the
deviable radiation, two facts which might be related.
They stated that it was too soon to accept Rutherford's
theory of a diffusing particulate radioactive emanation
since other equally satisfactory explanations could be
formulated. But they were not prepared to say what these
were admitting only that 'La radioactivit6 induite se
transmet dans l'air de proche en proche' from source to
object and insisting that this process might be connected
with the deviable radiation. The adoption and interpretation by Rutherford of the
results described in P.Curie's and Debierne's succeeding
note 'Sur la radioactivite induite et les gaz actives par
le radium',9 read a few weeks after their first, exemplifies
the closeness of and the differences between the paths
followed by the students of radioactivity. Two months
later Rutherford described the electrical examination of
183
what he called 'The New Gas from Radium'10
whose moderately
high molecular weight he had estimated with the aid of a
standard diffusion method; induced activity was caused by
'a positively charged substance' somehow manufactured from
this gas. His approach may be contrasted with the French
scientists' suggestion of a gas activated lox radium and
with their experimental attempts to grasp the role of the
medium through which induced activity was transmitted.
Curie and Debierne found that induced activations were
unaffected by the use of different gases or by evacuation
down to 1 cm. mercury. But in a high vacuum maintained
throughout the induction by continuous pumping, as they
stressed, activation did not occur and previously induced
bodies lost their activity. Suppression of activation
failed, however, when the evacuated vessel was simply
sealed and left. Curie and Debierne attributed this
result to the release of highly radioactive gases occluded
within the radium sample; when collected by gently heating
the specimen these produced spectacular effects such as
the luminosity of the entire containing vessel. Evidently
all this fits well with Rutherford's earlier ideas on
thorium emanation and with his as yet unpublished study
of the effects of heat upon the production of emanations.
Yet without again citing that theory the French team
spoke with justifiable reserve concerning the only explan-
ation of their results which they mentioned. It might be
supposed that 'des gaz ordinaires contenus dans lair
slactivent au contact de la matiere radioactive';11 the
activated gas could then excite solid bodies by contact.
But, as they noted, this accounted neither for the maximum
activity's independence of the pressure and nature of the
surrounding gas nor for the rapid transmission of the
activity along a capillary tube, which appeared to proceed
faster than ordinary diffusion would allow. Evidently
the idea of a heavy gas, on which Rutherford was shortly
to publish, was in even greater disagreement with the
apparently rapid transfer through gases than Curies's and
Debiern's own discarded conjecture. The French scientists
were to find their answer to the problems of the inductive
184
transmission of radioactivity by employing a totally
different medium.
Researches on this subject differed between Paris
and Montreal in technique as well as interpretation in
an unfortunate and perhaps unavoidable manner. P.Curie
and A.Debierne were moved to comment openly on the 'stat
deplorable' to which things had come in the laboratory:
the air had become so conducting that only 'des mesures
grossieres' with the electrometer could noftrbe made.12
They attributed this situation to the continuous formation
of activated gases rather than to dust as previously
assumed;13 in another context they reported induced
activities of up to 8,000 times the intensity of uranium
rays.14 The difficulties of repeating any of Rutherford's
work on thorium must have been great. This seems to be
confirmed by information recorded in a laboratory notebook15 of the Curies: the 'mouvement propre' of their weight-balance
piezo-electric electrometer varied from day to day during
some activation studies16 and sometimes made the radiation
from thorium impossible to measure.17 In compensation
however the extreme activity of the Curies' radium samples
soon led them to experimental discoveries which could
otherwise not have been made. Within two months of their complaint, Curie and
Debierne had been able to construct the most comprehensive
theory of radioactive phenomena yet achieved. Its experi-
mental basis developed as they turned from gases to water
as the medium of radioactive transmission. Debierne had
been one of the first to investigate the solution method
of induction in 1900; now with P.Curie in a note 'Sur la
radioactivity des eels de radium' of July 190118 he
explained that the heating of radium salts produced not
only active gases but radioactive water too. Here was
their clue. They followed it by showing that water could
also be activated simply by placing it in a dish within
the same sealed enclosure as a similar dish containing
the solution of a radium compound. Perhaps more revealingly
water could also be activated by immersing in it a sealed
185
celluloid capsule of the radium salt. Their interpretation
was that the celluloid 'joue is role d'une membrane semi-
permeable parfaite' allowing the activity but not the
radium to pass. Transmission did not occur through a
dry celluloid wall. Radioactive water lost its activity
within a few days when in a sealed vessel, much more
rapidly in an open one, and faster if the surface area
was greater. The fact that solutions containing radium
itself appeared to behave similarly, with the difference
that here the activities declined to a minimum but not to
zero, was the final point. The resulting theory was the
first which 'permet de coordonner' the rise, decay,
equilibrium and transmission of radioactivity. Curie's
and Debierne's fundamental assumptions were, firstly, that
'chaque atome de radium fonctionne come une source continue
et constants d'energie radioactive', and secondly that this
energy thence dissipated itself in two different ways:
1. par rayonnement (rayons charges et non charges d'electricit6); 2. par conduction, c'est-a-dire par transmission de proche en proche aux corps environnants par 1'interm6diare des gaz et des liquides (radio-activite induite).19
The authors made clear the analogy which they saw between
this formulation and expressions in use 'dans l'etude des
phonomenes calorifiques'; but other points were not so
plain.
Although they wished to elucidate 'le mecanisme de
la propagation de la radioactivite induite'2° Curie and
Debierne could or would give no details beyond the phrase
'de proche en proche', or as a translator put it 'from
particle to particle' .21 This remained so even after the
end of 1901 when they were exploring the spatial aspect
of radioactive transmission.22 Having excluded convection
and diffusion as major modes of transfer in favour of a
step by step process the impermeability of dry solid
materials would seem hard for them to explain. And for us
the explanation of the apparently rapid transmission along
capillaries is equally obscure. By means of their theory
Curie and Debierne pushed a multitude of observations into
the background. One may note their failure to comment
186
upon the different chemical properties of emanations and
induced activities, a problem which worried F.Giesel who
mentioned it to the Curies.23 Among other details of
which Curie and Debierne knew but gave no account were
the electrical concentration of induced radioactivity
and the characteristics of the complex radioactive rays.
In this respect it seems they placed the phenomena of
radioactivity in a hierarchy of importance which was
almost the reverse of that adopted by the prononents of
disintegration theories.
Evidently the authors had good reason to keep their
options open. This they did with the claim of July 1901
that the radium atom constituted a constant source of
radioactive energy 'sans qu'il soft nbcessaire, d'ailleurs,
de preciser vient cette energie',24 and in a footnote
they placed Marie Curie's speculations of January 1899:
the energy might have been previously stored, or derived
from an external radiation, or taken from the surrounding
medium, or produced 'par une modification du radium lui-
mame'. The last of these conjectures and then the first
were soon to become elevated in Paris and elsewhere to
positions of the highest significance. But P.Curie heaped
harsh criticisms upon those who did this.
With his work on the deviable rays from radium, his
studies of the chemical deactivation of uranium in 1900,
and some points from the above discussions of Curie and
Debierne, Becquerel combined theoretically a new discovery
of his own. The attempt thus to produce a complete
explanation of radioactive phenomena was nowhere ignored.
The researches on secondary radiation which he undertook
during 190125 had not turned Becquerel from his beliefs
that the primary phenomenon of radioactivity was the
emission of the deviable rays, that the consequent
undetectable loss of mass was the origin of the energy,
and that the non-deviable rays (probably meaning the
absorbable alpha rays but possibly gamma rays also) were
a type of X-ray produced by the primary rays. At the end
of that year he clarified his views 'Sur la radio-activite
de l'uranium'26 with a detailed description of the processes
187
involved. He considered that 'en se s6parant' the small
negatively charged corpuscles of Thomson's theory were
matched by large particles, oppositely charged, recoiling
at low velocity. These particles, which would not be
penetrating, formed the gas-like positively eleotrified
material emanation which would deposit upon all surfaces
except those similarly charged. Once deposited 'Ce depot
de matiere serait capable de se diviser A son tour en
particules plus petites qui traverseraient le verre'.
Thus in Becquerel's explanation emanations fade into
radiations in an interesting manner; however, among other
discrepancies, he missed the point that the emanations
themselves are electrically neutral. He appears not to
have anticipated the view which was later to become
important that the alpha rays consisted of rapidly moving
large particles; this had already been suggested in 1900 by R.J.Strutt27 on penetration evidence. But Becquerel's account contains the first published expression of the
conception that molecules suffer a mechanical recoil upon
the release of a corpuscle; such a notion was also adopted
by Rutherford during that same month as a new means of
explaining the electrical properties of thorium emanation.
As revealed in his paper Becquerel's most striking
advance lay on the chemical side of radioactivity. Persuaded
by the evidence of his own and of Crookes' observations he
had come to believe in 1900 that uranium owed its entire
radioactivity to a removable impurity. Since then, as
Becquerel stated, he had realised that such a conclusion
stood in contradiction to the fact that all commercial
samples of uranium salts of whatever purity were equally
active; it is noteworthy that this 'fact' was by no means
so clear to all.28 His observation of June 1900 that old fractionated uranium samples possessed identical activities29
may possibly have been a clue to it. However, Becquerel
did not explain the long delay indicated in his announcement
that now, eighteen months later, he had reexamined both
the deactivated uranium and the barium sulphate specimens
activated by precipitation from the solution of uranium.
188
Though the path to its attainment remains obscure
Becquerel's discovery that his deactivated uranium had
completely regained its activity whilst the activated
barium sulphate had entirely lost this power had a direct
impact. His interpretation of these results extended the
Curie-Debierne induction theory just so far as to make
contact with his own ionic speculations; he was doubtless
surprised when sparks flew. Becquerel's words reflect
his adoption of their distinction between permanent
primary radioactivity and temporary induced effects, but
his influential statements are in need of analysis:
Laperted'activite, qui est le propre des corps activ6s ou induits, montre que le baryum n'a pas entraine la partie essentiellement active et permanente de l'uranium.30
The active barium's decline was thus equivocally explained. Did he mean that the barium had merely become temporarily induced? Or was he suggesting that the barium had in fact
extracted one impurity, temporarily active, from uranium
leaving another, permanently radioactive, behind? Evidently
Crookes' preparation of an inactive Ur would fit neither
of these; but it was not to be Becquerel who proved him
wrong. One might expect that Becquerel's conjectures as to the means by which uranium spontaneously regained its depleted activity would indicate his preference. But this
is not the case; instead he employed what appears to be
an uneasy combination of the two. He compared uranium's
revival with the well-known rise in activity of freshly
precipitated radium-barium salts and to explain both of
these he proposed 'L'hypothese d'une auto-induction'.
Becquerel suggested that this could occur in mixtures of
active and inactive substances 'et mgme a une combinaison chimique de molecules'; this comment may be better under-
stood by replacing 'molecules' with 'atomes'. His final
remark on the point was perhaps intended to cover any
loopholes concerning the attribution of an element's own
activity: 'pour un corps purl elle equivaut a celle d'une transformation molgiculairel. If one similarly substitutes
atomic for molecular then there appears one of the vital points
against which the Curies reacted.
189
Certainly in their note 'Sur les corps radioactifs',31 read to the Academy by Becquerel himself a few weeks later
in January 1902, the Curies attributed to him a theory of
'transformation atomique'. This they attacked in both
specific and general terms but in doing so left some of
their own difficulties exposed. Their first point was
directed against the hypothesis that a process of auto-
induction operated to revive diminished activities.
'Certaines exp6riences, mal interpret-6es', they wrote 'conduirent a admettre une destruction partielle de la puissance du radium'. They insisted that on the contrary
each of the known radioelements had always exhibited the
same unvarying activity when placed in the same physical
and chemical state. It must be pointed out that this
straightforward statement immediately compounded the
problems of polonium which element they therefore committed
to the footnotes as an 'exception', branded 'une espece
de bismuth active'. The Caries concluded the argument
regarding induction with the claim that neither the laws
of dissipation of radioactive energy nor the effects of
physical and chemical state were known; this appears to
be a retreat from the assertions of the previous summer.
The second part of their reasoning was directed both
against the notion of the emission of material rays and
against the related concept of atomic transformation; it
was conducted in mainly energetic terms. If the source
lay within the radioatom in the form of potential energy
then the activity should decline. The Curies considered
that this was contrary to their observations. Alternatively,
the atom could be a transformer of external energy. By
stressing the scientist's ignorance of the medium surrounding
him the Curies in effect defended such a viewpoint
notwithstanding the evident violation of Carnot's principle.
They placed Becquerel's explanation of induced activity
and J.Perrin's theory of the origin of radioactive radiations
into the internal category and labelled both of these,
perhaps questionably, as theories of 'transformation
atomique'. Gone was the Curies' acceptance of the material
190
nature of cathode and radioactive rays on the grounds
that electrical charge had always been associated with
matter.32 Instead they pointed, without calculations,
to the negative results of their experiments designed
to detect a loss in weight from radium. It is remarkable
how completely the Curies' joint statements of 1902
contradict Marie Curie's own apparently favourable dis-
cussions of a disintegration hypothesis of atomic trans-
formation published but eighteen months before.33 The verdict seems now to have been that Becquerel's theory
was at best premature: their final word concerned the
procedures suitable for the attainment of scientific
knowledge and implied that there might be no truth whatsoever
in such hypotheses. The arguments of the Curies may appear
unhelpful but at least these ensured a thorough airing of
the issues. It is doubtful whether they stemmed the advance
of disintegration theories.
Becquerel, for example, repeated his ideas 'On the
radio-activity of matter' in a slightly abbreviated form
a few months later in March 190234 and maintained these
for several years. Before continuing the discussion of
comparable speculations which also appeared at this time
let us examine the prior statements of J.Perrin which
were critically cited by the Curies. In his popular
lecture of 1901 entitled 'Les hypotheses moleculaires'35
Perrin attempted briefly and largely qualitatively to
explain spectroscopic, chemical and radioactive phenomena
in terms of a singular atomic structure. Concentrating
the positive charge into one or more central 'soleils' he
made the arrangement and orbital motion of corpuscles
surrounding each of these account for valencies and
spectral frequencies; radioactivity received a brief
explanation: Si l'atome est tree lourd, c'est-a-dire probablement tree grand, be corpuscule le plus oloigne du centre - le Neptune du systeme - sera mal retenu dans sa course par l'attraction blectrique du reste de l'atome; la moindre cause l'en dftachera; la formation des rayons cathodiques pourra devenir tellement facile que la matiere paraisse spontanement radio-active... 36
191
Should this necessarily be called a theory of atomic
transformation as the Curies said? G.Martin, whose
similar view is described below, might not have done so.
One can ask the same question of Thomson's explanation
of ionisation, involving the temporary separation of a
corpuscle, which he held continuously from 1899. And
our answer may be provided by his own rejection for a
time of a transformation theory of radioactivity in
favour of a mechanism of minimal corpuscular ionisation.
Crookes too published an explanation of radioactive
phenomena which involved corpuscular dissociation without
atomic transformation. Its expression, early in 1902,
requires some clarification which can be obtained by an
examination of the development of Crookes' ideas. We
have seen37 that during 1898-9 he had proposed that the
energy of radioactivity came from the kinetic energy of
the surrounding air molecules. He then discussed with
Stokes experiments, such as the influence of air pressure
on radiation, which might serve to distinguish between his
own idea and the latter's uranium molecule 'wagtail'
hypothesis.38 By the end of 1900 they were communicating
on the deflecting effect of the magnet upon radium rays.
In his recent work on this subject R.J.Strutt39 had rather
confusingly used the term 'emanation' for what was more
often described as 'radiations'. Stokes adopted the
former expression and interpreted Strutt's results in an
individual manner. He considered that the 'emanation'
consisted of two different portions, namely 'rays', or
ether waves, and 'jets', or molecular projectiles.
Crookes replied40 that he did not now accept such a
dichotomy but was 'inclined to think that all the radio-
active actions are to be accounted for by the theory of
"bodies smaller than atoms"'. His conception of radio-
activity in these material terms involved an unusual
interpretation of Thomson's theory in which the corpuscles
were supposed to act in the manner of a gas, diffusing
slowly through certain materials:
It may be urged that Thomson's ultra-atomic particles are only existent theoretically, and
192
no instance is known of such a phenomenon as a gas or projection or emanation passing through matter ... Now however it has been shown by Villard (C.R. June 25, 1900) that hydrogen will pass through fused quartz at a red heat ... Now if a dense body like hydrogen gas will get through quartz, how much more easily will particles much smaller than the ordinary chemical atom get through glass, aluminium and black paper?41
Whilst some physicists had accepted earlier in 1900 that
materials were penetrated by high velocity radium projections
Villard's belief on the other hand was that hydrogen itself
rather than subatomic particles comprised the radiant matter of the vacuum tube.42 Crookes however, after seeing Rutherford's paper on the new gas from radium, in the June 1901 issue of Nature emphasised his point to Stokes: 'I
cannot agree that the chief radio-active body in pitchblende
is a gas, in the ordinary sense of the word'.43 Instead that position was held by the 'Thomsonian ultra-atomic' particles which, after emerging, temporarily behaved like a gas. Crookes' additional suggestion that some bodies
might be capable of 'temporarily fixing additional atoms of electricity - unstable perelectrides' then expelling
these surplus atoms of electricity came with an apology:
'Forgive my crude speculations. I feel as it were groping in an unknown laboratory in the dark'. His laboratory work may have furthered this feeling. Among other experi-
ments some attempts made during 1901-2 to prepare an
inactive thorium by the successive entrainment method are
recorded in his notebooks.44 These show 'EA' and 'PA' (electrical and photo activity) moving irregularly in
opposite directions as well as problems with leaking
electrometers.45 He felt sufficient confidence in some of
his results to publish early in 1902 a paper on 'Radio-
activity and the Electron Theory'.46 The view that 'Electrons emanating from radio-active bodies behave like
material particles and are impeded by the molecules of the
surrounding medium' Crookes now illustrated by the diffuse
photographic effects which he had obtained. In this manner
he explained emanations and radiations by the same means;
other researchers might have considered that he had not.
193
clearly distinguished these entities. Crookes accepted
Strutt's suggestion that the emission of subatomic
corpuscles was matched by the release of large positive
ions and likened this to his own earlier conclusions
concerning the electrical evaporation of metals. It
could be said that his theory included a minimal atomic
dissociation but that he was far from any consideration
of radioactivity in terms of atomic transformation or
irreversible disintegration. He believed until mid-1903
that the molecules of the surrounding air provided the
source of radioactive energy.
The young chemist Geoffrey Martin began his communi-
cations on radioactivity by expressing views akin to those
of Perrin and Crookes but he soon proceeded well beyond
them. The first of his series of letters to the Chemical News47 concerned 'Radio-activity and Atomic Weight'. In
it he broached the subject of a possible connection
between radioactivity and variable valency via a process
of ionic interchange, which he saw as the basis of both
phenomena. He thus arrived 'at the conception of very
heavy metals continually casting out into space light ions,
until finally their supply runs short or diminishes'.
Evidently no permanent atomic change was envisaged here:
if the exhausted metal were chemically 'treated with
another body full of such particles ... the heavy element
will abstract from it a sufficient quantity to replenish
its store, and thus the radio-activity increases again'.
One may possibly discern a movement towards his forth-
coming depiction of a more destructive atomic process in
Martin's letter on 'Prout's Hypothesis and Radio-active
Elements'.48 Though this simply raised the question of
whether heavy metallic impurities might be the cause of
some of the observed deviations of atomic weights from
whole numbers, it tended marginally to strengthen the
tenuous link between the subjects of the title. It may
also be recalled that R.J.Strutt published separate
articles on radioactivity and upon Prout's hypothesis in
1901. Martin's publication of most significance for the
emergence of atomic disintegration theories appeared
194
early in 1902 shortly after Crookes' paper on ultra-atomic
diffusion. Endowed with the title 'The radio-active elements
considered as examples of elements undergoing decomposition
at ordinary temperatures. Together with a discussion of
their relationship TO the other elements',49 its contents
may be described as a collection of speculations, some
well-worn others premature, all loosely linked to the
experimental basis of the hypothesis of subatomic electrons.
Martin considered that the long-sought laboratory evidence
which could combine with the work of Lockyer to support
the notion of a common 'protyle' was now at hand. In his
opinion decomposition of atoms occurred not only at the
high laboratory temperatures which caused any metal to
ionise gases but also spontaneously at room temperature.
He cited as experimental support the view of the Curies,
which they had themselves revoked shortly before, that
'radio-active matter is at ordinary temperatures giving
off electrons (and other particles?)', and put Russell's
studies of photo-active zinc and hydrogen peroxide into
the same supporting bracket. These points together with
the correlation of high atomic weight with both variable
valency and radioactivity took him beyond his former
position regarding radioactive atoms. He now supposed
that these atoms suffered 'incipient decomposition' and
were completely 'shattered' into positive, negative and
'inactive' particles; these latter 'which may be very small
indeed' comprised 'the unelectrified matter which composes
the bulk of the atom'. This statement shows that Martin
probably agreed with W.Crookes, L.Boltzmann, W.Sutherland
and others in his conception of the atom as a spherical
solid mass furnished with electrical or other appendages.50
His idea of atomic decomposition would therefore seem to
be more revolutionary than that deriving from the corpus-
cular theory of matter. Nevertheless Martin's comments
evidently covered but a small proportion of the known
phenomena of radioactivity. He later claimed priority
with the leading question 'Who first suggested that the
radio-active elements are elements undergoing decomposition
at ordinary temperatures?'51 But the appearance of his
195
paper in 1902 is perhaps best looked upon as part of a
general upsurge of discussion concerning the decomposition
of atoms.
During that year a subtly different set of ideas
appeared in a textbook of J.Stark, privatdocent at
G6ttingen University, entitled Die Elektrizitdt in Gasen.52
Stark advocated an electronic view of matter of the kind
then gaining ground among physicists; an approach which,
in part independently of radioactivity, gave hopes for the
transformation of the elements.53 Supposing that the chemical
atom consisted of equal numbers of negative and as yet
undetected positive electrons, each of which comprised a
vortex in the ether, he provided explanations of the
various phenomena of electrical conduction and chemical
valency in terms of electronic dissociation. As for the
emission of electrons by radioactive substances he conject-
ured in the first place that the electrification thus lost
could be regained by the capture of negative electrons from
the surroundings. Stark's second suggestion, that electrical
neutrality might also be maintained by the loss of positive
electricity, indicates his view that the emanations and
induced activity consisted of positively charged material
particles, presumably atoms or molecules.54 However, he
failed to make any theoretical connection between such
phenomena and his suggestive ideas on the pressing question
of the source of radioactive energy. Stark introduced his
answer to the energy problem by calling upon what were
becoming standard arguments, based on spectroscopic and
ionisation studies, against the indivisibility of the
chemists' atom. His conjecture, which was accompanied by
rough numerical estimates, that the high temperature of
many celestial bodies was due partly to the electronic
'Genesis der Atome'55 effectively inverted the view of
Lockyer and Martin that heat was the cause of elemental
dissociation. The naturally radioactive substances with
their continuous release of electrons and energy Stark saw
as remnants of this cosmical process. They were elements
which having been stable at the astronomical temperature
of their formation now possessed the property of slowly
196
dissociating and recombining into stabler forms.56 But whether these were chemically distinct entities he did
not say. As Stark completed his speculative exposition
at Easter 1902 the race towards a successful theory of
radioactivity was almost won. Rutherford and Soddy were
already watching the exothermal creation of new radioactive
elements in their laboratory.
2. A quantitative theory of atomic transmutation (1902)
I am now busy writing up papers for publication and doing fresh work. I have to keep going, as there are always people on my track. I have to publish my present work as rapidly as possible in order to keep in the race. The best sprinters in this road of investigation are Becquerel and the Curies in Paris, who have done a great deal of very important work in the subject of radio-active bodies during the last few years.
Thus wrote Rutherford to his mother in the first week of
190257 soon after seeing the latest paper of Becquerel;
he knew also that Crookes was about to publish.
Just six years earlier, shortly before the original
discovery of uranium rays, he had written of the new X-ray
photographs:
One of a frog is very good ... The Professor of course is trying to find out the real cause and nature of the waves, and the great object is to find out the theory of the matter before anyone else, for nearly every Professor in Europe is now on the warpath...58
197
Though no single scientist emerged to dominate the study
of X-rays E.Rutherford and F.Soddy, physicist and chemist,
not only stayed in the radioactive race but ran out clear
if disputed winners. The strong field comprised similar
mixed teams of P.Curie and A.Debierne, G.G.Stokes and
W.Crookes, also researchers of a single discipline such as
Elster and Geitel, or Becquerel. By late 1903 the leading
pair had forged sufficiently far ahead for one of its
partners privately to pour scorn upon such reputable
competitors.59 As Rutherford's letter shows, the situation
early in 1902 was quite different. The manuscript he had
just mailed to London was but the first of a score of
papers published during those two years, about half of
them jointly with Soddy, which established a position of
superiority only gradually.
During that period at McGill these researchers
developed a theory of atomic transformation which passed
rapidly through several phases. Fortunately Rutherford
and Soddy provided in their joint publications fairly
explicit discussions of experimental studies and theoretical
problems concerning the relationship between thorium and
ThX. This aspect of radioactivity provided them with the
most successful quantitative test of their theory. This
furrow has been ploughed deeper by successive writers who
have thereby followed the steps leading to the attainment
of the 'full' theory of 1903.60 Much is left to be said
even along these lines. However, my intention is to place
those thorium studies in a wider perspective by viewing
them both as a continuation of earlier developments and as
a part of the contemporary network of radioactive investi-
gations. Emphasis will be placed on the origins of the
theory of disintegration rather than upon its experimental
confirmation. We shall see for example that even before
the discovery of ThX a notion of chemical transmutation
in radioactivity had entered the laboratory; this was
itself related to prior discussions. It is notable that
the two known members of the uranium series and four or
five of the thorium sequence featured together in
Rutherford's and Soddy's earliest exposition of the
198
disintegration theory; that all of these contributed to
its origins as well as to its extensions is a possibility
which should not be ignored. The influence and implications
throughout the period of the theory of induction and of
the work of scientists such as Becquerel, Crookes, Curie,
Dorn, Giesel and others are also considered.
Rutherford perhaps felt that to stay the course of
radioactivity he would need more than minor chemical
assistance. By the end of May 1901 some of the most
interesting questions were chemical ones. That these had
been created by Rutherford is an indication of that
physicist's chemical leanings, the origins of which we
have sought in earlier Chapters to unearth. He had
subjected different thorium salts to the action of heat
and had concluded that the production of emanation was a
kind of chemical process highly dependent on temperature.
He was sure that radium emanation was in reality a non-
radium 'heavy radioactive vapour or gas'. And a notable
change in interpretation was that instead of viewing
excited activity as a deposit of thorium emanation
positively charged, Rutherford now considered that the
similar radium emanation 'manufactures from itself a
positively charged substance'.61 Whether or not this
material difference between emanation and active deposit
had come to the fore by virtue of evidence additional to
the chemical pointers of 1899 it is not clear. Certainly
by May 1901 there had been discussions with a chemist at
McGill but these may not yet have been constructive.
In the autumn of 1901 F.Soddy, Demonstrator in
Chemistry at McGill University since the summer of 1900,
joined Rutherford's investigations.62 However, prior to
their experimental union the partners had clashed mightily
at a McGill Physical Society debate in March 1901.
Rutherford commented in a letter to J.J.Thomson, mainly
concerning appointments, that in a forthcoming 'great
discussion' on the latter's physical corpuscular diss-
ociation theory 'we hope to demolish the Chemists'.63
According to Soddy's report to his biographer long after-
wards the physicists were quite unable to do this64 in
199
the face of his own vigorous debating-style attack.65
He spoke against the evidence for atomic dissociation
provided by Lockyer and pointed to the weakness in
Thomson's early statement on the mie ratio for cathode
ray corpuscles; the latter had stated that e might be
large as well as m small. Perhaps Soddy became familiar
with and less antagonistic towards the more recent
studies which provided separate estimates of e. His
later statement that he had always been sceptical of 'the
electrical theory of matter'66 may not exclude this
possibility; a purely electrical view of matter did not
become widely acceptable to physicists, including Thomson,67
until after 1901. Despite these theoretical disagreements
Soddy though not Rutherford's first choice as an
assistant68 no doubt seemed a good one since he was
involved in lecturing on gas analysis at McGill during
190169 and claims to have been familiar with this subject
and with the inert gases before leaving Oxford.70 A series of exciting discoveries and allied interpretations was
soon to make Soddy the champion of transmutation rather
than its challenger.
In a lengthy publication entitled 'The Radioactivity
of Thorium Compounds.I. An Investigation of the Radioactive
Emanation',71 whose abstract72 was read at the Ordinary
Meeting of the Chemical Society of London on 16th January
1902, Rutherford and Soddy described the many fruits of
their first few months of united labour. Prime among these
were two remarkable conclusions. Firstly, thorium emanation
belonged to the family of the recently discovered inert
gases. And secondly, discovered late in the day we are
told, it was not in fact thorium which produced thorium
emanation. These deductions constituted part of the
answers to the five largely chemical questions explicitly
posed; though one cannot apportion the authorship the
200
interested physicist was evidently capable of formulating,
if not of answering, each of these. Did the emanation
come from thorium itself or from a 'foreign substance';
could the almost non-emanating thorium oxide, rendered
thus by excessive heating, recover this property; was
the radioactive gas chemically similar to any known
matter; did its emission cause a loss in weight; was there
anything in the chemistry of thorium to account for its
'almost unique power' of giving an emanation?73 Many of
the experimental techniques involved in these new investi-
gations were modifications of or improvements on those
employed earlier by Rutherford. For example, in measuring
the conductivity produced by thin and thick layers of a
powdered thorium compound in combination respectively with
an air draught or screening, the researchers were satisfied
that their separate estimates of the direct radiation and
emanation emerging from the same specimen were accurate
to within one or two per cent. Their preferred method for
determining the emanation, which avoided waiting for
equilibrium, was to pass it at a known speed down a tube
along which a number of electrodes were spaced. The
successively lower electric currents detected at these
points gave a value for the emanation which agreed well
with the simpler arrangement; each method was comparative
in conception with a 10 gm. sample of thorium oxide taken
as the standard.74 These techniques, which can be traced
back to the time of Rutherford's first studies of thorium
emanation in 1899 or earlier, were vital for Soddy's
answer to the question of the chemical nature of this
entity. Able thus to estimate the emanation's quantity
by means of its radiations, while they lasted, Soddy found
that this gaseous material refused to combine with any
reagent. The conclusion which the investigators boldly
announced was that the emanation belonged to the recently
established75 family of inert gas elements.76
Whilst acquiring this understanding of its nature
Rutherford and Soddy enquired experimentally after the
emanation's source. In response to their own question
201
'Is the Emanating Power a Specific Property of Thorium?'77 they reported that samples of thorium sulphate from
opposite ends of a fractionation each exhibited the same
intensity of direct radiation and identical emanating
powers. The entailed affirmative answer, though soon to
be stifled, was backed by further evidence. For Soddy
was able in effect to smooth out the sharp differences in
emanating power between oxide specimens subjected to
different heat treatments. Such variations had contributed
to Rutherford's description of the phenomenon as a heat-
dependent chemical process. 'The Regeneration of the
Emanating Power by Chemical Processes'78 which they claimed to have achieved was impressive though irregular and
incomplete. The ordinary oxide (adopted as the standard
at 100%), after de-emanation by ignition (to 10% power),
dissolution, then reconversion to the oxide, exhibited a
partial recovery of its emanating property. Following
such a cycle via the sulphate thorium oxide turned out as
high as 40% effective, via the chloride 55%. But a
different chemical conversion tried by Soddy gave quite
different results which, however, tended to make the
situation clearer. Both ordinary and ignited thorium
oxides when dissolved then reprecipitated as the hydroxide
were endowed with enhanced powers of emanation. And in
addition these hydroxides' values rose spontaneously
during the course of a week or so from an initial in-
equality (hydroxide from normal thoria, 108%; from ignited
thoria, 128%) to exact equality at a high level (about
250% of the standard). Furthermore they were able to
clarify the effect of moisture79 thus freeing themselves
from the influence of Dorn's suggestion of a 'physikalisch-
chemischen Prozess'. Rutherford and Soddy were beginning
to realise that these chemical and physical influences
might only be superficial. They noted, for example, 'that
the cause of the emanating power is not removed by ignition,
but only rendered, for the time being, inoperative'.80 The
authors' statement that the evidence 'certainly seemed to
point to the conclusion that the power of giving an
202
emanation is really a specific property of thorium'81 is
doubly significant. Firstly its expression gives an
unusual illustration of conclusions which were already
withdrawn. Now the word 'specific' was practically
synonymous with the term 'atomic' which others such as
the Curies tended to use in this context. Thus, secondly,
the proof of a chemical transmutation may briefly have
become apparent.
It may well have appeared so to Soddy who remarked,
on looking back more than fifty years,82 that he had first realised in 1900(sic) that a genuine chemical transmutation
of thorium to an inert gas was at hand. In the published
paper one reads the cautious statement that though it was
'perhaps early' for theoretical discussion one of two
possible alternatives was:
to look upon the emanation as consisting of a gas emitted by the thorium compound. It is not necessary that such should contain thorium, it might conceivably be an inert gas continuously emitted in the radioactive state.83
But this is very similar to their better known assertions
made in the spring of 1902 which are generally taken to
mark the great innovation of atomic transformation. One
might argue instead that radioactive transmutation had
been conceived by Rutherford and Soddy as an experimental
reality before the end of 1901. Rutherford's recollection
long afterwards was that:
The great contrast in the physical and chemical properties of thorium X and the emanation gave us the first definite clue that radio-activity was a consequence of the successive transformation of elements and led ultimately to the disintegration theory...84
'Definite clue' is an ambiguous phrase; but Rutherford
unlike Soddy evidently refers to the period following the
time in about December 1901 when the idea of the direct
production of the emanation by thorium became untenable
and thorium X appeared.
In a section which forms an appendix to their first
paper Rutherford and Soddy admitted that 'since the
preceding account was written developments have been made
203
in the subject which completely alter the aspect of the
whole question of emanating power and radioactivity'.85
The revelation was twofold; its cause was their discovery
that both emanating power and direct radioactivity, the
latter so far unaltered in all experiments which affected
the former, were properties not of thorium but of a
different substance. This they labelled ThX. The conn-
ection with W.Crookes' earlier conclusions is an interesting
one which should perhaps be brought out. Rutherford and
Soddy stated, rather surprisingly it seems, that their
negative results in fractionating thorium sulphate were
obtained before they knew of his similar work.86 Yet it is true that almost everyone else who wrote on radioactivity
had referred to Crookes' important preparation of inactive
Ur and his discovery of UrX shortly after the announcement
in May 1900. Rutherford, however, left for New Zealand to
get married at just this time and although he may have
read the Curies' and Becquerel's papers to the Paris
Congress of August 190087 each of these referred to Crookes'
discoveries with uranium alone. Only Baskerville's dis-
cussion 'On the existence of a new element associated with
thoriuml88 published shortly before Soddy began these
studies, and Crookes' own publication, described the
latter's failure with thorium sulphate but his partial
success in fractionating the nitrate. However, Rutherford
and Soddy made no mention of this work on the nitrate; they
merely pointed out that the photographic methods used by
the above chemists were qualitative and incapable of dis-
tinguishing between thorium rays and those from the eman-
ation.89 Nevertheless, the McGill scientists may have owed
some debt to these nitrate studies; they were certainly
aware of them. In his paper Crookes had mentioned the
German commercial source of a highly purified thorium
nitrate; Rutherford wrote asking him to forward a request
for this material. Crookes replied welcoming Rutherford's
promised paper to the Chemical Society, stating that he too
was preparing a publication, and reporting the 'curious
circumstance' of Becquerel's finding that an old specimen ,90 of inactive uranium nitrate 'had reassumed its radioactivity.
204
At that time, in December 1901, Rutherford and Soddy were
themselves studying the spontaneous recovery or increase
of the emanating power of thoria. Now since in their
view uranium gave no emanation the question of the revived
activity of this element was an open one. Evidently there
was much to consider even beyond the late additions and
alterations which appeared; here they described to their
readers the successful outcome of a new search for ThX,
the hypothetical emanating and radiating constituent in thorium.
Crookes' attempts to correlate the electrical and photographic activities of thorium with its chemical
treatment, which had yielded only irregular results, were
thus overtaken at the end of 1901. But the passage of
Rutherford and Soddy was not smooth. Their emanation
studies began to take on a coherent form as 'it was beg-
inning to be realized' that emanating power depended on
the 'previous history', or mode of preparation, as well
as upon the chemical nature of a compound.91 But the continuing approach towards the origin of this power was
disturbed by further striking irregularities. As they
noted in their late addition, whilst the powdered crystals
of thorium nitrate were of surprisingly low emanating power
(1.8% of standard thoria), quantitatively prepared solutions
whose study they had newly taken up possessed a very high
power (about 300%) independent of dilution. They inter-
preted the phenomenon as a 'latent emanating power'92 of
thorium nitrate in the solid state. 'Simultaneously with
this observation' they remarked, it was noticed that
preparations of thorium carbonate varied enormously in
emanating power according to their method of preparation .93
They recorded a notable vagary, evidently one of many,94
in which Soddy precipitated thorium carbonate from the
nitrate by means of sodium carbonate, partially redissolved
the carbonate with nitric acid, presumably removed the
remaining solid, then reprecipitated the dissolved portion
as hydroxide using a solution of ammonia. 'The result was
remarkable: the carbonate had an emanating power of only
6 per cent, the hydroxide one of 1225 per cent of that of
205
the ordinary oxide'.95 It was perhaps equally remarkable
that the hydroxide's emanating power then decreased
spontaneously to 1/3 value after 14 days whilst the
carbonate's remained constant. They ascribed this result
to an 'accident' of the conditions but did not let the
matter rest. Repetitions of the procedure gave totally
different results: the carbonate and hydroxide precipitates
were of approximately equal low powers (about 15%) which
in the manner usually anticipated rose spontaneously during
a week or two to values of 100 to 300%. The first carbonate
of very low emanating power displayed no abnormality in its
direct thorium rays and was turned into a normal emanating
sample of the carbonate on redissolution in acid, followed
by reprecipitation. But we are told that 'The production
of preparations of such low emanating power led naturally
to an examination being made of the filtrates and washingsq6
which should contain no metallic substance whatever. This
examination was indeed a fortunate step. Its well-known
results have been seen97 as a turning point. For although
'they should be chemically free from thorium' the ammonium
nitrate/hydroxide filtrates possessed a definite emanating
power and, after evaporation, direct radioactivity too.
Subsequent closer tests of the filtrates yielded a mysterious
white phosphate precipitate, highly active and 'in very
appreciable quantities', but they were able to dismiss this
as an irrelevant impurity.98 'The evidence of long series
of experiments in two directions' afforded them 'little
doubt of the actual existence' though in 'altogether minute
amount' of 'a constituent ThX to which the properties of
radioactivity and emanating power must be ascribed'.99
There seem to have been several implications of this
conclusion. Evidently one of these was that the direct
radiations of thorium as well as its emanating power were
now in question. Another relates to parallel studies
under way in Paris. Curie and Debierne had announced in
July that ordinary water could readily acquire induced
radioactivity,100 or 'emanating power' in Rutherford's
and Soddy's terms. In the light of Curie's experiments
and theories it was surely to be expected that all
206
filtrates from thorium or radium would posses an emanating
power which might in turn be transformed into direct
radiation: Rutherford indeed found that the solid traces
in thorium filtrates emitted such rays. But the fact that
this activity could be made far higher than that of thorium
itself, by factors of up to 1800, appears to confirm ThX
after the fashion of its predecessors Po, Ra, UrX and
others. Rutherford and Soddy noted that its inconsistent
chemical properties, for example solubility in and prec-
ipitation by hydroxide or phosphate, could be explained by
entrainment of the minute amounts of ThX present. And they
made the identification of ThX safer by the technique of
penetration analysis of the radiations, a method which was
becoming increasingly important. The rays from the active
residue were identical with thorium rays and different
from the several other varieties; furthermore this residue
produced an emanation whose activity decayed at a rate
identical to that from thorium. Having thus attributed
the entire radioactive phenomena of thorium to ThX they
considered naturally that the preparation of a totally
inactive thorium would form a desirable confirmation. To
this end they reported their actual attainment, by repeat-
edly washing with water, of a 20% reduction in the activity of thorium.101 However, the path leading from this point,
like that leading to it, was not a straight one.
During the course of their later experiments of 1901
Rutherford and Soddy achieved the beginnings of a unific-
ation of the two major aspects of thorium's, now ThX's,
radioactivity. That the 'straight line' radiation of
thoria remained constant throughout wide fluctuations in
its emanating power had at first served 'to bring out the
fact' that these two powers were independent.102 The apparent difficulty of reconciling this with their con-
clusions that each was connected with the thorium atom or
molecule was soon to be eased. In directing attention to
the 'straight line radioactivity, which is generally un-
affected by these changes of conditions and previous
history' in order to follow 'the progress of the removal
of the active material', Rutherford and Soddy flatly
207
contradicted their initial statement with the comment
that 'The two phenomena are undoubtedly connected'.103
The empirical correlation, whioh had arisen from the use
of solutions, was soon to become of great theoretical significance. From the modern viewpoint one sees a con-
tinuous emission of emanation from ThX which is itself
continuously produced by thorium; when formed within the
various solid compounds of thorium the actual rate of
release of the gas is complicated greatly by a temperature-
dependent process of occlusion. By April 1902 these scientists suspected all this.104 But in December 1901 they thought that 'the surprising uniformity' of the
emanating powers of variously treated thorium compounds,
despite the known loss of a large proportion of the supposed
emanating source ThX made the process appear:
rather as the result of a dynamical change, possibly in the nature of a chemical reaction where the active mass of emanating material is a constant, than as the property of a peculiar kind of matter in the static state, additive with regard to mass.105
Here 'active mass' is a term drawn from chemical reaction
kinetics rather than radioactivity; 'effective mass' may be a clearer substitute.
The vision of a strange chemioal reaction which they
saw here was obscurely mirrored in Rutherford's parallel
and neglected researches on the excited radioactivity
produced by the emanations. During the Christmas vacation
of 1901 at about the time of completion of the joint paper
on thorium emanation and ThX he presented to the American
Physical Society in New York papers on the 'Transmission
of Excited Radioactivity'106 of thorium and radium, and
on 'Excited Radioactivity and Ionization of the Atmosphere,107 To the physicists Rutherford revealed some of the mechanisms
he had in mind. Regarding the question of the positive
charge of excited activity Rutherford noted his own earlier
208
explanation that this might be caused by condensation of
the emanation around the positive ions produced by its
radiation, and Thomson's alternative idea of an average positive charge left by the temporary loss of a corpuscle.108
The version he preferred, however, which could better
account for the suppresion of the charge effect at low
pressure was an extension of Thomson's view: the emission
of a material corpuscle or electron at its high velocity of 1010 cm./sec. would impart an opposite impulse to the
remaining positively charged molecule. This could fling
it contrary to the field against the positive electrode.109 Presumably this molecule would have to pick up another
electron or a negative ion before actually adhering to the 10 electrode to form the active deposit. Thus, like Becquerel,1
Rutherford saw the first step in the production of excited
activity as the violent emission of an electron. But he was as yet prepared openly to discuss the notion of a
minimal disintegration only. Rutherford thought that the
radiation from excited radioactivity was caused by the recoil vibrations within each deposited molecule. 'Da es unwahr-scheinlich ist, days innere Schwingungen von Molekulen
verschiedener chemischer Natur sowohl nach Charakter wie
naoh Dauer dieselben sind' it was to be expected that the
induced activities from radium, thorium and perhaps
atomospheric air would differ both in duration and pene-
tration, as they did.111 However, Rutherford's explanation
of the electrical character of its transmission left open
many questions regarding excited radioactivity.
These can be isolated and grouped into three areas.
Firstly there were problems concerning atmospheric excited
activity. Whilst a chemical audience read or heard that
the existence of an atmospheric emanation was most probable112
to the physicists Rutherford spoke equivocally. Although he
accepted that its electrical properties and the duration
and penetration of its radiation appeared to identify an
atmospheric deposit he queried, on the basis of two
observations, Elster and Geitel's assignment of the cause
of this deposit to an atmospheric emanation. The conduct-
ivity of a sealed mass of air failed to decline, remaining
209
constant for a month; and carbon dioxide exhibited the
same properties as air.113 The contradiction was soon to
be eased114 by C.T.R.Wilson's conclusions, based on
pressure-variation experiments, that the walls of the
vessel contributed to the 'Spontaneous Ionisation of
Gases'.115 But Rutherford's only visible step towards
clarification at the end of 1901 was his statement that
experiments were in progress to see whether any of the components of the atmosphere, prepared chemically, dis-
played sufficient ionisation and excited activity to
account for the observed atmospheric effects.116 Whilst it
is not entirely clear how these might affect the problem
such experiments seem relevant to statements published
with Soddy on thorium emanation. Their joint abstract
boldly denied that the emanation might be a mere sport of
induction:
The possible explanation that the emanation is the manifestation of excited radioactivity on the surrounding atmosphere was shown to be untenable by a crucial experiment.117
Vernon Harcourt who read this at the meeting and who had
been one of Soddy's referees at Oxford seems nonetheless
to have been unimpressed. He pointed in a different
direction to Russell's work on hydrogen peroxide emana-
tions.118 However, in the full paper Rutherford and Soddy
had dismissed the Russell effect as having no connection
with radioactivity and had written with more caution
regarding induction. They had performed certain experiments
which proved the amount of emanation emerging from a source to be independent of the type of gaseous carrier and others
which showed that the emanation differed from the trans-
porting medium in its resistance to any chemical attack.
Bitt they admitted that these facts still left open the
possibility that one of the inert gases known to be present
in the atmosphere might be 'rendered radioactive in the
presence of thoria'.119 It is of interest that the authors referred to experiments in progress which appear comple-
mentary to those mentioned by Rutherford in the context of
a supposed atmospheric emanation; they hoped to measure the
210
emanating power of a specimen placed in a current of gas
as free from air as possible. No results were published;
perhaps this test came to appear unnecessary during the
following months.
A second group of problems relating to excited
radioactivity and its transmission which existed at the
end of 1901 may be loosely described as chemical. Whether
Rutherford's complete avoidance of any description of
thorium emanation as an elemental inert gas was deliberate
one can only speculate. It is notable however that he
expressed doubt as to the 'Ausstramungsanschauung' or
emanation view of atmospheric effects in favour of the
'Elektronhypotheses.120 Yet he did not say what variety
of molecules in the atmosphere might be supposed to emit
the electron. In his account of 'Versuche fiber erregte
Radioaktivitdt' dated mid-January 1902121 Rutherford confirmed that the rate of decline of the radioactivity
excited by thorium suffered no change when this deposit
found itself in acid solution. Having deduced therefrom
that the decay was a process occurring within the active
substance he gave no further indication of what this
substance might consist. And, thirdly, the process of
radioactive decay was itself made to seem more complex by
his discovery122 that the weak excited activity produced
upon a wire by brief exposure to thorium in fact spontan-
eously increased in intensity for an hour or two before
declining at the known rate. The activity excited by
radium showed something similar.123 The curves were unaffec-
ted on heating the wire or plate to redness; nor could any
secondary emanation be detected. Rutherford concluded
without elaboration that this pattern of changing radiation
was caused either by a gradual molecular rearrangement or
chemical reaction, or a second excited activity produced
upon the surface by the first, presumably without an
intermediate emanation. Evidently the phenomenon of a rise
in activity could not be explained so readily as the usual
decay. It is interesting to speculate whether that dis-
covery had an influence upon the emerging theory of atomic
211
transformation which was to bring these diverse facts
rapidly to order. This spontaneous rise in activity was
the first of three which appeared almost simultaneously.
The second occurred in Becquerel's deactivated uranium.
And the third involved the vital extraction of ThX. All
three took their place in the famous joint publication
which followed within months.
Rutherford and Soddy probably read Crookes' letter
upon their return to the laboratory after Christmas 1901.
On checking their partially deactivated thorium, which
they no doubt would have done as a matter of course, they
found that like Becquerel's nearly inactive uranium it too
had regained its full direct radioactivity. That of the
extracted ThX had declined almost to zero. They may have
seen at the same time the paper which Becquerel read on
9th December 1901. Rutherford mentioned this scientist
when he wrote home on 5th January and in the second joint
paper with Soddy124 where credit was assigned for the
discoveries of the respective revival and decay of the
activities of deactivated uranium and activated barium.
Becquerel's paper contained far more than this. He was no
more satisfied than were Rutherford and Soddy merely to
report an empirical discovery; some of the ideas he expressed
may have seemed highly significant at the time. As we have
seen125 Becquerel was first to publish the valuable sugg-
estion that the mechanical recoil from a corpuscular
emission might play a part in radioactivity. He explained
radioactive induction in alternative ways either by direct
contact or by the deposition of the large positive particles
ejected on the emission of electrons, which remnants were
themselves subject to further division. The recovery of
uranium's activity he attributed to self-induction which
might constitute a 'transformation moleculaire'. By
ignoring his assumption of a radioactivity induced by
contact or direct rays one can extract the consequence that
the rise in uranium's radioactivity is entirely due to its
own disintegration into smaller atoms themselves radio-
active. Doubtless it could also have been deduced that
there might exist a linked series of distinct and
212
radioactive chemical products and that the apparently
constant maximum activity of uranium or thorium was a
resultant or equilibrium value of these changing
activities. By the time Becquerel's paper was published
Rutherford and Soddy already had an obscure knowledge
of such a chemical series, thus: ThX---) emanation—>first
active deposit (---? second active deposit); directly after
Christmas they added with their own discovery the vital
initial stage Th---ThX. Upon the basis of the quantit-
ative experimental examination of this step they were to
argue their theoretical case.
Thomson's unhappy news regarding Rutherford's
candidature for the Royal Society126 arrived at McGill early in May some days after Rutherford and Soddy had
sent off their second joint paper to the Chemical Society;
its confident title was 'The Radioactivity of Thorium
Compounds.II. The Cause and Nature of Radioactivity'.127
It is fortunate that they there included a brief history
of the events; (changes with the passage of time were now
of considerable importance. We are told that on reexamin-
ation after three weeks the thorium hydroxide which they
had deactivated to only 36% of the normal value had com-
pletely recovered, that the ThX residues 'had almost completely lost their original activity', and that 'At this
time, M.Becquerel'e paper ... came to hand':
A long series of observations was at once started to determine: (1) The rate of recovery of the activity of thorium rendered less active by removal of ThX, (2) The rate of decay of the activity of the separated ThX. 128
Using chemical methods similar to those employed previously
they obtained 'numerous series of observations made with
different preparations at different times'. Apart from
'the difficult questions' of 'initial irregularities' in
the first few days of each 3 to 4 week series, and an
appreciable unseparated or inseparable 'residual activity'
left in the thorium, these rise and decay curves matched
perfectly. They fitted respectively the related equations
213
ItiIo = e -Xt and It/Io = 1-e- >t with the same X. The
authors noted explicitly129 that these were identical in
form with the pair of equations which Rutherford had dev-
eloped in 1899 to describe the asymptotic rise of current
observed on steadily passing gaseous thorium emanation into
a vessel, and the geometric decline of activity which
ensued once this flow was stopped. The explanation which
they formulated may seem startling perhaps because it
could be 'put to experimental test very simply'. It was
this which made the Th-ThX relationship the main spearhead
of their theoretical claims. Like the arrival of particles
of emanation in the vessel 'the active constituent ThX is
being produced at a constant rate1130 and similarly its
activity then suffered a geometric decay. Thus the 'normal
or constant radioactivity possessed by thorium is an
equilibrium value', a balance of the two processes. One
of these effects, the decay of activity of the separated
ThX, was directly observable and the other, a continuous
growth of ThX itself in the thorium freed from it, almost
so. Having refined the separation techniques they proceeded
as follows. A double precipitation of thorium as hydroxide
from a solution of the nitrate left the ThX in the filtrates.
Evaporation of these gave a measure of the maximum or
equilibrium value of ThX's activity. A third precipitation
of the thorium after redissolution confirmed that it now
contained a negligible amount of ThX. Several thorium
hydroxide precipitates thus initially freed from ThX were
later dissolved and reprecipitated after different periods
of time from a few hours up to one month. The fact that
the new filtrates always possessed activities agreeing
with the normal recovery of deactivated thorium131 provided
a striking confirmation that 'The process of production of
ThX is continuous' and that this transformation was unaffected
by the separation procedure.
One of the major differences between the observed
measurements and those expected for a continuous arrival or
production of active material followed by a geometric decay
concerned the initial portion of each curve. The discrepancy
214
was complementary: the activity of freshly removed ThX
began by rising for a few days (by about 10%) before
decaying geometrically, and the deactivated Th actually
lost activity (about 15%) for the same period before
rising asymptotically. The thorium deactivated by
removal of all its ThX still possessed 46% of the normal
activity - a second discrepancy since according to theory
thorium should be inactive. Perhaps these were welcome
problems, for Rutherford and Soddy were able to use them
to justify their hypotheses at the same time helping to
tie together the entire incipient transformation series.
ThX was known to create excited radioactivity, via an
emanation, 'on surrounding inactive matter' so that the
46% 'residual activity of thorium might consist in whole
or part of a secondary or excited radioactivity produced
on the whole mass of the thorium compound by its association
with the ThX1.132 In brilliant style they tested this
supposition by preventing the ThX from producing any excited
activity. A series of 23 successive dissolutions and
reprecipitatione in 9 days to remove ThX from one tortured
thorium hydroxide specimen allowed the excited activity
initially present to decay completely, as the constant
minimum of their readings showed. This still left the
thorium with 25% of its normal activity - a discrepancy
smaller but harder to accomodate as will be seen. When
left to recover from this treatment the activity of the
specimen rose without the initial fall; to clinch the point
they showed that the 'difference curve' between ideal and
observed curves for ThX, both rising and falling, had a
half-value time of 12 hours which was equal to that for
the known decay of the 'ordinary excited activity'. There
was thus 'no reason to doubt that the effect is the same
as that produced by the thorium emanation, which is itself
a secondary effect of ThX'.133 The production of thorium
emanation from ThX took its place in the new scheme: just
as thorium produced the non-thorium material ThX, a 'further
transformation° of the latter resulted in the continuous
emission of a radiating inert gas; the sometimes highly
215
irregular results could readily be attributed to occlusion
or changes in crystal structure.134 Furthermore an analysis of the radiations showed that specimens from which the
emanation could not escape possessed the expected high
proportion of excited activity. Rutherford and Soddy were
not yet ready to say whether the production of emanation
imitated the primary change of thorium to ThX in proceeding
at a rate independent of the conditions;135 they were
prepared only to state that this applied to the decay of
the emanation's radiation. This aspect of thorium had
given rise to the first notion of its transformation to an
inert gas; but now the emanation took second place both in
sequence and in certainty. Rutherford and Soddy extended
their discussion beyond the confines of thorium: they went
so far as to suggest that the apparently constant activities
of uranium and radium too were the resultants of chemical
changes 'also independent of the conditions'. And describing
the radiated energy as a loss after each such change from a
supposed store within the system they came to the impressive
conclusion that 'All known types of radioactivity can thus
be brought into the same category'.136 It may be noted in
return that considerations of several varieties of radio-activity had contributed to the theoretical development.
It is true that the remarkable quantitative confirm-
ations with thorium and ThX increased the power of this theory far beyond any other. However, in its explanations
there were weaknesses. Some of these led to a broadening
of its success but others to criticism from without. In
a final discussion Rutherford and Soddy tried to justify
the applicability of expressions such as 'the chemical atom
in certain cases spontaneously breaking up with evolution
of energy' and 'sub-atomic chemical change'137 to radio-
activity but their chemical evidence was not strong. ThX,
the most important member of the thorium decay series from
a theoretical point of view, was perhaps also the least
certain. In their section on 'Chemical Properties of
ThX1138 radioactivity measurements correlated with precip-
itations formed the only evidence they could muster to
justify the statement that 'There can thus be no question
216
that both ThX and UrX are distinct types of matter with
definite chemical properties'.139 Though they believed
that 'transmutation' was the origin of these materials
such an expression did not appear in print; nor did they
describe these substances as 'elements' known or new. Yet
claims of the latter kind based on similar evidence had
played a vital part in radioactive studies since their
early days. Radium was the only thoroughly confirmed new
radioelement. It had been preceded on the scene by polonium
and had been followed by actinium and radiolead all of which
were of disputed status. As for UrX, Crookes and the Curies
thought it might be identified as actinium and Debierne
interpreted ThX similarly. As Rutherford and Soddy noted140
there was also the non-radioactive splitting of thorium by
Brauner and Baskerville to consider. Since 1899 each
claimant had to refute the possibility that his supposed
new radioelement was merely an induced activity. We have
seen141 how Hofmann defended his radiolead against
F.Giesel's accusation of induction. At the time of
Rutherford's and Soddy's first publication, in January 1902,
Hofmann reported his own results on the fractionation of
thorium. These led him to suppose that thorium itself was
really inactive and that it possessed only a temporary
radioactivity induced by uranium; he claimed to have success-
fully prepared inactive thorium from minerals which contained
no uranium. This work142 was not of a high electroscopic
standard143 and was not mentioned by Rutherford and Soddy
in their second publication. However, as with the emanation
previously, they were at pains to refute any suggestion
that the invisible ThX was no more than the manifestation
of a temporary activity induced by thorium upon some portion
of the neighbouring materials. They described such a view
as 'quite untenable'; if it were true any precipitation of
thorium should leave active residues in solution whereas
experiments showed that carbonate, oxalic acid, and
phosphate precipitated thorium leaving non-active solutions;
only ammonia was capable of the separation. It should
perhaps be recognised that this point taken in isolation
217
may have seemed less than convincing to others; for they
failed to dismiss explicitly the possibility that ThX was
a trace element in thorium which suffered temporary
activation by induction.
Rutherford and Soddy at this stage may themselves
have entertained the idea of direct induction in order to
explain a second questionable aspect of their theory.
What was the cause of 'The Non-separable Radioactivity of
Thorium',144 the 25% of the radioactivity remaining, when
both ThX and excited activity were absent, which did not
decay appreciably with time? One hypothesis of the two
they described involved the production of 'a second type
of excited activity' with a 'very slow rate of decay' by
ThX. It should be possible to observe this decay if
thorium were continually freed from ThX over a very long
period. Notably, their assumption that 'it will not be
possible to free thorium from this activity by chemical
means' suggests that by 'excited activity' in this instance
they meant a radioactivity directly induced into the
thorium. And since they referred to it as a 'second type
of excited activity ... similar to that known',145
Rutherford and Soddy may also have wavered towards an
induction view of ordinary excited activity. They appear,
at first sight, to have avoided the consequences of such a
belief by instead accepting a second hypothesis: the
initial transformation gave two products rather than one.
The constant residual activity would accordingly itself
be an equilibrium value and the other substance continually
produced by thorium should be chemically separable from it.
Magnetic analysis of the radiations tended to support this
view for the residual activity consisted only of non-
deviable rays whereas both ThX and the excited activity
emitted mixed radiations.146 But the main reason for their
conclusion regarding thorium's residual activity, as they
said,147 was the work of Soddy on uranium where both
emanation and excited activity were absent. His active
investigations of that element had probably been induced
by Becquerel. These were thus pursued throughout the
course of the ThX studies and contributed directly to the
218
early transformation theory generally associated exclusively
with thorium.
Soddy concluded his companion paper on 'The Radio-
activity of Uranium'148 by turning a current criticism
of his own emanation studies against the researches of
another; he asserted that the diffusing emanation from
uranium discussed by Crookes 'is not a radio-active substance
in the accepted sense of the word' but 'an agent similar to
hydrogen peroxide in its photographic action'. And Soddy
began this piece by announcing that the 'inactive Ur'
prepared according to Crookes was actually not inactive
electrically but only photographically; it emitted a non-
deviable and readily absorbed 'alpha' radiation and thus
possessed a 'residual activity' similar in character to
that of thorium. The same questions arose here:
1. Is this residual activity to be regarded as a secondary radiation produced by the presence of UrX? Or,
2. Is it caused by a distinct material substance capable of chemical separation? 149
According to the corresponding joint paper one might expect
definite answers; these were provided but on the basis of
insecure evidence. Soddy considered the first view to be
improbable. He kept uranium free of UrX for three weeks
by barium sulphate precipitations and then compared it
with Ur not so treated. The alpha rays in one sample had
thus been given three weeks to decay but Soddy could detect
no difference between the two. He concluded that 'it takes
at least a year to decay to half value' which was 'not in
accordance with what is known of the nature of excited
radio-activity'.150 But what indeed was known of excited
radioactivity? F.Giesel's comment151 which appeared two or
three months before that of Soddy illustrates the difficulty.
Having found that a radioactive preparation of lead was
still active after a year he yet refuted Hofmann's idea
of a new element; although induced activity was usually
'relatively soon lost' Giesel considered it possible that
the extreme conditions of induction - great excess of
radium and a year-long exposure - might result in an induced
lead of greater activity and longer duration than obtained
219
formerly. The fact that Soddy's argument requires the
existence of directly induced radioactivity seems to
indicate that he too believed in its reality at this time.
However, he had no particular interest in maintaining such a view.
For his second hypothesis regarding uranium's residual
activity involved the attribution of these alpha rays to
'a second distinct type of matter'. Soddy tells us it was
Rutherford's idea that the Curies' polonium 'fulfils in
almost all respects the functions of this hypothetical
constituent'. He expected that the known slow decay of its
activity would be observable 'after, but not before, its
separation from the uranium producing it'.152 One might represent the suggested process as Ur<roUrX . Presumably Soddy saw the constant level of activity, exhibited before
the hypothetical separation, as an equilibrium value. He
thus reawakened the polonium controversy after the Curies
had recently settled with Giosel that polonium was 'une esp6ce de bismuth actif'.153 W.Marckwald's paper of June 1902 'Veber das radioactive Wismuth (Polonium)'154 provided evidence which Soddy regarded as most important for his theoretical views.155 The method of electrolytic displace-ment in solution gave Marckwald his success in separating
from radioactive bismuth a particularly active substance
emitting only absorbable rays which were undiminished in
intensity after several months. According to the assumptions
of the Curies, to whom Marckwald referred, only true radio-
active elements possessed a permanent activity. Soddy's
theory on the other hand required polonium to be such an
element but with an activity which must decline. In his
paper on uranium presented in May 1902156 Soddy, however, reported that his attempts to separate a second constituent
either from uranium or from thorium had so far failed. His
continuing efforts to this end were soon to be overtaken by
the development of a third hypothesis which demanded their
failure as a logical consequence. But first let us consider
further ramifications which underlay the publication of the
original 'disintegration or transmutation' theory of
Rutherford and Soddy.
220
Neither of these crucial terms appeared in the second
joint communication which contained the core of that theory.
At the end of April 1902, when this paper together with
Soddy's on uranium were mailed, Rutherford caused Crookes
to become the first scientist outside McGill to see those
expressions on paper. Rutherford began his letter157 by
thanking Crookes for copies of the latter's two Royal
Society papers, which were probably those on 'Radio-activity
and the electron theory'158 and on 'The Stratifications of
Hydrogen'.159 In his discussion of vacuum-tube phenomena Crookes expressed the views that what he had once called
'Radiant Matter' now passed as 'electrons' or 'atoms of
electricity' which were the same as Kelvin's 'satellites'
or J.J.Thomson's 'corpuscles', that only a few of these were attached to each 'material nucleus or atom of matter'
to constitute a chemical ion, but that there was nevertheless
a'protyle' basis to matter.160 Perhaps these and earlier
conjectures played a part in persuading Rutherford that Crookes was a suitable recipient of the request 'to
facilitate the publication of the paper if difficulties
arise over "atomic views". The fact that publication was
achieved does not imply that Crookes accepted Rutherford's
conclusions, as will be seen. In his letter Rutherford
briefly summarised the equilibrium view of thorium's
activity, noted the similarity of uranium and radium to
thorium in this respect, and explained in a well-known
passage that:
All these processes are independent of chemical & physical conditions & we are driven to the conclusion that the whole process is sub-atomic. Although of course it is not advisable to put the case too bluntly to a Chemical Society, I believe that in the radioactive elements we have a process of disintegration or transmutation steadily going on which is the source of the energy dissipated in radioactivity.161
These words thus supplement the published 'General
Theoretical Considerations' regarding 'sub-atomic chemical
change'162 where the authors were doubtless also constrained
by the attack which the Curies had recently launched upon
Becquerel. Rutherford's usage of the expression
221
'disintegration or transmutation' may perhaps be interpreted
on the physical side by comparing it with the respective
alternatives of emission or rearrangement of the electrons
comprising the corpuscular atom.163 On the chemical side
the phrase seems to fit Rutherford's and Soddy's published
statements that thorium might undergo the subatomic version
either of a chemical 'decomposition' or a idepolymerisations
to produce respectively either two or one non-thorium
radiating substances.164 From the advanced position which
they held in April 1902 Rutherford and Soddy could see
novel problems and fresh ways of solving them.
The difficult question of residual activity, which had
led to the idea of 'decomposition' into two products, was
soon to be given a definitive answer. Attained apparently
by the head and not the hand the revised transformation
theory which they put forward was both a solution and a
synthesis. In November 1902 the second of Rutherford's
and Soddy's joint papers, slightly altered in structure,
appeared for the first time in a leading journal of physics.
To their account of 'The Cause and Nature of Radioactivity.
Part II'165 was appended a brief section which contained a far-reaching theoretical modification. Trenn166 has
argued cogently against Romer that the new view arose within
weeks, rather than months, of the submission of the old.
One may also note that the revision contains an elementary
error; it may therefore have been hurriedly penned. Possibly
one of its several consequences initially led them to it.
Instead of assuming 'as the simplest explanation' that
radiation was 'preceded by chemical change' their new inter-
pretation was that 'Radioactivity may be an accompaniment
of the change'.167 The apparently accidental correlation
between the radiating and the emanating powers of ThX, which
both suffered a geometric decay to half-value in 4 days, now
followed logically from the new theory; two pairs of decay-
rise curves thus became one. Instead of viewing the decay
in Hertzian or vibrational terms, as the loss of excess
energy mainly in the form of soft X-rays from a freshly
formed product, they now saw the phenomenon in a quite
different light. The decline of activity obeyed 'the simple
222
law of chemical change' according to which a single
substance is transformed at a rate directly proportional
to the amount present. The observed 'decay' was thus
essentially a dissipation of the substance itself; a lasting
view. On its basis they tried to explain the seeming
permanence of the residual activities: 'In the primary
change the amount remaining is infinitely great compared with
the amount that alters in a short time, and therefore the velocity of reaction is constant'.168 But this account seems unfortunate since the amount of material present in any
reaction following that law makes no difference to its
proportional rate of disappearance; reasons for the dis-
tinctive slowness of the primary change were hard to find.
Nevertheless the success of the newer theory is further
marked by its consequences that both uranium and thorium
should themselves in fact be truly radioactive. Accordingly,
Soddy's as yet unsuccessful path to an inactive uranium
should not exist, and that to a second 'decomposition'
product need not.
It would also seem to follow as a probable implication
of the accompaniment theory of radioactivity that each
radiating material should be in the course of producing a
substance different from itself. The statement of Rutherford and Soddy169 that the changes in uranium detectable by radio-
activity 'appear to be at an end' with those causing the
radiation of UrX is thus an interesting one. For it appears
to suggest either the production of an inactive UrY or,
since UrX was exceptional in giving for a period of 'many
weeks' only the 'cathode rays', a total corpuscular dis-
integration reminiscent of that proposed by Becquerel. When
applied to thorium the assumption that each active element
was creating another forced open the gateway to disintegration
series of increasing length and complexity. At the same time
the theory of induction began to recede. Rutherford's and
Soddy's first complete and public rejection of the
'Radioactive Induction' by irradiation or contact accepted
by others appeared early in 1903.170 However, the worried note which Soddy wrote to Rutherford in the autumn of that
year gives an indication both of their deep involvement
223
with the question of induction and of how difficult it was
to disprove its existence. Soddy's discovery that a sealed
glass tube of radium emanation produced upon a brass
electroscope temporary radioactivity lasting two or three
days provoked his opening remark:
The new fact, concerning which, or the possibility of which, you were always harping when I was prone to be too positive in the statement of the disinteg-ration theory has I fear arrived.l71
He could evidently see no fault in his own technique and
managed to turn the result against a rival, as was his wont,
wondering 'how the dickens P.Curie managed to measure the
decay of the Ra emanation by this means ... unless the
effect is peculiar to brass'.172 Nevertheless, the theory
of induced or artificial radioactivity died completely
within the next few years to be resurrected decades later in
entirely different circumstances.
In mid-1902, only a few weeks after he had framed with
his partner the new accompaniment version of the transform-
ation theory, Rutherford made an explicit denial of directly
induced radioactivity in so far as it related to 'Excited
Radioactivity and the Method of its Transmission'.173 It
has not been clearly recognised that in this same publication
the near full-grown flower of the final modification of the
disintegration theory was already evident. Rutherford's
investigations of the comparatively long-standing questions
of thorium emanation and its active deposit here provided
a pictorial account of the disintegration process. Such
studies had led during 1899-1902 to a situation in which
three related points required explanation, the almost
complete concentration of the excited activity upon a
negative electrode, the suppression of this effect at low
pressures, and new experiments174 which showed that even
with powerful electric fields some few percent of the
carriers moved in a direction contrary to that of the
majority. To recall and to expand upon the developments
of those years, Rutherford had relinquished his own early
condensation hypothesis for an extended version of an idea
of J.J.Thomson. The latter had privately suggested in
April 1901 that the loss of an electron left a positive
224
charge whose mean magnitude depended on the speed of
neutralisation by negative ions; the pressure-dependent
balance between the number and mobility of these ions in
turn determined this speed. Rutherford's additional
contribution, made by December 1901,175 was to suppose that a recoil at the moment of separation of the electron
gave some carriers a sufficient velocity to reach the
repelling electrode. This mechanism satisfied all of the
above three points. However, at its very time of initiation
one sees the beginnings within the scheme of a serious dis-
crepancy. The problem was the lack of a corpuscular type of
emission from certain radioactive substances in particular
the emanations which theoretically needed it most. Thus in
May 1901 Rutherford had published the remark that radium
emanation emitted absorbable X-rays (alpha rays) without
mentioning any other radiation. Moreover, discussion of
the nature of the rays from the two emanations was con-
spicuously absent from his later account, dated March 1902,
of research performed with Miss Brooks on the 'Comparison
of the Radiations from Radioactive Substances' although both
substances were actually listed for examination.176 And in June, as an explanation of atmospheric excited activity,177
Rutherford again invoked the hypothesis of 'the expulsion
of a negative electron' from some aerial 'carrier' giving
as its only justification the fact 'that all the radioactive
substances, thorium, radium, and uranium, as well as the
excited activity due to thorium and radium, possess the
property of spontaneously expelling electrons'.178 But this
claim rings rather hollow in view of earlier, contemporary
and later events; and it was soon to disintegrate completely.
At the end of July 1902 Rutherford finally confessed 'I was
at first inclined to suppose that the particle expelled from
the emanation was a negative electron' but that 'a close
examination' had detected none.179 The gamma rays might have
answered here, see below, but presumably he could not detect
these either. Still requiring such an emission of negative
particles from the emanation he found it in a new inter-
pretation of the alpha radiation. We are told that he had
been recently led 'by a mass of indirect evidence', largely
225
concerning absorption characteristics, to the conclusion that
alpha rays were not after all soft X-rays. They consisted
instead of streams of rapidly moving and as yet undeflected180
particles of atomic size.181 Rutherford noted the earlier
suggestion of Strutt, taken up by Crookes in his recent publi-
cation, that the alpha rays might be positively charged atoms
and mentioned Wien's work on canal rays which also were pos-
itively electrified.182 However, he used the recent results
of electrolytic studies on radium solutions as support for
the adoption of the negative charge which his theory required.
The alpha rays were thus negatively charged atoms ejected
from radioactive substances. One can observe the considerable
contribution of the chemical transformation theory to inter-
pretations of the phenomena of 'Excited Radioactivity and the
Method of its Transmission'. For Rutherford now understood
that like thorium and ThX the emanation 'consists of matter
in an unstable state' which in producing the material excited
activity suffered a 'chemical change'. Conversely, a crucial
influence of studies of excited activity upon that theory is
illustrated by Rutherford's remark that 'The change consists
in the expulsion of a negative particle from the neutral
molecule'.183 This statement provided a final, permanent and
mechanical link between chemical change and radiation within
the accompaniment theory of atomic disintegration.
How Rutherford succeeded during the autumn of 1902 in
demonstrating the characteristic positive electrification
of the atomic particles constituting alpha rays instead of
the expected negative charge is a well known story.184 His
'Magnetic and Electric Deviation of the Easily Absorbed
Rays from Radium' was announced early in 1903. It was closely
followed by the extension of these results and ideas towards
the important generalisation that in the various series of
changes the alpha rays 'are in all cases the first to be
produced'.185 It appears, however, that other striking
experimental discoveries which arose at this time had a
more profound influence than such developments upon the
views of contemporary scientists regarding the mysteries
of radioactivity.
226
CHAPTER 5
RECEPTIONS, GENERALISATIONS, SPECULATIONS
1. Reception of the disintegration theory (1902-3) The year 1903 was especially important ... Pierre Curie demonstrated the astonishing discharge of heat by this element [radium], which nevertheless remained unaltered in appearance. In England, Ramsay and Soddy announced a great discovery. They proved that radium continually produces helium gas and under conditions that force one to believe in an atomic transformation ... It furnished us the first example of a transformation of atoms.l
This later account of Marie Curie hints at the fact that
she and Pierre were not easily persuaded of the validity
of the disintegration theory of radioactivity and indicates
the evidence which to them appeared crucial. But the story
of that year is more complex than this record shows.
Important and spectacular experimental discoveries were
announced in rapid succession: the condensation of the
emanations in November 1902, described more fully in 1903,
by Rutherford and Soddy; deflection of the alpha rays in
February 1903 by Rutherford; the spontaneous emission of
heat from radium in March 1903 by P.Curie and A.Laborde;
scintillation effect of the radiations in March 1903 by
W.Crookes and others independently; and the production of
helium from radium, in July 1903. Although the last of
these certainly affected the views of the Curies it seems
that the previous discoveries and experiments were not
without influence. In tracing the development of this
period, partly chronologically and in part by taking up
parallel threads, we shall see that scientists with
different interests received these discoveries and the
associated theoretical advances in various ways. The
arguments presented by Rutherford and Soddy during 1902
were intricate, contained late appended revisions, and
could easily be misunderstood. The new discoveries
immediately aroused a tremendous scientific and general
interest in radioactivity. This in turn may have hastened
the publication of the powerful and lasting notions at
227
which Rutherford and Soddy had arrived before the spring
of 1903. Although by that time they had the theoretical
side effectively sewn up it was necessary for them to
campaign vigorously against misinterpretation and opposition
during the rest of that year; this they did in what was more
than a mere defence.
The Curies were acknowledged authorities on radio-
activity. The acceptance of the basis of the disintegration
theory by them seems therefore particularly significant.
In tracing its course we shall see that this acquiescence
involved the almost complete overthrow of their own published
views of 1901-2. This occurred despite a tendency, discern-
able after their attack upon Becquerel's disintegration
theory in January 1902, to keep their theoretical conclusions
as general as possible. In the period following that event
P.Curie attempted to remedy his proclaimed ignorance of the
experimental laws of dissipation of radioactive energy.
Moving away from the study of spatial arrangements, which
may not have provided regular results, he investigated more
closely the effects of time and temperature. Curie began
to eliminate the problematical effect of a cooling jacket
in absorbing the relevant rays by the ingenious method of
determining conductivities produced not in the air but in
the surrounding liquid itself. Thus in February he
announced, in a paper on 'ConductibilitO des dielectriques
sous l'influence des rayons du radium et des rayons de
Rantgen',2 that radium rays remained constant over a small
temperature range. By November he had extended the study
to highly accurate determinations of the rate of dissipation
of activity excited by radium. His researches 'Sur la
constante de temps caractoristique de la disparition de la
radioactivit6 induite par le radium dans une enceinte
fermee'3 showed that the decay of the rays emerging from a
sealed tube followed closely the equation I = I0 e-t/T
with a half-period of 3 days, 23 hrs., 42 mins. He thought
that this law's independence of conditions such as the
nature of the walls of the vessel and the gas within was
sufficiently firm to provide 'La mesure absolue du temps'.4
228
That this also applied whilst the vessel was maintained
at widely different temperatures, from -180° with liquid air to +450° in an oven, was a point of great theoretical
importance to Curie. For it served both to confirm his own
position and to rebut that which he attributed to Rutherford
and Soddy. In a paper delivered in January 1903 Curie
revealed his views 'Sur la radioactivite induite et sur l'emanation du radium'5 which were based upon further
studies of the abnormally rapid dissipation of induced
activity from open vessels; this was apparently the only
means by which the decay law could be altered. He considered
that the radium atom, the unchanging source of radioactive
energy, gave no direct rays at all but merely an 'emanation'.
Only if this were unable to escape such as in a solid radium
compound would it 'se transforme sur place en rayonnement de
Becquerel'. Evidently the emanation occupied a position of
great importance within Curie's theory. It is notable
however that he explicitly rejected Rutherford's usage of
the term. Although he adopted the expression 'emanation'
as 'commode' Curie restricted the meaning to 'Ponergie
radioactive &Ilse par les corps radioactifs sous la forme
speciale sous laquelle elle est emmagasinee dans les gaz
et dans le vide'.6 Citing Rutherford's and Soddy's revised
paper on thorium emanation of November 1902 he remarked
that there was insufficient evidence to establish 'l'existance
dune emanation de matiere sous sa forme atomique ordinaire',
firstly because spectroscopic evidence was lacking, and
secondly since the emanation vanished spontaneously from
sealed tubes containing it. This last comment is interesting
in that it seems to impinge only upon the second version of
Rutherford's theory;7 the first had assumed a loss of
radiation8 rather than the disappearance of emanation to
be the major cause of the observed decay. Whether Curie
distinguished between the two versions, both of which were
contained in the paper he cited, it is not certain. Curie
concluded on experimental grounds that after energy had
been released by the radium atoms to their surroundings:
dans les gaz l'energie transmise de proche en proche est emmagasinee sous une forme speciale
229
qui so dissipe suivant une loi exponentielle en provoquant la radioactivite des corps materiels.9
Thus he provided an alternative explanation of the results
which Rutherford saw in terms of material emanations and
active deposits. The observed invariability of the decay
law over a wide temperature range allowed Curie a final
criticism:
Je considere aussi come peu vraisemblable que les effete qui accompagnent l'existence de l'omanation aient leur origine dans une trans-formation chimique.10
This too was misdirected but again perhaps understandably
since the particular paper to which Curie referred con-
cluded with the late theoretical discussion of whether the
radiation accompanies or precedes 'chemical change' without
mentioning that this was not a normal chemical change.11
Rutherford's reply in an open letter comprising 'Some Remarks
on Radioactivity'12 corrected Curie's criticisms. Rutherford
pointed out that the chemical change conceived was not
'ordinary' but 'sub-atomic', that he had described both the
condensation and the diffusion of the emanation some months
before, and that the rays from the emanation were themselves
material consisting of 'heavy charged bodies'. It is notable
that a condensation of the emanation explains perfectly the
single outstanding exception to his own theory which Curie
reported. After keeping the sealed tube at the temperature
of liquid air he found that the standard rate of decay
occurred only after the recovery of the activity from an
abnormally low value. Curie vaguely attributed this to the
effect of temperature on the walls of the glass vessel.
And his description of the remarkable spontaneous rise (for
some 30 minutes) of the activity induced by very brief
exposure to radium, given in the succeeding publication of
February 1903,13 demonstrates the hopeful flexibility of
Curie's view. He thought that the explanation of the
initial increasing portion of what should be a decay curve
might lie 'dans la presence et dans la transformation d'une
certain quantite d'emanation'. Rutherford's explanation
of this phenomenon had come to be couched in terms which
230
appear similar to those of Curie but which in fact possessed
a much more material meaning. But even before the publi-
cation of Rutherford's informative reply Pierre Curie had
made a dramatic discovery of sufficient force to produce
the beginnings of a shift in his own outlook.
The path to Curie's revelation of 'La chaleur degagee
spontanement par les eels de radium', in a note published
jointly with a younger assistant A.Laborde in March 1903,14
was via the latter's apparently successful attempt to detect
a mechanical pressure produced by radium radiation.15 These results were instead seen by Pierre Curie as explicable in terms of a small temperature difference, known to affect
delicate weighings. This was directly confirmed by means
of a sensitive thermometer. And they announced that a
radium-barium chloride sample (about 4% Ra) remained
permanently at a temperature 1.5 degrees above its surround-
ings; background variations were but 1/100 degree. The rate
of heat production, easily measured both by electrical
comparison and calorimetrically, was incredible yet had to
be believed. 1 gm. of radium gave 100 calories per hour;
or as they significantly put it 1 gm.-atom of radium con-
tinously released in each hour as much heat as the combustion
of 1 gm.-atom of hydrogen in oxygen. Thus Curie could
reason with conviction that 'Le degagement continu d'une
telle quantito de chaleur ne pout s'expliquer par une
transformation chimique ordinaire'.16 If this was intended
to be an additional argument against Rutherford the effect
was perhaps the opposite; the points that Curio made were
somewhat similar to those already aimed at himself by
Rutherford and which were still on their way. If one sought
the origin of the heat in 'une transformation interne' this
must be more profound than a chemical change and might be
due to 'une modification de l'atome de radium lui-mime'.
However, since no change in the spectrum of radium was
visible each atom must change 'avec une extreme lenteur'.
Thus the energy in such a transformation would be
'extraordinairement grande'. His figure was in fact some
60 times greater than Rutherford and Soddy's forthcoming
231
'under-estimate' based upon alpha-ray ionisation measure-
ments.17 Curie failed to make it clear whether he still
preferred the alternative hypothesis, which he again
mentioned, that 'le radium utilise une energie
extOrieure'.18 Within three months he had seen Rutherford's
letter, had himself demonstrated the condensation and
diffusion of the emanation,19 knew of the scintillation
effect of the alpha rays and had accepted their atomic
nature. Some of this experimental evidence served to move
him theoretically one step further. The concluding remark
of his lecture at the Royal Institution2° on 19th June 1903
was that the two competing hypotheses regarding the source
of the energy - 'un "element en voie d'evolution' or an
unknown external radiation - 'ne sont pas du reste incom-
patibles'. Perhaps this was the effect Soddy desired as
he wrote to Rutherford regarding their general attack, to
be published in May, against the theory of induction:
'I feel it would be unwise to get Curie into a position
he was unwilling to go back on, before he has seen all our
evidence'.21 Certainly the Curies were not easily moved.
For in that June lecture Pierre repeated his own ideas on
the transmission of radioactive energy through gases together
with some of his reservations on the material nature of the
emanation. And Marie, in her D.Sc. thesis on 'Radio-active
Substances' which was probably completed in May,22 aimed a
blow at the most vital area of the disintegration theory,
namely 'ThX'. She asserted that this was no more than the
manifestation of an activity induced by thorium upon some
inactive substance whose chemical properties might be
temporarily or permanently altered to give the results
obtained by Soddy; she did not omit these comments from
the 1904 edition of her work despite making other important
changes. However, the balance of Pierre Curie's newly
attained compromise of mid-1903 was soon to be tilted, by
the weight of still more experimental evidence, towards
the idea of atomic change.
F.Giesel, a figure not without influence in the early
field of radioactivity, was a representative of those who,
232
even before the striking discoveries of 1903, accepted
the existence of atomic disintegration as well as radio-
active induction. Perhaps wisely however he offered no
coherent theory. The essential part played by the Curies'
induction theory in Giesel's chemical controversies over
polonium and radiolead during 1901 has been described above.23 The German chemist's concern with the problems
of the theoretical side of radioactivity is illustrated
in his letter to the Curies of March 1902.24 Having mentioned his new radium-barium bromide fractionation, which was
shortly to supersede the chloride method of Marie Curie,
he commented upon Pierre's experiments on the transmission
of radioactivity through water. Giesel suggested an idea
later to be extended by the Curies, that radium might release
energy only in the form of a Rutherford emanation which
would in turn produce direct radiations as a secondary
effect. Regarding induced activities he expressed unease
at the possibility that all such manifestations, of which
he had seen many, might be due to traces of known active
substance; as he admitted, the Curies' demonstration that
the activity induced by soluble radium chloride was itself
insoluble constituted an exception to this.25 In a paper 'Veber Becquerelstrahlen und die radioaktiven Su.bstanzen'26 published some months later Giesel advocated a material
interpretation of thorium emanation; he compared it to the
odour emitted by Musk. It is interesting that he pointed
to the characteristic chemical properties of induced radio-
activity whilst still expressing uncertainty as to whether
this was a deposit of the primary substance. By October
1902 Giesel appears to have combined the Curies' current
view that the emitter of radioactive energy was the atom,
'die Arbeitsmaschine' driven by an unknown power, with the
prevalent physical theory of electronic dissociation:
das Atom dabei nicht bestehen bleiben kann, sondern sich in noch west kleinere Theilchen aufliisen muss, in Ionen (oder Elektronen), welche als Zwillinge mit + und - Elektricitat geladen zur Welt kommen.27
233
He was also inclined favourably toward Rutherford's
opinion that since the emanation came from radium itself
its study might clarify 'die inneren Vorggnge im Radium-atom'.28 On the other hand his classification of radioactive
substances in no way agreed with Rutherford's disintegration
theory. Giesel divided these materials into three groups
according to their radiation characteristics: intensely
active and constant, feeble and constant, and weakly to
intensely active with declining radiations.29 His attribution of the activity of the entire third group to inductions by
the permanently active elements, and his placement of
polonium in this category invoked again the comment of
Rutherford that radioactive induction did not exist.30 After this Giesel quietly dropped the idea to work like others
within the disintegration theory. These developments high-
light the difficulties in understanding induced radioactivity
which also troubled others during 1902-3.
J.J.Thomson too expressed an idea of direct induction
or self-induction by radiation which was also to be dismissed
by Rutherford in this case by straightforward experimental
means.31 However, Thomson's suggestion of April 190332
marked not the beginning but the end of a scientific struggle
between the two. For more than a year they had differed
over the emanations which had played so vital a part in the
development of the disintegration theory. Whilst the Curies
steadfastly reiterated that the emanations were a special
non-material form of energy and spoke against Rutherford's
and Soddy's belief that these were a particular kind of
matter, J.J.Thomson came to adopt a quite distinct position.33
At first he had accepted Rutherford's idea that thorium
emanation was a radioactive material, and had contributed
welcome suggestions regarding its gaseous nature and the
origin of its acquired positive charge. After some three
years of harmony disagreement arose over Elster and Geitel's
234
invocation of a third radioactive emanation as the cause
of the temporary activity produced on negatively charged
wires in the atmosphere. The tension was almost at its
greatest when in May 1902 Thomson wrote to Rutherford of his own experiments on this phenomenon:
These results make me doubt whether Eleter and Geitel's induced radioactivity is really due to some rare substance; it seems to me it is probably made from wind and water! C.T.R.Wilson has discovered that freshly fallen rain is radioactive.34
But worse was to come as the criticism tended to expand.
Let us briefly follow the tale up to this point and on to
its conclusion. Some earlier experiments of Thomson's
research students were directed to show what could be done
with ordinary materials. For example Wilson announced in
1899 that large uncharged (non-radiating) nuclei could be
created in gases by irradiation.35 By the end of 1901 he had demonstrated an apparently 'Spontaneous Ionisation of
Gases'36 and J.C.McLennan had written 'On a kind of Radio-
activity imparted to certain Salts by Cathode Rays'.37
Thomson's experiments 'On Induced Radio-activity'38 which he described without interpretation in March 1902 appear
to continue this trend. Although a negatively charged rod
did not become active in a sealed vessel of air, which one can see might be explained by the limited quantity of
emanation therein, when the air was continuously irradiated
by X-rays with all that implied, then the rod did become
active: it is notable that this was entirely contrary to
Rutherford's statement of 1899.39 Furthermore, as Thomson reported, chemical substances especially hydrogen peroxide
produced large currents when absorbed on paper in a layer
around the rod; this may possibly be interpreted as a
renewed link between radioactivity and Russell's earlier
photographic work. By May 1902 the 'continuation of the
experiments' to a related subject showed that 'The Increase
in the Electrical Conductivity of Air produced by its
passage through Water,40 could be as great as 10 to 12 times
the initial value. Such observations led him to compare
explicitly 'the "emanation" from radio-active substances'
235
and ordinary air 'put in this highly conducting state'
simply by bubbling. Writing to Rutherford on various
matters at this time Thomson gave some details of the
ionic mechanism which he saw operating here:
I think the effect is due to excessively minute drops of water so small that they fall with extreme slowness, & that around each drop there is a layer of ionised gas which is pulled off by the electric field.41
He openly linked this only with 'atmospheric electricity',
but the direction of his reasoning is suggested by questions
regarding Rutherford's emanations in the same letter: Have you tried whether the emanation is longer lived when it is in a solid or liquid than when in the air; if you let it bubble in very small bubbles through water for a minute will it lose half its radio-activity as it would in air; it seems as if the ability of the emanation to get through a great many layers of paper rather pointed to the conclusion that the emanation does not fade away so rapidly in solids as it does when free.42
Despite the compliment that Rutherford's explanation of
radioactivity 'clears up a great deal of obscurity', which
would help Thomson's forthcoming book, the latter evidently
remained suspicious of the emanation aspect. Besides the
above discrepancies and queries it is also possible that
he had noticed Rutherford's and Soddy's easy recognition
of an atmospheric emanation in their first joint paper43
without the promised44 tests. It seems that Thomson found
many of Rutherford's results not unrepeatable but too easily
so. He wrote again some days later45 mentioning Wilson's
radioactive rain and ascribing Elster and Geitel's 'rare
substance' to 'wind and water'. Then into print went
Thomson's 'Experiments on Indueed-Radioactivity in Air,
and on the Electrical Conductivity produced in Gases when
they pass through Water'46 which extended and united the
two main aspects of his previous researches on radioactivity
mentioned in the title of his paper.
Thomson argued squarely both that the existence of a
radioactive component in the atmosphere was 'possible'
but 'not necessary', and that 'negatively electrified
surfaces may become radio-active without the deposition
236
upon them of substances having specific radio-active
properties'.47 Let us consider firstly the emanation side
of the study. The air which Thomson passed through water
in various ways attained an increased electrical conduct-
ivity by factors as great as twenty and retained this
property for some days; it followed that 'In the air
modified by passing through water there must be a continuous
production of ions'.48 And he appears to have cast aspersions not only upon the hypothetical atmospheric emanation but
also at Rutherford's thorium and radium emanations. Though
no-one seems to have known it, Elster and Geitel, the cited
targets, had already moved out of range49 before Thomson launched his attack. It therefore fell entirely upon
Rutherford. Thomson asserted that although certain extremes
of heat and cold destroyed the artificial conductivity an
electric field did not:
Thus, in this respect, the modified gas resembles a gas mixed with the 'emanation' from thorium. Rutherford has shown that in this case the conduct-ivity is not destroyed by a strong electric field.50
Regarding the second major aspect of the discussion, that
of surface induction, he explained the 'induced radio-
activity caused by negative electrification' within the
modified gas in a manner ironically reminisoent of Elster
and Geitel's newly favoured hypothesis. Thomson supposed
that positive ions in the gas adhered to the negatively
charged wire causing the corpuscles there present to be
accelerated into the surrounding gas as 'cathode rays'.
Thus was the wire radioactive. He postulated a similar
mechanism involving minute water drops surrounded by
positive ions to explain the lasting conductivity of the
modified air. Some positive ions, for example those from
flames, did not produce such effects; but an electrode
polarised in solution did so. Evaporation to dryness
satisfied Thomson that no active material was present in
the water which he had used for spraying and bubbling.51
The inference that all of the emanations and induced
activities were not each 'a special kind of matter', as
Rutherford and Soddy claimed, was quite clear. The
237
implications left ThX isolated, exposed and highly
vulnerable. But Thomson had not expressed the criticism
very forcefully in that direction. Perhaps this was
fortunate. For by the end of 1902, only six months after
the completion of his paper, he had adopted a separation
of the atmospheric emanation hypothesis from the trans-
formation theory of radioactivity which he now fully
supported. He had evidently not heeded Rutherford's dig
that 'although the amount of excited activity ... varies
greatly with the weather and amount of wind' its decay law was always the same.53 But perhaps Thomson had come to
appreciate the revised discussions of 'The Cause and Nature
of Radioactivity'54 before writing his piece on 'Becquerel
Rays' for Harpers Monthly Magazine.55 In that article
Thomson publicly recognised in the disintegration theory
both a solution of the energy problem and an explanation
of the chemical manifestations of radioactivity:
what is the nature of this energy, and how is it stored? A satisfactory answer to this question has, I think, been given by some quite recent researches made by Professors Rutherford and Soddy in Montrea1.56
Having described the separation of ThX from Th, the
recovery and decay of activities, the continuous production
of ThX and the equilibrium nature of thorium's constant
radiation he concluded:
We see now the source from which the energy required to sustain the radiation is derived; the radio-active substance is undergoing a continuous transformation into a state in which it has less energy ... Ordinary thorium is thus steadily being transformed into the active thorium X, while this is continually passing into some inactive form.57
That Thomson envisaged some form of chemical transmutation
seems clear. For he suggested that this final inactive
substance might be detected in thorium minerals 'by ordinary
chemical means'. Most significantly, despite the confusion
in the last words of the above extract, Thomson now accepted
that the emanations were inert-gas elements58 produced from
ThX; and he pointed to the presence of radioactive elements
in all helium-bearing minerals. Conversely he also repeated
his recent claim regarding atmospheric excited activity
238
that 'this induced radio-activity'59 could be explained
otherwise - in terms of ionic clusters and layers. The
skepticism of Thomson, thus narrowed, was soon to disappear
completely along with the experimental basis of his own
conclusions.
In a long letter sent to Thomson just after Christmas
1902 Rutherford persuasively described his recent successful
condensation of the emanations using liquefied air.60 'The experiment is an extremely simple one to show and works
like a charm'; the gas leaving a spiral tube at that
temperature 'had not a trace of emanation in it' and, on
warming, the emanation 'comes off in a rush - all at once
apparently or at any rate within a degree'. Delicately
Rutherford phrased his remark that 'Anyone whodoes'nt (sic)
believe it is a gas after such an expt. is difficult to
convince'.61 Furthermore he had 'proved' that much of
Wilson's '"spontaneous" ionization' of air was due to a
penetrating radiation from outside the containing vessel,
from the walls of the room itself. Thomson presented and
may have been influenced by J.C.McLennan's paper on
'Induced Radioactivity Excited in Air at the Foot of
Waterfalls'62 also dated December 1902. The latter had
written to Rutherford in October63 concerning high voltage
experiments at Niagara and others in the laboratory which
involved the dropping of water through thorium oxide.
McLennan confided that these 'would explain the radio-
activity of rain found by Wilson' and 'seem to point
against J.J.'s results'; but on repeating Thomson's own
experiments, McLennan 'found exactly what he found'.
That the conflict was soon resolved for the latter is
indicated by his conclusion, published in April 1903, that.
'the consensus of opinion' appeared to be that atmospheric
excited radioactivity 'is due to the presence in the
atmosphere of some peculiar constituent similar to the
emanation from thorium'. By that time in April 1903
when he replied to Rutherford Thomson too had fitted into
this category of opinion. He announced, or admitted:
I have found a radio-active gas in Cambridge water, or rather in that part of the water
239
which comes from deep wells. Dewar liquefied for me the gas extracted from the water... 64
He made the first open statement to this effect soon
afterwards;65 his ionic condensation theory of radio-activity was not heard of again. However, it is not
true that he gave up the belief that radioactivity was
a property of ordinary matter. Papers on this subject
had already begun to appear and the discussion which ensued
during the next few years similarly involved both Thomson
and Rutherford.66 This provides one reason for saying that
Soddy's claim of June 1903, that 'Professor J.J.Thomson
and Sir William Crookes have both recently abandoned their
former theories in favour of the new hypothesis',67 is
something of an oversimplification at least with regard
to the former. Besides, in 1898 he had been the first to
ascribe uranium's energy source to an internal rearrangement
of its atom. It is hard to decide whether Thomson can be said to have retained or revived this conjecture. But it
is clear that he now attempted to strengthen it with a
rough calculation of the possible magnitude of the available
atomic energy: if the radium atom were totally corpuscular
and each negative charge of 3.4 x 1010 e.s.u. were 10-8 cm.
distant from an equal positive charge then a 1% reduction in
the intrinsic energy would suffice to maintain its heat
production for 30,000 years.68 To Crookes, however, Soddy's
straightforward interpretation fully applies. Crookes too
had proposed a theory of radioactivity in 1898 and indeed
abandoned it in mid-1903. But, as the placement of the
younger chemist's claim in The Times Literary Supplement
hints, not before the skeletal explanations of that public
figure had been well aired.
240
In the spring of 1903 Soddy sent to Rutherford from
London an illuminating letter oontaining his comments on
their forthcoming publication:
wish you immediately to get into thinking array & to consider it & this letter. Events are moving rapidly here. The announcement of the heat given out by Curie has created quite a furore in the Press, & in yesterday's Times Johnstone Stoney had a letter which I have enclosed. Ramsay told me & from his attitude seemed to think it quite possible.69
Soddy confessed that he was unable to convince Ramsay that
the surroundings were not the energy source and in relation
to these matters he continued:
I mention this to show we are on a flood-tide of interest & I do not want to delay (1) If there is a controversy all our papers should be out. They all predate recent developments ... (2) The fewer 'grand-savants' who make asses of themselves the better for our (personal) relations with them afterwards.70
He also feared that Becquerel, who had made certain claims
concerning the deflection of the alpha rays, might come
out with a theory 'of his own'; in a way he was right.
The events which prompted Soddy's urgent message had
begun with the delivery on 16th March of Curie's paper on
the heat of radium and continued with Crookes' announcement
on the 19th of his discovery of a remarkable scintillating
phosphorescence produced by that substance. Crookes
attributed this phenomenon to the individual impacts of a:
bombardment of the screen by the electrons ([footnote] Radiant matter, satellites, corpuscles, nuclei• whatever they are they at like material masses) hurled off by radium with a velocity of the order of that of light.71
The 'furore in the Press' began with The Times' editorial
of 25th March entitled 'The Mystery of Radium' which
summarised both Crookes' very beautiful demonstration' of
the scintillation effect and the discovery of P.Curie:
M.Curie, a French physicist of the highest reputation and attainments, has made a communication to the Academy of Sciences which would have been received with absolute incredulity had it been offered on less unimpeachable authority.72
241
For radium, as they put it, produced heat spontaneously
'without combustion, without chemical change of any kind,
and without any change in its molecular structure'.73 The conclusion that radium could 'gather up and convert into
heat some ambient form of energy with which we are not yet
acquainted' was clarified by Crookes the next day, after
Punch had interpreted his discovery in its own terms:
'On Ions'. - Such was the subject of Sir W. CROOKES' most recent lecture. Were they Spanish? Pickled? Boiled or fried...? They were made 'visible'. This was hardly necessary, as in such a case the evidence to the eyes would be less convincing than that to the nose.74
In a letter to The Times of 26th March Crookes explained
that the source of ambient energy need not be a mystery.
Explicitly reviving his theory of 1898 he again referred
to the large store of kinetic energy in the surrounding
air and suggested that radium might use the faster molecules
in the manner of Maxwell's 'Demons'. Crookes made no mention
of the disintegration theory about which Rutherford had
written to him a year earlier. Despite the success of the
transformation theory in explaining the chemical, radiant
and electrical phenomena of radioactivity the supposition
that the considerable energy involved was stored within
the chemical atoms was the subordinate hypothesis with the
least direct supporting evidence. Rutherford and Soddy had
turned the observed indestructibility of the ordinary chemical
elements to their own theoretical advantage; but this move
implied that the atoms of every element contained such a
reservoir. The notion of an internal store was evidently
at first unacceptable to Crookes but his own answer to 'The
Mystery of Radium' was immediately questioned by others in
the flurry of correspondence which followed the editorial
and his letter in The Times. 'Ignoramus'75 pointedly mention-
ed the known constant intensity of radium rays in vacuo. On
the other hand G.J.Stoney in his opening letter76 not only
stated his agreement with Crookes but claimed to have employed
just such an explanation in 1893. Stoney's 'Suggestion as
to a possible Source of the Energy required for the Life of Bacilli, and as to the Cause of their small Size'77 had been
242
that such organisms might be penetrated by 'swifter moving
molecules' and could thus abstract the energy of formation
of organic compounds from the surroundings, which would
become slightly cooler as a result. The process 'therefore-
belongs to the recognized exceptions to the Second Law of
Thermodynamics'. In Stoney's opinion the restoration by
molecular collisions of radiated energy 'whenever the motion
of the electron has transferred energy from the molecule to
the aether' was another exception.78 This is the point which
seems relevant to radioactive radiation; it indicates how
the new energy problem was seen by some in terms of an old
but still living enigma.
Crookes, Stoney, and 'Ignoramus' were joined by others,
some anonymously, as they continued the correspondence into
mid-April; the names of Rutherford and Soddy received a
mention79 before Crookes withdrew from the argument describing
it as unfruitful.80 But perhaps it was not entirely so, for
Crookes soon changed his view on the source of the energy.
Despite the latter's protest that the fast-moving molecules
in a 'vacuum' could well provide radium's energy81 one might
say that the comments of 'Ignoramus' among others made their
mark. For in his note with Dewar 'On the effect of extreme
cold on the emanations of radium' read to the Royal Society
during the next month82 Crookes described the use of his new
'spinthariscope',83 whose scintillations he now took to be
caused by the impact of 'positive atoms'. His intention
was to test the effect of a vacuum on radium, the source of
these particles. The 'very good vacuum' obtained by using
Dewar's liquid air or hydrogen as condensing agents did not
diminish the scintillations nor did the low temperatures
thus provided; though the screen lost its fluorescing ability
when allowed to become cold. The knowledge that such extreme
experimental conditions ought to affect the availability of
gaseous kinetic energy may have helped to turn Crookes away
from his attribution of radioactive energy to this source.
His stated intention of following up 'the important dis-
covery' by Rutherford and Soddy of a condensable emanation
from radium salts indicates another likely influence.84
243
In expressing his 'Modern Views on Matter: The Realisation of a Dream', 05 some days later, Crookes revealed an important change in his ideas. He now believed that in addition to
the process of ultra-atomic dissociation, which he had
postulated in 1902, radioactivity involved an atomic trans-
formation. To the historic discovery of radium he credited
the coalescence 'into one harmonious whole' of the 'isolated
hypotheses' of 'ultra-gaseous' matter, electrons, subatomic
particles, X-rays, and 'the emanations from uranium'.86 The twin threads of electrical theories of matter and notions
of the complexity of the chemical elements, both of which
ran from Davy and Faraday via himself to contemporary
studies, he saw thus united. But, apart from the miscon-
strued 'emanations from uranium', Crookes' own seemingly
attractive joinery appears in its context less than perfect.
In the previous year Crookes had believed that the protyle
atoms of matter were attended by comparatively few electrons87 But now in mid-1903 he considered that 'the electron would
be the "protyle" of 1886, whose different groupings cause
the Genesis of the Elements'.88 According to what he called 'a Darwinian development of chemical evolution',89 the elements were formed in order of increasing atomic weight
presumably correlated with decreasing thermal stability.
He stated that radium exhibited 'a spontaneous dissociation'
and that its atom 'might be actually suffering a katabolic
transformation'.90 Radium, outshining uranium in this
respect and being thus the least stable element, ought
therefore to have the highest atomic weight. Probably for
this reason Crookes chose to adopt the tentative spectros-
copically extrapolated estimate by C.Runge and J.Precht91
of 258 ignoring Marie Curie's correct gravimetric result
of 225. Crookes' only hint as to the cause of atomic dis-
integration was that since radium held the position 'next
after uranium' in order of original creation and present
instability it would be sensitive to 'our terrestrial
sources of heat'.92 Crookes may have derived this idea from
0.Lodge whose positive and negative electronic atom of
electromagnetic mass ho described.
244
It is one indication of the speed of developments at
this time that Lodge had already changed his mind about
each of these subjects. In his address similarly entitled
'Modern Views on Matter',93 delivered one week after that
of Crookes, Lodge placed reservations both upon the
existence of the positive electron94 and upon external
influences on atomic disintegration.95 And it is a corres-
ponding sign of rapid movement that he attacked the position,
from which Crookes had already shifted, of supposing that air
molecules supplied radium's energy. Lodge followed closely
the theory of Rutherford and Soddy, probably as revealed in their May publication on 'Radioactive Change'.96 In terms bolder than theirs he proclaimed that 'The transmutation
of elements' through 'temporary transitional forms' was
a process 'going on before our eyes';97 the loss from an
atom of radium with atomic weight 225 of a projected portion
with atomic weight 2, which caused it to become the unstable
emanation, was 'the main fact of radio-activity'. Such
comments mark the decisive end to a period of six months,
following his perusal in November 190298 of Rutherford's
and Soddy's paper 'On the Cause and Nature of Radioactivity',
during which he entertained both alternatives for the energy
source. Lodge may also have been influenced by J.J.Thomson
whose estimation of the internal energy of an electronic
atom he appears to have reproduced.99 These two physicists
together with Crookes comprised an important trio of converts
from a variety of opinions to the disintegration theory of
radioactivity. But the major authority in the field lay
with the Curies who had other ideas.
Marie Curie's criticism of the chemical evidence for
radioactive transformation had appeared in May 1903 and
Pierre Curie, his rival induction theory in difficulties,
had moved to a position of compromise regarding the energy
source by June. Let us again take up their story and its
connections. The announcements made by Soddy and Ramsay
in July created an impression both deep and wide; upon the
Curies the impact was almost conclusive. Soddy now partnered
the well-known discoverer of terrestrial helium100 and the
245
inert gas family of elements.101 A preliminary notice
reported simply that the 'Gases Occluded by Radium Bromide'
contained helium.102 Their subsequent description of
'Experiments in Radio-activity, and the Production of
Helium from Radium'103 evidenced a considerable development.
By means of low-temperature purification techniques they
had succeeded in following the first ever spectroscopically
traceable chemical transmutation. Not only had they
watched the characteristic yellow line of helium appear,
after some days, in a tube of radium emanation but they
had at last seen a glimpse of the emanation's own spectrum.
Writing to Dewar on 22nd July regarding the publication
of joint experiments on the heat of radium Pierre Curie
told him that he was '1'ennemi des publications hatives'
which also was why Rutherford and Dorn 'ont public avant
moi des °hoses quo j'avais faites avant eux'.104 Curie did
not include the production of helium from radium, of which
he was already informed, among those things. But perhaps
he recognised the 'presence simultanee dans certains
mineraux de l'uranium, du radium et de l'helium' as such.
For in a review of 'Recherches recentes sur la Radioactivito'
written during the following months he claimed to have been
impressed with this fact 'des le debut de nos recherches'.105
The proposed transmutation suitably confirmed106 sowed the
seeds of change which can be seen still preserved in Curie's
written words. In the main his review repeated earlier
statements for example that the emanation was not 'un gaz
materiel ordinaire' but one of the forms of radioactive
energy.107 One can see that this could form a gulf between
radium and helium. However, in a final section Curie
credited the new experiments with 'une importance fonda-
mentale'. He accepted that'L'helium pourrait, d'apres
cola, etre l'un des produits de la desagregation du
radium'.108 At the same time his recognition of the work
of Kaufmann seems to have removed the basis of Curie's
objection to Becquerel's ballistic hypothesis of the beta
rays; Curie now seemed happy to grant the possibility of
an electronic-electromagnetic theory of matter.109 His note
246
'Sur la disparition de la radioactivity induite par le
radium stir les corps solidest 110 shows that by the spring
of 1904 he was actively using the transformation aspect
of the theory 'de M.Rutherford' in his quantitative research.
Marie Curie was similarly influenced by the helium
experiments; she reported these and mentioned the dis-
integration theory in the 1904 revised edition of her thesis.111 But her work gives a curiously patchy impression.
She largely retained the induction theory, including her
view of ThI, though now admitting the emanation as a
material gas.112 Whilst she was persuaded that 'trans-
formations atomiques' indeed occurred in radioactivity
her final word took the form of a strange though perhaps
not unique defence of the unchanging radium atom:
Au lieu d'admettre que l'atome de radium se transforme, on pourrait admettre que cet atome est lui-mAme stable, mais qu'il agit sur le milieu qui l'entoure (atones materiels voisins oil other du vide) de mani4re a dormer lieu a des transformations atomiques.113
Marie Curie also appears to have attempted to ensure that
she had the last word by asserting that Rutherford had
'franchement adoptee'114 one of her own hypotheses of
1898-9; this she repeated115 in 1906 when there was no
alternative to the disintegration theory of Rutherford
and Soddy.
The aspects of the radium-emanation-helium trans-
formations which thus impressed the Curies also invoked
a wider interest, a second wave of publicity. Ramsay was
applauded as he made the first public announcement of the
creation of helium in mid-July at the Dinner of the Society
of Chemical Industry.116 In the same week The Times117 noted that Sir W.Huggins had found helium lines in 'The Spectrum
of the Spontaneous Luminous Radiation of Radium at Ordinary
Temperatures1118 - an apparently independent discovery,
though based on hints from Rutherford's recent papers.
Huggins seems now to have speculated in a manner reminiscent
of the dissociation hypothesis of his old rival Norman
Lockyer and privately noted elsewhere that a xenon line
247
was identical with one of the radium spark spectrum lines.119 Unfortunately, however, just as the first of a series of
letters on 'Radium and Helium' appeared in The Times
Huggins had to admit that radium's luminous spectrum con-
tained not helium but nitrogen lines,120 a discovery
notable in itself. 'Verily this is the summit of fame'
wrote Soddy as he sent 'a cutting from the current no. of
Punch' to Rutherford.121 The relevant extract may have
been the interesting disclaimer that:
'RADIUM' wishes it to be distinctly understood that he can throw no light on the present political situation. He adds that there is no affinity between him and TIM HELIUM, M.P. 122
Or perhaps, following Soddy's own popularising article in
The Times Literary Supplement on 'Possible Future Appli-
cations of Radium',123 the cutting may have been the
illustration of a subtle connection between 'SCIENCE AND
MATRIMONY' which appeared in the same issue of Punch:
He (the accepted one, enthusiastically discussing their projects for the future). 'I think it would be a splendid idea, when we marry, to have the Kitchen fitted with a Radium Cooking Range!!'
The Betrothed (who doesn't believe in long engagements, very sweetly). 'Er-ye-es, Darling, but if Radium does not come into use - say, in one month's time from to-day, we won't wait for it, dear, will we?'124
Articles by scientists and others125 concerning the physical,
chemical, technological and medical implications of radio-.
activity proliferated in non-technical magazines during
1903. The extent of popular interest in the Curies, perhaps
not matched in French academic circles, can be judged by
Pierre's pained declaration to Ramsay early in 1904:
Nous avons etc terriblement deranges dans ces derniers temps par les journalistes, lee gene du monde, les excentriques de toutes les especes male vous connaissez vous-meme ces visiteurs encombrants.l26
According to Soddy's complaint, two months earlier, Ramsay
himself enjoyed this kind of action:
Ramsay has again been interviewed by the Daily Mail. I can't quite understand it. Sometimes wonder if he forsees the great commercial advantage in the future of being known as the expert on radium, & has done it from this motive or from pure lack of judge-ment & consideration.127
248
However, the students of radioactivity were evidently most
concerned with the professional side of their reputations.
This is shown for example by Rutherford's remark that 'a
photo of my noble self' in Harper's Magazine gave him 'as
much advertisement as is good for me', but that 'These
things ... don't count scientifically for it is work that
tells'.128 Nevertheless one can argue that events in the
broader arena did in certain respects affect those in the
smaller. Such an effect may possibily help to explain why
feelings about priority ran particularly high during the
second half of 1903.129 And a belief in such an influence
would certainly account for Soddy's further comment to
Rutherford at the end of the year that Ramsay's Press inter-
views 'must prejudice our case with the real scientists'.130
The publicity at least ensured that the case was brought
to the attention of such persons, but their verdicts were
not uniform.
Rutherford enjoyed more success in his campaign among
the physicists than did Soddy with his fellow chemists.
J.Larmor in his capacity as Secretary of the Royal Society
had written to Rutherford back in April when the first
furore arose:
I am glad to hear that you are coming in May: you may be the lion of the season for the newspapers have suddenly become radioactive. I see you again monopolise most of the Phil.Mag. 131
The editors of The Electrician were in favour of the new
theory.132 They had commented after 'The extra meeting of
the Physical Society, convened last Friday [5th June] at
University College to meet Prof.RUTHERFORD', which 'was a
crowded and enthusiastic gathering', that:
the suggestions put forward by Prof.RUTHERFORD in explanation of some of the mysteries of his subject have special value, and must carry great weight.133
At that gathering Rutherford had received the praises of
Lodge and had also answered the doubts repeated by
J.D.Everett134 and expressed by S.P.Thompson concerning
the source of radium's energy.135 It was the chemist
T.M.Lowry who at this physicists' meeting made perhaps the
249
most outspoken objections to the transformation theory.
He attacked directly the weakness of its experimental
foundations. In return Rutherford pointed to the
inadequacy of Lowry's substitute. But the latter's last
words on the matter had not yet been heard. Nor had those
of his senior associate at the Central College South
Kensington, H.E.Armstrong.
As a chemist and a founder of the atomic disintegration
theory of radioactivity Soddy seems to have experienced a
division of his loyalties. Perhaps he still sympathised
with those who believed in the indivisible chemical atom
as he himself had done but two years earlier:
Having failed utterly as I can see to make you realise the width of the gap between our recent work and anything preceding I do not intend to attempt it in this letter ... I must say I sometimes feel however as if I had been a traitor to my own camp and let you ... in by a back door. But for me the chemists' fraternity would have continued to smile hard and long for many a year.136
And as a confrontation with the figure described by his
excellent biographer as 'the foremost British chemist of
the time'137 loomed near Soddy organised the tactics.
Referring to Armstrong's and Lowry's recent publication
on radioactivity he wrote to Rutherford:
I think they, being chemists, are my fair game & I hope to get an opportunity of asking some questions if they get up on their feet at the B.A. Otherwise I think they are best ignored altogether. So perhaps you will leave them to me if they try to interfere. I shall only engage them under provocation.138
Armstrong, a chemist interested in physical aspects of his
subject, was one of the few scientists to set out fully an
alternative to the disintegration theory. In a paper with
Lowry on 'The phenomena of luminosity and their possible
correlation with radio-activity'139 he attempted to explain
radioactivity 'from the chemists point of view' and to
bring it within the boundaries of his own field. Having
outlined the relationships of triboluminescence, floures-
cence and phosphorescence to the dynamic equilibria of
organic compounds he compared Th and ThX, as 'isodynamic
250
forms of thorium', with the forms of nitrocamphor whose
rate of interconversion followed a 'simple logarithmic
law'. He asserted that this explanation was 'at least
as rational as one which assumes that nature has endowed
radium alone of all the elements with incurable suicidal monomania'.140 This bark, which Soddy found noteworthy,141
may derive its bite from Crookes' well-phrased suggestion
of a 'fatal quality of atomic dissociation'.142
Armstrong's opinions on radioactivity may be understood,
in part at least, by viewing them as an extension or con-
tinuation of earlier convictions. Though once a supporter
of Lockyer's dissociation hypothesis in its early days,143
Armstrong later took every opportunity to criticise various hypotheses of dissociation. He attacked the electrochemical
molecular or ionic dissociation theories of the 1880'9,144
and poured scorn upon the corpuscular atomic dissociation
theory of the late 1890' s.145 Similarly, in discussing 'The
Conditions determinative of Chemical Change and of Electrical
Conduction in Gases, and on the Phenomena of Luminosity' in
1902146 he argued that the occurrence of ordinary reversible
oxidation effects made Crookes' explanation of vacuum tube phenomena in terms of radiant matter or electrons unnecessary.
On the traditional them© of 'The Classification of the
Elements' Armstrong argued along two familiar lines. He believed in the genetic relationship and complexity of the
elements but asserted that 'no direct evidence acceptable
to chemists has been adduced which in any way justifies
the belief that the elements are decomposable'.147 Though
Armstrong's expressions might appear very similar to
Crookes' current and earlier views the meaning of the
term 'decomposable' constituted a point of distinction
which radioactivity soon brought to the fore. In addition
Armstrong appears to have been the author of a series of
anonymous personal cum scientific attacks in Crookes' journal
upon Ramsay and his researches on the inert gases.148 One
• would expect this to have a bearing upon Armstrong's picture
of the radioactive emanations which had been placed within
what was for him an ill-conceived family.149 It seems
251
probable also that his view of Ramsay's helium transformation
was a jaundiced one, which may account for his continuing
resistance to the disintegration theory even after that
most impressive demonstration.
It was indeed necessary for Soddy to speak against
Armstrong at the meeting of the British Association in
September 1903 as the former had anticipated. The unusually
lengthy discussions which followed Rutherford's paper150
have been seen as marking a turning point. An account of
'How the "Newer Alchemy" Was Received' describes the way in which 'The opposition, brought into the open, was all but
demolished by the strength of the demonstrated support for
the theory'.151 It is to be noted that Armstrong's subsequent
three year absence from public discussions of the sabject152
fits with this interpretation but that Soddy's reference
several months later to 'the I expect numerous class of
unconvinced chemists'153 apparently does not. Perhaps the
members of such a chemical class were able to ignore the
electrical results as foreign subject matter, were left
unconvinced by the chemical evidence for transformation, and
accounted for the most recent demonstrations by R.Meldola's suggestion that radium was in fact an unusual chemical com-
pound containing helium.154 Though F.Richarz, disciple of
H.Helmholtz155 and similarly interested in the borders
between physics and chemistry, stressed the 'Analogien
zwischen Radioactivitgt and dem Verhalten des Ozons'156
such physical interpretations were largely extinct by the
end of 1903.157 As for the chemists, The Electrician seems
to have portrayed them as a single group and pointed with
relish to the disparity between the hypotheses of Meldola
and Armstrong.158 There was an air of editorial disappoint-
ment that W.C.D.Whetham (at that time reading the proofs
for Rutherford's book on Radio-activity) in his reply -Go
Meldola merely summarised:
the case for the transmutation hypothesis, from the point of view of the physicists. We should have been glad if a physical chemist so well known as Mr. Whetham had given us, rather, a glimpse of the arguments pro and con which arise in the chemical mind... 159
252
Thus physicist baited chemist across the gulf which divided
them. Radioactivity which might in theory have formed a
bridge of harmony instead served some as a route of attack.
But the response grew faint.
A few dissenting chemists such as W.Ackroyd of the
Halifax Borough Laboratory, who continued to advocate an
external source of radioactive energy,160 and the famous
Marcelin Berthelot who turned from his hopeful 'Etudes sur
to radium' of 1901161 to reservations concerning 'Emanations et radiations' in 1904162 made their voices heard. But
they were outnumbered by those such as F.Giesel, W.Marckwald,163 G.Martin, A.Debierne,164 Ramsay, Crookes and Soddy who
favoured or employed the disintegration theory. If Soddy's
'unconvinced chemists' formed a majority it was largely a
silent one; it may nevertheless have influenced future
recruitment and prospects.
2. The mechanism of radioactivity (1903-4)
Although Rutherford and Soddy were credited with the
discovery of the first chemical evidence of the subatomic
nature of radioactive change they held no monopoly of dis
cussions concerning the mechanism of the process. This
applied both when disintegration was supposed to involve
the emission only of corpuscles or electrons and after
Rutherford had in 1902 recognised as its most striking
253
feature the high-velocity alpha 'projection of a heavy
charged mass from the atom'.165 One can well understand
the closing comment of the latter's earliest exposition
of the disintegration theory: 'Nothing can yet be stated
of the mechanism of the changes involved'.166 We recall that two years earlier he believed that an all-electronic
atom contained insufficient energy to support the observed
radiation; the newly advocated reduction of mass to elec-
tricity167 may not have eased the difficulty. In his first
announcement of the magnetic and electric deviation of the
alpha rays Rutherford made clear his modified view:
There seems to be no doubt that the emission of 3 rays by active substances is a secondary phenomenon, and that the 0( rays play the most prominent part in the changes occurring in radioactive matter.168
His increasing hope of discovering the mechanism of these
changes is indicated by the further comment that:
The power possessed by the radioactive bodies of apparently spontaneously projecting large masses with enormous velocities supports the view that the atoms of these substances are made up, in part at least, of rapidly rotating or oscillating systems of heavy charged bodies large compared with the electron. The sudden escape of these masses from their orbit may be due either to the action of internal forces or external forces of which we have at present no knowledge.169
From the time of publication of these statements early in
1903 various physical scientists sought to grasp by moans
of hypothetical atomic structures the underlying mechanisms
and causes of disintegration; this phenomenon was by some
emphasised less strongly than other areas of chemical
physics. In the case of radioactivity the design of suitably
unstable model atoms which would produce the successive
elements, appropriate rays and stable end point remained as
a recognised problem for decades. The questions of the
origins of these atoms and of the causes of their peculiar
law and rates of decay similarly stood unanswered. Before
discussing the contributions of physicists, who indeed
made most of the running with this approach, I shall briefly
examine relevant views held by some members of the chemists'
fraternity.
254
In his report on 'Inorganic Chemistry' for the year
1904 to the Chemical Society of London P.P.Bedson17° gave pride of place to the latest publication of D.Mendeleef,
Professor of Chemistry at St.Petersburg. The propounder
of one of the earliest periodic tables had now, some thirty
years later, extended the system in An attempt towards A
Chemical Conception of the Ether.171 He had added a zero
group which included Ramsay's five new inert gases172 and which was headed by the 'ether' as the lightest and most
inert element. The noble gases were thus doubly linked
with radioactivity, though Mendeleef made no mention of the
emanations. For the concluding passages of his little
book173 dealt with explanations of radioactivity in terms
of the ether. This substance in his opinion possessed an
'individualised attractive capacity, a mean between gravity
and chemical affinity'174 which caused its condensation
upon the heaviest atoms, those which exhibited the 'photo-
radiant' and ether 'emission' phenomena of radioactivity.
Similar attractive forces were assumed to explain the known
solubilities of the other inert gases in water. Mendeleef's
views were not entirely out of the run of current opinion.
The idea of a zero group had commonly attended attempts to
incorporate the inert elements into a periodic scheme. And
he was not alone in proposing the existence of a chemical,
material ether. C.F.Brush, for example, announced in 1898
the detection, by thermal conductivity measurements, of
'Etherion: A New Gas' consisting of one or more entire
groups of new elements all much lighter than hydrogen and
probably filling 'all celestial space'.175 However, its
properties were soon ascribed by Crookes176 and Dorn177 to
nothing more than water vapour. The leading spectroscopist
W.N.Hartley proclaimed in a review of his 'branch of
chemical physics' in 1903178 that 'atoms of definite groups
of chemically related elements are composed of the same kind
of matter in different states of condensation'; and spec-
ulated that the 'molecules' of matter in the state of
greatest attenuation 'may be imagined to constitute the
ether'.179 And Marie Curie's theoretical attempts to
255
preserve the stability of the radium atom in 1904 involved
an action of this element upon the 'other du vide' to
provide 'transformations atomiques';180 possibly some form of condensation was envisaged here. A reviewer in the Chemical News181 described Mendeleef's work as speculation
'pure and simple' yet put forward somewhat similar ideas
himself; and Bedson, looking back to the reception of
Mendeleef's earliest periodic system, warned against too
dogmatic a critique of the newest one.182
Mendeleef made clear his hopes of creating explanations uniting physics and chemistry183 but he rejected equally
strongly the suggestion of any single material or other
basis of chemical matter. He considered that he had now
provided a superior alternative to the protyle or electron
theory having abandoned that development as atrophied more
than a decade earlier.184 Mendeleef did 'not require the
recognition of a peculiar fourth state beyond the human
understanding (Crookes). All mystical, spiritual ideas about ether disappear'.185 He considered that the 'series
of recently discovered physico-chemical phenomena', partic-
ularly radioactivity, which had prompted his publication
and had 'caused many to return to the emission theory of
light, or to accept the, to me, vague hypothesis of electrons'
were best explained in terms of 'the entrance and egress of
ether atoms' and the 'familiar conception of an etherial
medium transmitting luminous vibrations, &c.'186 Such a conception owed much to the condensable ether of low density
of Kelvin187 whose case will be taken up shortly. Each of
these men died in 1907 whilst in opposition to the disin-
tegration theory. The example of Mendeleef may thus seem
to be a negative one, the more so since material ethers
faded from science during the early decades of the twentieth
century. Yet his low opinion of the hypothesis of electrons
was shared not only by chemists who like him rejected the
closely related disintegration theory but by some who
accepted the latter with enthusiasm.
W.Ostwald began his Faraday Lecture in the spring of
1904 with an unusual description of that 'venerated master'
256
as a past leader in the theory of force or energy. He
ended the address with a discussion of radioactivity which
was similarly inclined. Ostwald had fitted the notion of
spontaneous transmutation with his well known energetic conceptions of 'Elements and Compounds1188 in which 'Chemical dynamics' had 'made the atomic hypothesis unnecc-essary' for deducing the laws of chemical combination.189
He therefore depicted the chemical elements in terms of an
energy curve in the form of a series of 'stalactites'.
'The elements with the highest combining weight' Ostwald
represented by the shortest or rudimentary stalactites on
the 'sloping ceiling'; along these a drop of water would
run at varying speeds of its own accord. The heavy elements
were thereby endowed with the temporary existences required
by a theory of stepwise transmutations. He claimed an
independent realisation of the idea that their known stability constituted an argument for, rather than against, the
presence of a large store of energy within the elements.
In his view the attainment of artificial transmutation was
prevented only by the practical impossibility of concen-
trating sufficient power.19° Ostwald's approach gave no clue as to the subtlety of the eventual attainment of this end.
Its limitations are exemplified by his description of the
complex radiations merely as 'intermediate forms' of energy;
and by his attribution of both the radioactive and chemical
stability of helium to the same 'exceptionally long stalactite' when a distinction had already been recognised,
for example in the case of radium. The importance of
energetics for radioactivity cannot be denied on that
account since many problems of radioactivity and atomic
structure from that time onwards were expressed in such terms.191 However, considerations of energy were never
taken as a complete description of any atomic process; not
even by Soddy who pointedly refrained from depicting any detailed atomic structure for almost a decade.192
One of the most interesting features of Soddy's view
of radioactivity is his attitude towards the corpuscular or
electronic theory which became during the period under
257
oonsideration the very foundation of many physicists'
understanding of chemical matter. It appears that his
more or less continuous skepticism came to a form of
fruition in 1904 when it became almost constructive. The
clash between Oxford chemist and Cambridge physicist at
McGill in mid-1901193 illustrates Soddy's early hostility
to the corpuscular theory. It is possible that he had
acquired these views whilst at Oxford; one of his referees
there was soon to question the validity of his electrical-
emanation studies. During the course of the pioneering
work with Rutherford the antagonism was suppressed within
or absent from Soddy's mind; in their joint publications of
1902 on thorium the emission of corpuscles was cited as
evidence in favour of the occurrence of subatomic chemical
change. However, the earlier attitude began to reappear in
Soddy's lectures and writings of 1903-4 after his return to
England. During the course of his dozen lectures on
'Radio-activity'194 Soddy described quite fully both the
current cathode and beta ray researches and the theoretical
reasoning which led to an electrical explanation of mass
or matter. 'How far these calculations possess a real
meaning I am not in a position to say' he remarked with the
qualification that the electronic hypothesis despite its
many 'doubtful points' was yet 'necessary to assist the
mind in forming concrete mental pictures of the various
relations between matter and electricity'.195 In the ampli-
fication of his lectures in book form196 Soddy went a stage
farther as he closed similar discussions with a ohemist's
claim for independence: It may at once be pointed out that the theory of atomic disintegration, to which, in the succeeding chapters, the study of radio-activity will lead, is independent of the electrical or electronic view of atomic constitution. It postulates no view of atomic structure beyond the original conception of Dalton... 197
He commented that the theoretical dependence of the
electrical mass of an atom upon its internal energy served
to show: how useless it is to attempt to find numerical relations between the atomic weights of the
258
elements of the Periodic Table. It is notorious that all such efforts have been fruitless, but it is only recently that the reasons for the failure have been indicated.198
Now the Cavendish laboratory had quite recently continued
the discussions 'On a General Numerical Connexion between
the Atomic Weights'.199 Such acidic remarks200 can therefore
be interpreted either as another assertion of independence
from the Proutian theme or as an attempt to confound the
physicists with their own labours.
Against only one aspect of the physical view of radio-
activity were Soddy's criticisms effective, but here they
were particularly so. From the accepted theory it followed
that prior to its sudden disruption an atom of uranium for
example enjoyed a long period of quiescence. Physicists
had come to realise this via an interesting route, shortly
to be discussed. By 1904 they were generally satisfied
with the assumption that the period of temporary atomic
stability must be explained by a slow progression in the
atom's internal corpuscular pattern. Soddy considered
such a picture to be totally untenable. His attack was
based upon two main considerations. Firstly, a very slow
approach towards the point of disintegration should be
accompanied by similarly gradual alterations in the physical
and chemical properties of an atom. If this were so then
'it should be easy by chemical analysis to separate the
homogeneous elements into groups'. But apart from the con-
fused case of the rare-earths there was no evidence of any
Each success.201 This argument seems to impinge upon
Rutherford's suggestion202 that an outlying electron might
be the continuous cause of a disintegration. Against the
possibility that any such cause could exist Soddy put
forward a startlingly straightforward argument which could
not be disregarded: the asymptotic law of radioactive
decay could accomodate no stage of a definite timespan.
Although 'the average life' of its atoms was a specific
feature of each radioelement the law implied that 'some of
the atoms break up in the first second' yet others survived
almost for ever.203 He himself went beyond 'the original
259
conception of Dalton' to assert that individual differences
exhibited by different atoms of the same element were man-
ifestations of an 'extremely rapid motion' of 'the internal parts of an atom':204
the internal movements of the atom must be highly irregular and cannot follow a definite sequence if the law of radio-active change is to hold good. The unstable position appears to be rather the result of a chance collocation of the parts than to be due to the operation of any simple law. An analogy might be drawn from the kinetic theory of gases, in which certain of the molecules are regarded as possessing momentarily much higher and others much lower temperature than the average, and the acting causes are so complex that, although the proportion of the whole at any temperature may possibly be calculated when the total number of systems is exceedingly great, the individual history of any one molecule is quite indefinite. In a radio-active substance a definite fraction of the total assumes a peculiar orientation and disintegrates in each second, but the life of any single atom is quite indefinite. The causes at work appear to be so complex that the results can only at present be described as 'chance' or 'accidental' happenings, in the sense of being impossible to predict.205
One may also quite readily devise kinetic models to imitate
aspects of radioactivity for example by depicting an atom
as a vessel containing a number of gas molecules; the diff-
usion or chance escape of a molecule through a minute hole
in the vessel would represent one disintegration. A large
number of such atoms constituting one radioactive sample
would exhibit the required geometrical decay law yet would
allow the life of an individual to range unpredictably
between zero and infinity. Soddy gave no such crude pictures.
But he was of the opinion that the comparison of radioactivity
with gas kinetic theory 'is important' in the further respect
that:
it suggests the question whether all atomic properties are not really average properties, the individual atoms continually passing with great rapidity through phases varying widely among themselves in chemical and physical nature.206
This suggestion was in accord with some contemporary dev-
elopments in both of the areas specified in the title of
Soddy's new post of 'Lecturer in Physical Chemistry and
Radio-activity in the University of Glasgow' .207 His
260
conolusions were unaffected by Bragg's notable discovery,
announced in late 1904,208 of the definite and by no means
irregular velocities of the alpha particles emitted by
each element in a decay series.209 As Soddy had stated,210
his hypothesis was at least consistent with the known law
of decay whereas the current theory of the physicists was
not. Though Soddy's criticism of the electrical explanation
of radioactivity was indeed accepted by his former physical
partner the electronic theory in general did not succumb to
his incursions. J.J.Thomson's treatment of thermal and
electrical conduction in metals211 indicates the possible
compatibility between kinetic and electronic theories of
atomio structure. However when Thomson later defended his
notion of radioactive decay it was on quite different
grounds. Since the disintegration theory of Soddy involved
irregular and unpredictable movements it may appear to
provide an uncertain foundation for experimental advance.
The physicist E.von Schweidler firmly grasped this nettle
with the first formulation of a clear statistical-probabilistic
approach. In his paper at the 1905 Congress of Radiology in
Liege he showed that the law of geometric decay was deducible
from the probability equations for random processes. His
major point giber Sohwankungen der radioaktiven Umwandlung'212
was that as it declined the rate of disintegration should
undergo fluctuations according to probability predictions;
these, he estimated should lie within the grasp of actual
electroscopic detection. Thus in Vienna where Boltzmann
(d.1906) had done so much with the statistical theory of
gases213 this old approach opened up and contributed to the
development of a new experimental path which led quickly
back to Rutherford. Soddy's anticipations may have influenced
the latter who, however, credited Schweidler with the innov-
ation.214
On the other hand it could be argued that a probabilistic
interpretation of radioactive decay pre-dated even the
expressions of Soddy. His conclusions may be seen as the
ultimate internalisation of the 'kinetic' explanation of
261
radioactivity which had in 1898 been among the earliest
to be suggested. One of the turning points in this minor thread came in mid•-1903 at a time when the discrete nature of atomic disintegration was becoming appreciated. The
surrounding molecules of gas became, for some, not the
primary source of the energy released in radioactivity but
the detonators by impact of internal atomic explosions.
The statements of Lodge exemplify this stage. At the end
of 1902, after reading the latest publications of Rutherford
and Soddy, his interpretation of radioactivity apparently
included both internal and external factors: in the case of massive molecules their mutual collision or agitation under the influence of ordinary temperature is sufficient to shake away some of the loose electrons, which then fly off tangentially with whatever orbital velocity they may have had: giving rise to phenomena recently discovered under the name of radio-activity..215
He may have been the first to make the valuable suggestion,
linking the recent results of Curie and Rutherford, that the
heat produced by radium was a result of self-bombardment by
its 'massive' alpha-projectiles. Yet in his letter on
'Radium Emission' 216 which contained this point Lodge still put forward the alternatives of an 'assumed necessary
stimulus, or external supply of molecular energy'. Thomson's
article published one month later, at the end of April 1903,217
shows that despite his specification of an internal energy
supply he too favoured the notion of a kinetic stimulus:
Suppose that the atoms of a gas X become unstable when they possess an amount of kinetic energy 100 times, say, the average kinetic energy of the atoms at the temperature of the room. There would, according to the Maxwell-Boltzmann law of distribution, always be a few atoms in the gas possessing this amount of kinetic energy; these would by hypothesis break up; if in doing so they gave out a large amount of energy in the form of Becquerel radiation, the gas would be radio-active, and would continue to be so until all its atoms had passed through the phase in which they possessed enough energy to make them unstable... 218
A similar 'law of distribution' if applicable to the non-
gaseous radium atoms would account for their passage too
'into some other configuration'. A response to these views
262
was provided by Lodge, whose brief 'Note on the probable
occasional instability of all matter'219 directly followed
his acclamation of Rutherford's exposition of the disint-
egration theory at the London Physical Society in June.220
Considerations of radioactivity appear to have changed
Lodge's view concerning Larmor's well-known solution to
the problem of the theoretical loss of energy from any atom
containing orbiting electrons. This development in return
allowed the electron theory to impinge strongly upon radio-
activity at a point where it had been unconsciously held at
bay. In previous months Lodge had supposed that incessant
radiation exchanges made unneccessary Larmor's proposal of a
zero vector sum of the electrons' accelerations;221 the
process of radioactivity was quite distinct and involved the
release of orbiting electrons by molecular collisions.222
But Lodge was now prepared to insist upon the importance for
radioactivity of the 'radiation or loss of energy' which
'must occur from every atom'.223 Calculation showed that an
electron suffering this loss would move inwards at increasing
speed until, as its velocity approached that of light, the
mass 'becomes suddenly infinite or very great'. It was this
effect which in his opinion constituted the likely cause of
the breaking up of an atom. Like Thomson he understood
that it was 'only a question of time how long an atom shall
last before it reaches this stage'. The significant dep-
arture by Lodge is to be found in his concluding comment
directed specifically at Thomson that 'the slight constant
radiation-loss seems competent to bring about instability
and decay irrespective of collisions, and therefore indep-
endently of any Maxwell-Boltzmann law'.224 Lodge's discussion
had an influence not so much for his explanation of dis-
integration by increased electronic mass, for which there
were alternatives, but for two more general proposals.
Apparently following one of these, Thomson indeed dispensed
with the probabilistic analysis of radioactive decay. Perhaps
gladly so; even Soddy's counterblast implied that the stat-
istical method was no more than a temporary and sometimes unavoidable substitute for definite knowledge. Nevertheless,
263
considerations of that kind had formed a continuous theme
in radioactive studios from almost their beginning.
Lodge's most influential proposal was to the effect
that a steady radiation-loss, understandable in terms of standard electromagnetic theory, was the prelude to dis-
integration. For a time it raised hopes of picturing and
predicting changes within the chemical atom, which at that
time were not high. It appears that the approach initially provided by the electron theory towards a complete account of spectral patterns was offset by the growing complications
of magnetic studies. For those who attempted to design
electronic models of the atom there was also the fundamental
difficulty concerning the nature of the positive charge.
J.H.Jeans was one of those who included shells of the un-detectable positive electrons in the hypothetical model he constructed to explain the above phenomena and others,
though not radioactivity. His appeal of 1901 that this theory of 'The Mechanism of Radiation'225 be not even 'judged as an attempt to attain to ultimate truth' has been
described as a typical disclaimer of the period.226 Though radioactivity placed additional demands upon the proponents
of atomic models its exciting new facts were attended by
experimental certainty. In conjunction with the theoretical
points made by Lodge in 1903 these results perhaps contributed
to the optimism of some expressions which appeared in 1904.
H.Nagaoka indeed hoped that 'The rough calculation and rather
unpolished exposition' relating to the 'Kinetics of a System
of Particles illustrating the Line and the Band Spectrum and
the Phenomena of Radioactivity'227 'may serve as a hint to
a more complete solution of atomic structure'.228 The Electrician's editor heaped an unwonted amount of praise upon Thomson's 'Structure of the Atom'.229 The task of 'handling mathematically the swarm of flying electrons'
constituting the material atom was 'obviously a formidable one'. Yet Thomson had 'made a huge stride towards the goal'.230 He had developed 'lucidly and with great perfection,
a wonderful theory of the chemical elements', an account of
the 'main laws of the line spectra of a series of elements'
and finally 'the most suggestive conception yet offered of
264
the mechanism of the radio-active elements'.231 Yet in the same breath Thomson's electronic arrangement was
described as 'an apparently highly artificial conception'.
Rutherford, who was impressed by these models, did not long
maintain his current confidence that he knew the 'probable
... primary cause' of atomic disintegration.232 In fact deficiencies in the various systems were not hard to find.
One may discern a subsequent withdrawal to more reserved
attitudes as some of the contemporary criticisms found their
mark.
Between the time of his death in 1907 and his earliest
published comments on the disintegration theory in late 1903
Kelvin's interpretation of radioactivity fluctuated con-
siderably. One might say that his ideas moved in an ellip-
tical path since they appear similar at each of these dates.
Kelvin's 'Contribution to Discussion on the Nature of the
Emanations from Radium',233 read by Lodge at the British
Association meeting of 1903, has sufficient faults possibly
to have embarrassed the observer. He was the only disting-
uished physicist to reject, with Armstrong and Lowry, the
theory of Rutherford and Soddy.234 Kelvin's individual views
were that the gamma radiation was 'merely vapour of radium',
that the alpha rays were atoms of radium or molecules of
radium bromide which apparently also comprised the emanation,
and that the experiments which purported to show a loss of
weight from active materials235 were acceptable. He attrib-
uted the extreme activity of radium to its possession of an
abnormally large quota of 'electrions' neutralising the
positive atom. 'But' he noted 'this leaves THE mystery of
radium untouched'. Kelvin's atoms provided no obvious energy
source: he considered it 'utterly impossible' that the known
emission of heat 'can come from a store of energy' in the
radium.236 He wrote in similar vein to J.Dewar237 and
W.Ramsay238 describing as 'utterly improbable' the hypothesis
of 'evolution in the atom or transformation of its substance'.
Instead it was 'absolutely certain' that 'energy must somehow
be supplied from without' possibly by means of ether waves.239
A.S.Eve has noted240 that Kelvin courageously 'abandoned
265
his theory publicly at the 1904 British Association' to
'fall in line with Rutherford's ideas'. But though true
this was not the whole, nor the end, of that story.
Kelvin indeed constructed his 'Plan of a Combination
of Atoms having the Properties of Polonium or Radium':241
(1) To store a large finite amount of energy in a combination having very narrow stability. (2) To expend this energy in shooting off with very great velocity, vitreously and resinously electrified particles.242
But when interpreted in the light of the atomic theory
described in his paper 'Aepinus Atomized'243 the conversion
seems marred by a heresy, or contradiction. Kelvin's model
atoms of 1901 had consisted of ponderable but interpenetrable
spheres244 of various sizes, positive electricity distributed
uniformly within. One or more potentially mobile but normally
static negative 'electrions' occupied each sphere. However,
out of line with the unifying ideal of the electron theory,
he felt that one could not assume that electrical forces
alone operated between atoms:
we must keep ourselves free to add a repulsion or attraction according to any law of force, that we may find convenient for the explanation of electric, elastic, and chemical properties of matter.245
Thus Kelvin's atoms were distinct from each other in several
respects namely, size; 'quantum' of positive charge and
whether or not completely neutralisable by electrions; and
finally 'it is possible that the differences of quality are
to be wholly explained in merely Boscovichian fashion by
differences in the laws of force between atoms'.246 In 1901
Kelvin appears to have allotted one distinct atom to each
chemical element, certainly at least for 0,N,H,C1,C,S and
Na;247 and in 1904 he indicated his retention of these
earlier notions.248 But his plan for polonium contained no
less than sixteen atoms, and that for radium two of different
sizes. Kelvin's statement that these substances differed
from 'ordinary matter' only in the high degree of their
'shooting'249 thus calls into question his conception of a
chemical element.
266
This inconsistency may possibly have been a cause of
the reactionary trend to be seen in his succeeding state-
ments. Thus his 'Plan of an Atom to be capable of Storing
an Electrion with Enormous Energy for Radio-activity'250
of 1905 involved considerations of thelwork-curve' within
a single atom only, albeit of a different, onion-skin,
design. And after opening a public dispute in 1906 con-
cerning among other points the manner in which 'radium'
could be said to contain helium251 Kelvin reverted to views similar to those he held in 1903. His final state-
ments that the energy was drawn from external heat and that
heating effects were mainly produced not by alpha particles,
which were charged radium atoms, but by emitted electrions252
isolate Kelvin from current research. Despite the evident
flexibility of his approach Kelvin was never quite able to
countenance the transmutation of atoms.253
Whilst Kelvin directed his theoretical considerations
specifically upon radioactivity two other physicists
incorporated the subject instead as a more or less important
secondary feature of atomic structures which were designed
primarily to explain other phenomena.
I propose to discuss a system whose small oscillations accord qualitatively with the regularity observed in the spectra of different elements and by which the influence of the magnetic field on band- and line-spectra is easily explicable. The system here considered is quasi-stable, and will at the same time serve to illustrate a dynamical analogy of radioactivity, showing that the singular property is markedly inherent in elements with high atomic weights.254
With these words H.Nagaoka introduced what he described as
a new version of an old story. He took a single 'positively
charged particle' surrounded by a revolving circular ring
of equally spaced electrons to be perhaps 'the most easily
conceivable' system for mathematical treatment; actual
chemical atoms would possess a number of concentric rings one
corresponding to each of the different spectral series
exhibited.255 Nagaoka saw a connection between this spectral
hypothesis and radioactivity in the example of radium.
Since its spectrum appeared simpler than those of elements
267
of comparable or oven lesser atomic weights then the radium
electrons must be arranged in fewer and therefore larger
rings. His dynamical analysis indicated that the more
electrons there were in a ring the greater its susceptibility
to the disturbances which might lead to disruption. Further-
more, as he noted, elements of high atomic weight were most
likely to contain 'massive rings' and consequently to exhibit
radioactivity, the manifestation of instability. This reason-
ing which evidently explains the relative activities of
uranium and radium may appear promising. However, the spec-
tral correlations, electrical neutrality, and mechanical
stablility of the structure were all put in doubt by the close questioning of G.A.Schott who claimed to have rejected
such a system as both unstable and 'not worthy of publication'
some five years earlier.256 He had deduced that the theoret-
ical vibrational instability attributed by Nagaoka to large
rings in fact extended much farther and applied to almost
all rings bar a handful of the smallest variety. In any
case it seems that Nagaoka came close to refuting himself
in mentioning Sir Oliver Lodge and the different problem of
the radiation loss from an orbiting electrical charge. For
he neither followed Lodge in considering this as a cause of
instability nor did he attempt to neutralise the difficulty
in the vectorial manner of Larmor and Thomson. He merely
stated that the loss from a 'Saturnian system' should be
'properly compensated' but did not say how this might be
arranged. Other difficulties of Nagaoka's system relating to
radioactivity were less readily avoidable than this. The
'disintegration of the ideal atom', he thought, involved
the breakage of a ring when its electrons 'will disperse in
various directions with great velocities, and the positively
charged particle at the centre will also fly off'.257 His
failure to indicate whether or how changes in the central
particle could explain the release of two heavy positive
particles, for example from radium, may relate to the
current problem of understanding the nature of the positive
charge. On the other hand there appears to be a quite
268
definite conflict with the known fact that alpha and beta
rays were emitted separately; a solution would seem hard
to find. In considering the stages leading to disinteg-
ration Nagaoka assumed that a ring was subject to 'resonance'
which 'in course of time, if the disturbance be persistent,
will acquire such an amplitude as to break the ring'.258
He was prepared to name as initiators of the resonance both
vibrations of other rings within the atom and incident
electromagnetic waves. But his discussion regarding such
an external cause seems only to weaken the case. He argued
indeed that since the destructive higher harmonics could
be excited by light of short wavelength it followed that
'actino-electric action259 may be the result of the des-
truction of atoms' under the combined influence of an electric
field and incident radiation; semiconduction effects, known
also to be produced by these forces, might be similarly
explained. Nagaoka's optimistic attempts to reunite dis-
parate phenomena in this way were short lived, thanks largely
to Schott. Yet they bore some resemblance to the better-
appreciated efforts of Schott's former professor.
J.J.Thomson too placed radioactivity in an important
supporting role in a plan of atomic structure. Issued in
late 1903, his paper on 'The Magnetic Properties of Systems
of Corpuscles describing Circular Orbits'260 contains a partial defence of such systems by examining:
problems ... met with when we attempt to develop the theory that the atoms of the chemical elements are built up of large numbers of negatively elec-trified corpuscles revolving around the centre of a sphere filled with uniform positive electri-fication.261
Regarding this theory he confided to Lodge some months later
that: I have ... always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity, and to replace it by some property of the corpuscles.262
This attitude allowed Thomson to go beyond the ponderable
positive atom adopted by Kelvin during 1902-4 which was
superficially similar to his own. Thomson was thus able to
269
explain physical phenomena, including radioactive trans-
mutations, in almost exclusively electronic terms; the
positive charge was a more follower of the encampment of
corpuscles.
The first point which Thomson considered in his paper
on magnetic properties was the problem of radiation loss.
His analysis showed that when corpuscular velocities were
email compared with the speed of light the radiation dimin-
ished very rapidly as the number of particles increased.
For example, 6 corpuscles rotating at 1/100 the velocity of
light emitted elliptically polarized radiation at only 10-16
of the intensity for a single corpuscle.263 Rather than
claim permanence for the arrangement, as he might,264 Thomson
instead used the very small theoretical lose in a remarkable
attempt to solve two further problems of the system which
at the same time united all three. He confirmed W.Voigt's
deduction that the magnetic properties of corpuscles set
out in rings also tended to nullify each other; but in this
case cancellation was complete. The hypothetical system
failed to display the known magnetic properties. The master-
stroke which might have succeeded was Thomson's proof that
this conclusion did not apply if the system were losing
energy:265 such a dissipation he related to radioactivity:
suppose the atoms of a substance, like the atoms of radio-active substances, were continually emitting corpuscles; the velocity of projection ... being, however, insufficient to carry them clear of the atom ... then, if the motion of the corpuscles were not accompanied by dissipation of energy, the corpuscles would not endow the body with either magnetic or diamagnetic properties; if, however, the energy of the corpuscles was dissipated during their motion outside the atom, so that they ultimately fell with but little energy into the atom, a system consisting of such atoms would be paramagnetic.266
He suggested that if the 'energy of projection were derived
from the internal energy of the atom' then experiment should
reveal a higher temperature within iron than brass. Such
results were never reported; corpuscular theory, magnetism
and radioactivity were not to be connected in this way.
Thomson waited for the major sequel 'On the Structure
270
of the Atom: an Investigation of the Stability and Periods
of Oscillation of a number of Corpuscles arranged at equal
intervals around the Circumference of a Circle; with
Application of the results to the Theory of Atomic
Structure'267 to make clear the link which he envisaged
between a tiny continuous radiation loss and the projection
of particles from the atom. In this paper he gave explan-
ations of the periodic table, chemical valency and affinity
including the inert gases, and spectral formulae. He again
compared the basic structure with Mayer's magnets268 as in
1883 and 1897; the rotations imparted to the latest models
tended to stabilise movements from the plane. And this
was the clue to Thomson's view of the 'Constitution of the
Atom of a Radioactive Element'.269 Standard dynamical
analyses showed that the speed of rotation could be critical
for such stability. With 4 corpuscles, for example, the
planar arrangement would be more stable than the tetrahedral
only if the angular velocity exceeded a definite value
depending on corpuscular charge, mass and number, and the
atomic radius; below this value the stabilities would be
reversed. And at this value 'there will be what is equiv-
alent to an explosion of corpuscles'. The increased kinetic
energy 'might be sufficient to carry the system out of the
atom, and we should have, as in the case of radium, a part
of the atom shot off'. The approach of a long-lived atom
to this value was provided by the radiation loss which
allowed velocities 'slowly - very slowly' to diminish.270
'I think a spinning top is a good illustration of the radium atom' he told Rutherford.271 Thomson did not mention Lodge's
more extreme alternative. On the basis of the former's
suggestions the approach of a corpuscle's velocity close to
that of light appears unneccessary. However, the emission
from radium of beta rays travelling at 90% of this speed
was well known.272
Rutherford continued to follow the electronic tradition
and cited the discussions of Lodge and Thomson on several
occasions. The high velocities and independent emission of
the beta rays may have been reasons for his leaning towards
the hypothesis of Lodge during 1904. Of late Larmor's
271 'lion of the season' Rutherford was now leader by far of the field in his incorporation of these and other facts,
many of his own making, into a coherent theory of radio-
activity. His attempts on this basis to depict the con-
stitution of the chemical atom suffered from the same
fundamental problems as those of his fellows. Yet to the
experimental scientist such diseases were fortunately not
fatal.
3. Conclusion
Early in 1904 Rutherford saw the radium series
extending to at least273 seven members. The succession of changes, with its radiations and half-lives, proceeded from
Radium (alpha, 1500 years274) to Radium Emanation (alpha,
4 days), followed by 'Emanation X' (alpha, 3 mine., soon
afterwards called radium A), 'Second change' (no rays,
36 mins.), 'Third change' (alpha, beta and gamma, 28 mins.), 2 'Fourth change' (alpha, beta, 200 years) and 'Final product'.75
Upon a theory framed only two years earlier was this new and
increasing knowledge founded. As might be expected Rutherford was not content to rest upon this base but
attempted to move towards a deeper synthesis. He argued
that the quite different six-membered thorium series and
272
uranium trio bore a resemblance to the above radium
sequence in one important respect:
The and probably also the rays of the three radio-elements thus only appear in the last of the series of radio-active changes. It is remarkable that the last change, which is readily detected by the radio-active property, should in each of the three radio-elements be accompanied by the expulsion of a single electron with great velocity... 276
Rutherford's subsequent Bakerian Lecture of mid-1904 contains
a description of 'The Succession of Changes in Radioactive
Bodies'277 which was the most detailed physical represen-
tation of the process of disintegration yet to appear; its
expression can be seen as a peak of confidence. The 'single
electron' of the 'last change' had become the mechanical key which released the apparently more important 'groups of
electrons' or alpha particles. According to Rutherford
events took the following course:
It may, perhaps, be supposed that occasionally one of the outlying revolving electrons, comprising the radio-atom, lapses into a position which results in a slow loss of energy ... in the form of radiation.278
In the ensuing situation of instability an alpha particle would fly off 'with its great orbital velocity, but the
atom still retains the disturbing cause' so that the required repetition would result. Meanwhile, and here Rutherford
acknowledged his debt to Lodge, the electron's velocity
would be increasing slowly until 'finally in the last stage a sudden lapse into a new state' ejected another alpha particle together with the rogue electron. Radium C
(RaC,half-life 28 mins.) and thorium B (ThB, 55 mina) were the crucial substances concerned here. The residual atom then 'adjusts itself again into a position of more permanent
equilibrium'279 corresponding to the longer lived product
RaD (40 years). That this latter material and UrX emitted
only beta rays seems not to fit in with this scheme.
Nevertheless Rutherford was sure that 'The experimental
evidence as a whole points strongly to the conclusion that
the change in which the /3 rays appear is far more disruptive in character than any of the preceding ones'. For not only
273
wan the accompanying alpha particle from RaC more pene-
trating than its predecessors, but recent electrolytic
results on ThB could be interpreted as a revelation of,
to use an anachronism, fission products. These were 'to
be expected' from the 'violent character' of this particular
change.280 Though branching disintegrations and high alpha
velocities were later confirmed for both RaC and ThB, and
the actinium series seems to have fitted quite well into
the scheme by 1905,281 yet during that year the fairly clear
theoretical picture became much obscured.
As the expanded second edition of his book shows,
Rutherford continued to think it:
probable that thep particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages. A dis-cussion of this question will be given with more advantage later (section 270) when the general question of the stability of the atom is under consideration. 282
He did not, however, provide the promised discussion; nor
did he again describe this electron as 'outlying'. One
reason for this may possibly have been the chemical
implications; another may be that such a description begged
the question of the cause of the electron's initial or
occasional 'lapse'. Hence Rutherford settled for his earlier
argument that although the law of decay could not itself
make a distinction negative experimental results283 made it
likely that rather than any external detonator it was 'forces
inherent in the atoms themselves' which brought about their
instability. And he repeated his view that:
It seems probable that the primary cause of the disintegration of the atom must be looked for in the loss of energy of the atomic system due to electro-magnetic radiation.284
But, as we have seen, Soddy's criticisms involving the
law of decay struck an area which no physicist, save perhaps
Nagaoka and Kelvin, had covered. To summarise the problem,
if the vector sum of accelerations of a system of electrons
is zero then the arrangement is stable, as are the atoms
of ordinary chemical elements. If the sum is not zero then
decay should occur: but not according to the observed law.
274
For this implied that some atoms disintegrate immediately
after their formation. To this argument Rutherford
responded with an unhappy compromise, outlined in his
published Silliman Lectures of 1906,285 before withdrawing
to a safer position. He still thought it most probable that
radiation loss was the 'primary cause' of disintegration286
but was aware that a steady drain from all radioatoms 'is
contrary to the observed law of transformation'. In attempt-
ing to avoid the difficulty Rutherford employed for the
first time the following argument:
We thus arrive at the conclusion that the configuration of the atom which gives rise to a radiation of energy only occurs in a minute fraction of the atoms present at one time, and is probably governed purely by the laws of probability.287
To this statement he appended a suitably revised version of
the single-electron theory in which 'one of the electrons
may take up a position in the atomic system which leads to
a radiation of energy'.288 Unfortunately such a hypothesis
harboured unmentioned problems and contradictions. For
since some atoms exploded in less than a second this energy
loss could neither be slow nor regular, nor could it con-
sistently be described as the primary cause of atomic dis-
integration.
The fundamental question of the processes leading to
disintegration was to be explicitly raised on numerous
occasions during the succeeding decades, which saw the dev-
elopment of the nuclear atom, the displacement rule and the
discovery of artificial transmutation. Even in the 1920's
when probabilistic interpretations of atomic structure began
to come into their own it remained unanswered.289 By the
end of 1906 despite his announcement of a new and suggestive
correlation between rate of decay and alpha velocity, which
could readily be related to the underlying concept of atomic
stability, Rutherford had reached the final retreat:
In the absence of any definite knowledge of the causes which lead to the successive disintegrations of the atom, it does not seem possible at the present time to give any adequate explanation of the modes of transformation observed in radioactive matter.290
275
The words with which he concluded that discussion of 'The
Velocity and Energy of the."( Particles from Radioactive
Substances' are of great interest:
A study of radioactive phenomena has emphasized the importance of the a particle as one of the units of which the heavier atoms are built up, and it is not improbable that the < particle may play an equally important role in the constitution of other atoms besides those of uranium, thorium, radium and actinium.291
This statement appears particularly significant when com-
pared with the closing comments of his Silliman Lectures
which had appeared shortly before:
It appears by no means improbable that the so-called radioactive bodies may differ from ordinary matter mainly in their power of expelling o< particles above this critical speed. Ordinary matter .., might be emitting a particles at a rate comparable with uranium ... and ... may be undergoing slow atomic transformation of a character similar to radium, which would be difficult to detect by our present methods.292
For these thoughts exemplify two aspects of what may be
seen as one broad theme which at that time seemed much more
than the mere speculation it had earlier been. A third
facet, which can be termed the 'cosmical' completes a trio
each complex member of which was to suffer a different fate.
Each area of this general idea that the phenomenon of radio-
activity was possessed of a universal nature had caught the
scientific imagination towards the end of the period of our
main concern at the time when the disintegration theory was
making its initial impact.
Early in 1903 Rutherford293 and Soddy cryptically
revealed their appreciation of the cosmical relations of
radioactivity. These authors were interested to point out
that Lockyer's views on Inorganic Evolution agreed with
their own on subatomic change. However, they effectively
reversed his dissociation hypothesis by noting that 'he
regards the temperature as the cause rather than the effect
of the process'.294 One striking implication of this was
made clear by W.E.Wilson's estimate, using Curie's figures
for the heat from radium, that the presence of this element
in a proportion of only 3.6 gm. per cubic metre (about
2.5 p.p.m., of the order of that in pitchblende) 'would
276
suffice to supply the entire output' of heat from the sun.
His brief letter to Nature on 'Radium and Solar EnergY'295 was followed by those of others, who hastened to take up
the exciting corollary of vastly increased astronomical
time scales, under headings such as 'Radioactivity and the
Age of the Sun'296 and 'Radium and the Geological Age of the Earth',297 as well as to point out difficulties.298
Rutherford, surveying these discussions shortly afterwards,
expressed the opinion299 that the time scale of the sun
might be 'from 50 to 500 times longer' than Kelvin's
estimates based on the energy of gravitational contraction
from a dispersed state. And he followed Joly's view that
the physicists' assessment of the earth's quiescent life-
period might now be stretched sufficiently to fit the min-
imum of 100 million years required by the biologists and
geologists against whom Kelvin had long argued. To geology,
radioactivity made a positive contribution via the minerals
which had from the first constituted the source of radium.
Their composition was both empirically studied and theoret-
ically explained on the basis of the disintegration theory
in the search for the parents and inactive descendants of
radium, which had begun in earnest by 1904.300 And a success-
ful approach towards the relative and even absolute dating
of minerals was one of the 'Cosmical Aspects of Radioactivity'
most confidently described by Rutherford in his departing
lecture to the Canadian Royal Astronomical Society.301
A.Schuster, introducing his own speculations on
'Cosmical Radio-activity', joined one aspect of the universal
theme with another in a statement typical of the period:
The fact that every physical property hitherto discovered in one element has always been found to be shared by all suggests the possibility that radio-activity may be a common property of all matter.302
By that time, in the autumn of 1903, Crookes, Kelvin, Lodge,
Thomson and others less well known in the field303 had already
assumed as much. However, the attitude of some experimental
students of radioactivity was more cautious. The Curies in
1900304 and Marie Curie again in 1903305 admitted their
predilection for 'the idea that it was scarcely probable
277 that radio-activity, considered as an atomic property,
should belong to a certain kind of matter to the exclusion
of all other', but then made the point that observation
showed any general activity to be less than 1/100 that of
uranium. By the time the later statement was made
R.J.Strutt and others had independently directed their more
sensitive experimental attention to the 'Radio-activity .of Ordinary Materials'.306 This, Strutt claimed, was small
(1/3,000 Ur) and variable but definite. His comment that to give such an effect one part of radium in three hundred million would suffice indicates the drift of his inter-
pretation. Moreover, the emanations and their active dep-
osits were known to be present in the atmosphere so that.
a minute surface activity of all solid materials was nat-
urally to be expected.307 But did any of the observed activity
in fact belong to the materials themselves? This question was
seen to.be of some importance with regard to general support
for the electron theory and, perhaps even more so, in rel-
ation to the theoretical problem of atomic stability which
beoame acute during 1903-4. As Rutherford succinctly
remarked:
According to the modern views of the constitution of the atom, it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be.308
J.J.Thomson indeed entertained notions which implied that ordinary chemical atoms were not permanent. We have con-
sidered the theoretical analyses of 1903 which induced him
to suppose that even the force of magnetism might derive
from the atoms' internal energy which would be finally dissipated as heat. And in a paper describing experiments
'On the presence of Radio-Active Matter in ordinary sab-
stances',309 read early in the following year, appeared his
conclusion that the ordinary material of the walls of a
closed vessel emitted, in addition to the effects of the
ubiquitous atmospheric emanation, a specific radiation of its
own. This too he thought 'involves a continual transformation . of the internal energy of the atom into heat',310 so also did
the normal dissociation and recombination of the gaseous ions
by which the radiation was detected. In this way Thomson
278
gave the universal radiation drain implied by his corpus-
cular atomic theory some experimental substance before the
appearance of his theoretical discourse 'On the Structure of the Atom'.311 We recall that during 1902 Thomson had
insisted that radioactivity was due to ionic interactions
of ordinary materials in opposition to the view of Rutherford and Soddy that special kinds of matter were
involved. And in some sense the difference of opinion was
maintained. However, N.R.Campbell in whose hands Thomson loft the subject obtained seemingly positive evidence of
'The Radiation from Ordinary Materials',312 though not of
the hoped-for heat emission. This turned Rutherford's strong
reservations of 1904313 gradually314 to complete acceptance.
Thus Rutherford proclaimed in 1906 that Campbell's results afforded:
very strong proof that ordinary matter does possess the property of emitting ionizing radiations, and that each element emits radiations differing both in character and intensity.315
Now this evidence did not stand alone. Rutherford had com-
bined it with two independent observations, which were
becoming clear during 1903-4, under the familiar 'general
principle' that 'every physical property discovered for one
element has been found to be shared by others'. The existence
firstly of quite rapid 'rayless changes' and secondly of a
rather high 'critical velocity' below which any emitted alpha
rays would be undetectable316 each implied that continual
unseen transformations might be universally proceeding. All
this evidently seemed convincing to Rutherford. But the
direct evidence soon faded into irregularity317 and, as one
can see, the latter two points merely allowed of a possib-
ility.
A further side to the universal view of radioactivity,
which may be termed the 'Proutian', has been introduced by
quoting Rutherford's words of 1906. Rutherford had by 1904 moved towards what he saw as a specifically radioactive
development of 'Prout's hypothesis,318 based on the observed successive release of alpha particles or helium from radio-
atoms. There was in his opinion 'no reason to suppose' that
279
radium was 'not an element in the ordinarily accepted sense
of the term' so that 'the radium atom is built up of parts, one of which, at least, is the atom of helium..319 He was able readily to combine this with the current electronic
version of Prout's hypothesis by asserting that the alpha
particles were themselves 'groups of electrons'. The radio-
atom thus consisted of electrons and large groups of elect-rons.320 W.H.Bragg accepted such ideas as the theoretical
basis of his experiments 'On the Absorption of the «Rays'321
and took them farther. In a letter to Rutherford he argued
that the alpha particle 'or some submultiple of it' might be
a 'common constituent' both of radioactive substances and of ordinary ionisable gases:
Then the electrons would be as it were the soldiers of the army, but the o< particles would be the regi-ments. Might not this account for the atomic weights having a leaning to whole numbers?322
Rutherford in turn extended his previous discussion with.
the remark that 'many of the elements differ in their atomic
weight by four - the atomic weight of helium'.323 Yet his continuing caution that the helium atom was but 'one of the
secondary units with which the heavier atoms are built up,-324
is to be noted. So too is his newly circumspect statement that the 'atoms of all bodies are built up, in part at least, of electrons'.325 If he now felt concern about the problem of the positive charge326 then this was soon to be justified.
In J.J.Thomson the persuasive force of Prout's hypo-thesis was manifested most clearly. Its effect on him
dates back to the 1880's when Crookes was proclaiming as
protyle the sun-element helium, then no more than a gleam
in the spectroscopist's eye. And its influence runs through the entire first decade of radioactivity to Thomson's
demolition in 1906 of his own all-electronic atom. 'The
Number of Corpuscles in an Atom'327 turned out to be a mere one thousandth of the number required to account for its
mass. At the opposite end of the discharge tube, whence he had in 1897 first extracted the corpuscular isubstanceq28
280
Thomson believed ho had found an alternative. The deflect-
able 'Rays of Positive Electricity'329 produced from a
variety of different elemental gases were apparently mainly
composed of streams of the alpha particles of radioactivity
together with hydrogen atoms. Thomson's dualist Proutian
interpretation of this particular result was unfortunately
most impermanent.330 Yet the idea provides one thread by
which to unravel the material upon which Lodge based his
judgement: regarding Rutherford's Radio-activity he wrote
'Scarcely anything to be found in this book was known
10 years ago'.331
281
NOTES FOR CHAPTER 1 (pages 8-47)
1 W.C.D.Dampier-Whetham, A History of Science, London, 1929, 382; D.L.Anderson, The Discovery 6f-The Electron, Princeton, 1964, 16; L.Badash, RutherfoW:Tia Boitwood: Letters on Radioactivity, London, 1969, Introduct on, Trig-eTimond Soientific Revolution', p.lf.; M.P.Crosland, The Science of Matter, London, 1971, 32; M.J.Nye, BWIecular Reali15.71Bndon, 1972, p.x.
2 L.Badash, The Early Developments in Radioactivity, with Emphasis on Contributions from the United States, Dies., Yale Univ., 1964, p.xiii, citing Maxwell, Papers, 244. L.Badash, The Completeness of Nineteenth Century Science, Isis, 1972, 63, 48-58, has since expressed a modified 1-7174.
3 Maxwell, ibid. 4 Page v. 5 Page 303. 6 RI Lib.Sci., (1889), 3, 481-92, 492. 7 W.McGucken, Nineteenth:-Century Spectroscopy, Baltimore
and London, 1969; S.G.Brush, The Development of the Kinetic Theory of Gases.VIII.Randomness and Irrevers-ibility, Arch.Hist.Exact Sci., 1974, 12, 1-88.
8 L.P.Williams the of VI7torian Science, Victorian Studies, 1966, 9, 197-204, 198-9.
9 Mme.S.Curie, Les Rayons de Becquerel et le Polonium, Rev.Ggn.des Sal., 1899, 10, 41f., Jan.; Oeuvres, 60-76.
10 Ibid., Oeuvres, 75. 11 RY-Eib.Sci., (1897), 5, 36-49, 30th Apr. 12 This. 787 13 D.M.Knight, Atoms and Elements, London, 1967, ch.3. 14 T.W.RichardsTriaTF-Weights, Chem.N., 1900, 81, 113. 15 W.V.Farrar, Nineteenth-century speculations on the
complexity of the chemical elements, Brit.J.Hist.Sci., 1965, 2, 297-323, 303f. Also relevani7-2.11/4/6731,-THe theory of the elements and nucleosynthesis in 19th century, Ch ia, 1964, 9, 181-200, but this has some anachronistic n erpretations.
16 T.H.Levere, Affinity and Matter. Elements of Chemical Philoso 1600-1665, Lon:Eli77971. R.Pox Tscusses this in re a on to The caloric Theory of Gases from Lavoisier to Regnault, London, 1971, ch.4, 6E767--
17 W.MoGuoken, Nineteenth-Century Spectroscopy, p.xi, lf. 18 Rep.Brit.Ass., 1666, 556-76. 19 IbidT7-561. 20 Ibid. 21 ysia. 22 Genesis of the Elements, RI Lib.Sci., (1887), 3, 403-26;
Presidential Addressed to-MliEical Society of London, J.Chem.Soc., Trans., 1888, 53, 487f.; 1889, 55, 256f.; Maugura-Address as President of Institute c77 Electrical Engineers, delivered 15th Jan.1891.
Notes for Chapter 1, p.8-47) 282
23 See e.g. E.von Meyer, A Histo of Chemistry, London, 1891, 349-50; also R. K. De kiisyk pecTF-oscopy and the Elements in the Late Nineteenth Century: the Work of Sir William Crookes, Brit.J.Hist.Sci., 1973, 6, 400-23, who discusses disagreements with French speotroscopists.
24 See A.E.Woodruff, William Crookes and the Radiometer, Isis, 1966, 58, 188-98; also G.G.Stokes, Mem.& Correa., 2, 8-408; S.G7Srush (ed.), L.Boltzmann, IdEfEiTtsWITTis
Theo , London, 1964, 25. 25 On t e fractionation of yttria, Rep.Brit.Ass., 1886,
586-90; also Genesis of the Elements, loc.cit., 405-16. 26 Genesis of the Elements, loc.cit., 411=27 27 Rep.Brit.Ass., 1886, 586-90. 28 Genesis 63Whe Elements, 421. 29 W.V.Farrar, op.cit., 319. 30 Kelvin, in G.G.Stokes, Papers, 5, p.xxxi; R.T.Glazebrook,
Report on Optical Theories, Rep.Brit.Ass., 1885, 157-261, 211.
31 Hereafter the name Kelvin is used, to avoid confusion with other scientists.
32 G.G.Stokes, Pa ers, 4, 373. 33 A.J.Meadows, Sc ence and Controversy. A biography of
Sir Norman f..6CITre7717c -gon, 1972, 169. 34 Researches in spectrum analysis etc., Bakerian Lecture,
Phil.Trans., 1874, 164, 479-94, 491. 35 =Stokes, Papers,-W7 365-6. 36 See e.g. On a certain reaction of Quinine, J.Chem.Soc.,
May 1869; Papers, 4, 327-33 37 On the Nature of the Röntgen Rays, Wilde Lecture 1897,
Papers, 5, 273. 38 G.G.Stokes, Mem.& Correa., 1, 406-7. 39 W.H.Brock, Lociq'ei-FIETIThe Chemists: the First Dissoci-
ation Hypothesis, Ambix, 1969, 16, 81-99. 40 W.McGucken, Nineteenth-Century Wectroscopy, 83-101. 41 A.J.Meadows, Science and Controversy 49tc.,ch.6; see also
C.L.Maier, ThITRn6f—gpectroscopy iE7The acceptance of an internally structured atom, 1860-1920, Dies., Univ. Wisconsin, 1964, 186-202 on 'Reactions to TBWEyeris dissociation theory'.
42 W.McGucken, 22.oit., 98; G.D.Liveing and J.Dewar, Collected Papers on Spectroscopy, Cambridge, 1915, 79, 139.
43 Proc.Roy.Soc., 187, 61, 148-209; C.L.Maier, Diss., 202-37, Mrs-Fribes Lockyer's later dissociation theory put forward about this time. 'Enhanced lines' indicate dissociation, but not now into products common to different elements.
44 See above p.18. 45 Quoted in M.W.Travers, A Life of Sir William Ramsay,
London, 1956, 100. 46 Ibid., 110, 154. 47 Phil.Mag., 1901, 1, 311-4; C.L.Maier, Dies., 105-6, 192,
men ions the ProuTian expressions of tEi.:7-s-pectroscopist J.R.Rydberg about this time.
48 The Ultra-Violet Spectra of the Elements, RI Lib.Sci., (1883), 3, 257-67, 259.
49 Quoted in D.M.Knight, Atoms and Elements, 130. 50 RI Lib.Sci., (1888), 3, 472-5.
Notes for Chapter 1, p.8-47) 283
51 W.V.Farrar, Nineteenth-century speculations etc., 317. 52 Proo.Camb.Phil.Soo., 1898, 10, 38-40, 28th Nov. 53 17RanTriiri317WaFEurg, Ueber die specifische Wgrme des
Quecksilbergases, Ann.d.PhT, 1876, 157, 353-69. 54 Liveing, 22.cit., E752, 39- O. 55 G.J.Stoney, b?-the Kinetic Theory of Gas, regarded as
illustrating Nature, Phil.Mag., 1895, 40, 362-83. 56 See Section 4 below, p.32-6. 57 Liveing, 22.cit., n.54. 58 Ibid. 59 kelvin, Nineteenth Century Clouds over the Dynamical Theory
of Heat and Light, RI Lib.Sci., (1900-1), 5, 324-58, 335. 60 J.J.Thomson, RecollectIZEs and Reflections, Toronto,
1937, 341. 61 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge,
1942, repr. London, 1969, 62 G.G.Stokes, Wilde Lecture 1897, Papers, 5, 257-8. 63 J.N.Lockyer, On the Chemistry of e otTest Stars,
Proc.Roy.Soc., 1897, 61, 148-209. 64 W.H.Brock, Lockyer aria-the Chemists etc., 98-9. 65 A.Schuster, Note on the chemical constitution of the stars,
Proc.Roy.Soc., 1897, 61, 209-13. Appended to Lockyer's paper, loc.cit., n.63.
66 A.J.Meeff&TrE; -ecience and Controversy etc., 152-3. 67 J.J.Thomson, T-stoTirreciTims and Reflecil7ns, 341.
G.Fitzgerald, 0.Lodge, W.Sutherland, thought the effects purely electrical.
68 Electric Discharge through Gases, RI Lib.Sci., (1894), 4, 282-90.
69 R.F.Schaffner, Nineteenth-Century Aether Theories, Oxford, 1972, 76.
70 On these aspects of Faraday's work, see L.P.Williams, Michael Faraday, London, 1965.
71 Preface to the let ed., 3 ed., 1892, p.viii. 72 M.Faraday, Thoughts of Ray Vibrations, Phil.Mag., 1846,
28, 345. 73 Fe-scribed by K.F.Schaffner, 22.cit.; and E.T.Whittaker,
History of the Theories of Aether and Electricity, 2 vols., London, 175111 1, ch.4f. /ST-Eiglyses of the latter are criticised by the former (p.viii).
74 Rep.Brit.Ass., 1885, 157-261. 75 Ibid. 260. 76 Ibid., 261. 77 Dated let Jan. 1885, Correspondence of J.J.Thomson, CUL. 78 Ibid., 1. 79 Phil.Trans.A., 1894, 185, 719-822. 80 Ibid., 719.- 81 Ibid., 806-22, dated 13th Aug. 1894. 82 H.A.Lorentz, La thOorie electromagn6tique de Maxwell et
son application aux corps mouvants, Archives Neerlandaises des Sciences Exactes et Naturelles, 1892, 25, 363f.; Pa ers 164-343-
83 .H rosige, Electrodynamics before the Theory of Rela-tivity. 1890-1905, Jap.Stud.Hist.Sci., 1966, 5, 1-49; id., Origins of Lorentz' Theory °lc-Electrons and the Tincept of the Electromagnetic Field, Hist.Stud.Phys.Sci., 1969, 1, 151-209.
Notes for Chapter 1, p.8-47) 284
84 R.McCormmach, H.A.Lorentz and the Electromagnetic View of Nature, Isis, 1970, 61, 459-97; id., Einstein, Lorentz, and the-EIT)(3tron Theory, Hist.STEd.Phys.Sci., 1970, 2, 41-87.
85 R.McCormmach, ibid., Isis, 1970, 463-4. 86 Versuch einer THI;7rie7d7gFt elektrischen and optischen
Erscheini'ngen in bewegten Kbrpern, Leiden, 1895; Pa errs, 5, 1-137.
87 On the Influence of Magnetism on the Nature of the Light emitted by a Substance, Phil.Mag., 1897, 43, 226-39, section 17.
88 J.J.Thomson, Cathode Rays, RI Lib.Sci., (1897), 5, 36-49, 49.
89 R.Hertz, London, 1893; repr. New York, 1962, 20. 90 Stuttgart. 91 Curie papers, BN, dossier 9, contains her brief notes
on the book. 92 CR, 1897, 125, 1165-9. 93 Mi;ude, op.-CIT., 589-90. 94 G.G.Stoki7-Un the Change of Refrangibility of Light,
Pa ers, (1852), 3, 259-413, 388-97. 95 Drude, op.cit., p.vi. 96 L.Boltzmann, Vorlesungen liber Maxwell's Theorie der
Elektrizitat und des Lichtes, 2 vols., Leipzig, 1891-3. 97 H.Pofncare, ETJEFIFitt, et Optique. Lee theories de
Maxwell et la thoorie electromagnetique de la lumiare, 2 vols., Paris, 1890-1.
98 Stokes, Mem.& Corres., 1, 250-1. 99 RutherfoRT Papers, 25. 100 2 vols., Oxford, 1892. 101 Recent Researches in Electricity and Magnetism,
Oxford, 1893. 102 Rutherford, Papers, 25, 26, 34, etc. 103 J.J.Thomson, Recent Researches etc., ch.1, 3-5. 104 Ibid., ch.2, 44-6. 105 Ibid., 5; see Section 4, p.38-44, below. 106 London, 1889; 2 ed., 1892. 107 Ibid., 184. 108 Later criticised by P.Duhem, see H.R.Post, Atomism
1900, P sics Education, 1968, 3, 1-13, 6. 109 Pa ers, . 110ee ection 4 below, p.32-3. 111 O.Lodge, Modern Views of Electricity, 1889, ch.10. 112 Ibid., 266-7. 113 TM., 267. 114 rua. 115 Ibid., 301-2. 116 Ibid., 250. 117 172TBrit.Ass., 1891, 574; quoted in J.T.Merz, A History
European Scientific Thought in the Nineteenth Ugntu, (1904), repr. New York, 1965, 2, 193.
118 Gam. money, Of the 'Electron' or Atom o? Electricity, Phil.Mag., 1894, 38, 418-20.
119 b.145dge, Modern Views etc., 1889, 250. 120 Trans. Royal Dublin Society, 1891, 4, 585. 121 W.McGucken, Nineteenth-Century Spectroscopy, 110-6. 122 Ibid., 122-6. However, see Section 2 above,
T7M-2, on Liveing.
Notes for Chapter 1, p.8-47) 285
123 Rep.Brit.Ass., 1874, 22; title only. 124 Phil.Mag. 71881, 11, 381-90; read 16th Feb.1881
Ta-Rokir Dublin Society. 125 Ibid., 385. 126 TurI., 387. 127 W.McGucken, Nineteenth-Century Spectroscopy, 188-9,
202-3. 128 I.e. slowly exchanging energy on collision. 129 Ibid., 376. 130 Ibid., 377. 131 Ibid., 378-9 132 17Tehuster, The Kinetic Theory of Gases, Nature,
1895, 51, 293. 133 H.EberT7 Phil.Mag., 1894, 38, 332-6. 134 G.J.Stoney, Of the 'Electron' etc., Phil.Mag.,
1894, 38, 418-20. 135 H.Eberi7 Electrische Schwingungen molecular Gebilde,
Ann.d.Phys., 1893, 49, 651-72. 136 U77..Utoney, op.cit.-- 137 Id., Of the KfnliFfc Theory etc., 1895, 378-9 138 He Modern Development of Faraday's Conception of
Electricity, J.Chem.Soc., 1881, 39, 277-304. 139 C.A.Russell, he HisiOTY of Valency, Leicester, 1971,
ch.l3, 265; see J.R.Partington, Histo of Chemistry, London, 1964, 4, ch.21, for stud es of electrolysis during those years.
140 See above, p.34-6. 141 J.C.Maxwell, Treatise on Electricity and Magnetism, 3 ed.,
Oxford, 1892, 1, 380. 142 Ibid., 383. 143 Teliholtz, 22.cit., 1881, 302-3. 144 W.C.D.Whetham,17eatise on the Theo of Solution,
Cambridge, 1902, provides many re erece ns. 145 Rep.Brit.Ass., 1885, 723-72.
146 Ibid., 723. 147 }ep.Brit.Ass., 1894, 482-93. 148 Ibid.-7TO: 149 Hereafter referred to as Kelvin. 150 See R.H.Silliman, William Thomson: Smoke Rings and
Nineteenth-Century Atomism, Isis, 1963, 54, 461-73. 151 Atom, Encyclopaedia Britannica, 1875; Papers, 2, 445-84,
473-6. 152 Elasticity viewed as possibly a mode of motion, RI Lib.
Sci., (1881), 3, 136-7. 153 Kelvin, On the molecular dynamics of hydrogen gas etc.,
Papers, (1896), 5, 350-3. 154 London, 1883; repr., London, 1968. 155 Ibid., 1. 'Strength' = mean velocity of rotation x section
area. 156 Ibid., 2. 157 Ibid., 109-14- 158 Ibid., 119. 159 Ibid., 108. 160 Cathode Rays, Phil.Mag., 1897, 44, 293-316, 313-4. 161 Shown by W.Mc6EFFen, Mineteenth7Uentury Spectroscopy,
174; and A.Romer, Experimental History etc., Isis, 1942, 34, 150-61, 151.
Notes for Chapter 1, p.8-47) 286
162 J.J.Thomson, Treatise on the Motion of Vortex Rings, 1883, 120.
163 J.J.Thomson, On the Chemical Combination of Gases, Phil.Mag., 1884, 18, 231-67.
164 W7073twald, Lehrbuch der Allgemeinen Chemie, 2, 745; see J.J.Thomson, Reply to Prof. Wilhelm Ostwald's criticism on my paper etc., Phil.Mag., 1887, 23, 379-80.
165 J.J.Thomson, Applications of-DYFismics to P iras and Chemistry, London, 1888; from leo res-ae were the Cavendish. D.R.Topper, Commitment to mechanism: J.J.Thomson, the early years, Arch.Hist.Exact Sci., 1971, 7, 393-410, has discussed tharggig6TUf Thomson's work.
166 Phil.Mag., 1883, 15, 427-34. 167 Tura., 428. 168 Ibid., 432. 169 R-MSilliman, William Thomson: Smoke Rings etc., 472;
also see above, p.39. 170 J.J.Thomson, On the illustration of the properties of
the electric field by means of tubes of electrostatic induction, Phil.Mag., 1891, 31, 149-71.
171 Oxford. 172 Ibid., 3. 173 Ibid., 43. Different ether-motion theories were used
5T-Fthers, e.g. see Heaviside in Schaffner, 208-9; also Section 3 above.
174 Recent Researches etc., 5. 175 Ibid. 176 Ibid., 44. 177 Ibid., 45. 178 777Thomson, Phil.Mag., 1895, 40, 511-44. 179 Ibid., 513. ---- 180 37,(7i-A.Romer, Experimental History etc., 157. 181 See T.Hirosige, Electrodynamics etc.1890-1905, p.18-20;
also A.Romer, op.cit., 156-7. 182 On the other hand, as early as May 1897 the physical
chemist W.Nernst cited Wiechert's dicovery of subatomic charged particles and discussed its possible application to electrochemistry. G.V.Bykov, Historical Sketch of the Electron Theories of Organic Chemistry, Chymia, 1965, 10, 199-253, 200-1, considers that the app cation of ele-aron theories to chemistry began in 1897; we have seen that there were earlier attempts on these lines.
183 See W.McGucken, Nineteenth-Century Spectroscopy, 211-2; G.E.Owen, The discovery of the electron, Ann.Sci., 1955, 11, 173-82, 177-9.
184 7.J.Thomson, On the cathode rays, Proc.Camb.Phil.Soc., 1897, 9, 243-4.
185 J.J.Thomson, The Röntgen Rays, Nature, 1896, 53, 581-3. 186 W.C.R8ntgen, Ueber eine neue ArT-VaRStrahlen,
Sitzungsberichte der physikal.-medicin. Gesellschaft, WUrzburg, 1895, 177=41, 134.
187 J.J.Thomson, op.cit., 1896, 581. 188 J.J.Thomson, TheOntgen Rays, Nature, 1896, 54,
302-6, lecture delivered 10th Jun. 189 Ibid., 304-5.
Notes for Chapter 1, p.8-47) 287
190 J.J.Thomson, Longitudinal Electric Waves, and Rtintgen's X Rays, Proc.Camb.Phil.Soc., 1896, 9, 49-61; also E.Rutherford, On tEeTTlectriTICation of Cases Exposed to Röntgen Rays, Phil.Mag., 1897, 43, 241-55, Note by J.J.Thomson, 255.
191 J.J.Thomson, Cathode Rays, Phil.Mag., 1897, 44, 293-316. 192 Ibid., 310. The work of Lenard referred to was probably
Te-Be'r die Absorption der Kathodenstrahien, Ann.d.Phys., 1895, 56, 255-75.
193 Thomson, 22.cit., n.191, 312. 194 Ibid., 313=47-- 195 Ibid., 312-3; on evidence of electric strength
UrRases. 196 J.J.Thomson, A Treatise on the Motion of Vortex Rings,
1883, 1. 197 W.Kaufmann, Methode zur exacten Bestimmung von Ladung
and Geschwindigkeit von Becquerelstrahlen, Phys.Z., 1901, 2, 602-3. J.J.Thomson was himself investigating whetheY. corpuscles 'have masses other than electrical' in 1901: letter to Rutherford dated 15th Feb.1901, A.S.Eve, Rutherford.Etc., 76.
198 A.S.Eve, Rutherford.7T6., Cambridge, 1939, 39; letter dated 30tEM=76.
199 In CUL, Add.mss.7653, and Royal Society Library, London; notebooks for the period 1896-1904, CUL, contain material generally similar to that published.
288
NOTES FOR CHAPTER 2 (pages 48-118)
1 W.C.R6ntgen Ueber eine neue Art von Strahlen, Dec.1895; G.Sarton, The discovery of X-rays, Isis, 1936-7, 26, 349-64.
2 T.41asser, Wilhelm Conrad Rönt en and the Early History of the Röntgen Rqys, Illino s, 377-3M.
3 W7CTRUntgen, op.cit., 139. 4 A.Romer, Acciairif-and Professor Röntgen, Amer.J.Phys., 1959, 27, 275-7.
5 A.H.Becquerel, Recherches sur une propriete nouvelle de la matiere etc., Paris, 1907:-
6 mar, 3. This account is repeated by O.Lodge, Becquerel Memorial Lecture, J.Chem.Soc. 1912, 101, 2005-42, 2032-8; also T.W.CHeIigis, A Liort Hil-517 of Radio- activity, pub. The Engineer, 1951, ; .BeTTianarSur l'origine de la decouverte de la radioactivity, CR, 1946, 223, 698-700, from personal memory support by his oWITTaboratory notes, dates Becquerel's interest in the photographic effects of pitchblende to 1893-4.
7 H.Poincare, Les rayons cathodiques et les rayons Röntgen, Rev.Gen.des.Sci., 1896, 7, 52-9, 56, 30th Jan.
8 S.P.Thompson,-agET7Vrable and Invisible, London, 1897, 260.
9 J.J.Thomson, Longitudinal Electric Waves etc., 1896, 60-1, 27-29th Jan.
10 A.Broca, L'Oeuvre d'Henri Becquerel, Rev.GOn.des Sci., 1908, 19, 803-13.
11 E.N.Harvey, A Histo of Luminescence from the Earliest Times until I9 , fgaelphia, 1957, 390-1.
12 7,7EaTikeWITle and G.F.Kunz, The Action of Radium, R6ntgen Rays and Ultra-violet light on minerals, Chem.N., 1904, 89, 1-6.
13 Tie above Chapter 1, Section 3, n.94. 14 E.N.Harvey, History of Luminescence, 363-4. 15 See above Chapter 1, Sections 1,4. 16 E.N.Harvey, 2.cit., 359. 17 Ibid., 364. 18 TRT-1885, 101, 1252-6. 19 711, 1891, 113, 618-23, 623. 20 VR, 1891, 112, 557-63. 21 TEid., 5637-- 22 17147Harvey, Historyof Luminescence, 364. 23 CR, 1896, 122, -1. A translation of this and three more
"Fac BecquereiTs first papers, and others, with commentary, are provided in A.Romer, The Discovery of Radioactivity and Transmutation, New YoT.E7 1964.
24 .C.R7 1696, 122, 662, 695, 790, 791, Mar. Henry became arector 0-The Laboratory of Physiology of Sensations at the Sorbonne in 1897.
25 CR, 1896, 122, 321-4, 10th Feb. 26 Tad., 27 *are-above, p.48-9, n.6.
Notes for Chapter 2, p.48-118) 289
28 G.H.Niewenglowski, Sur la propriete qu'ont les radiations emises par les corps phosphorescents de traverser certains corps opaques a la lumiere solaire, et sur lee experiences de M.G.Le Bon sur la lumiere noire, CR, 1896, 122, 385-6, 17th Feb.
29 CR, 1877 122, 420-1, 24th Feb. 30 E7Becquere17Sur les radiations invisibles emises par
les corps phosphorescents, CR, 1896, 122, 501-3, 2nd Mar. 31 L.Badash, Becquerel's 'Unexposed' Photographic Plates,
Isis, 1966, 57, 267-9, and A.Romer, Discovery of Tia-TOactivity, 9, have stressed the unusual aspects of developing unexposed plates, and the former, of work-ing in the laboratory on a Sunday. It is to be noted however that Becquerel indicated that he worked on Sunday 29th Mar.1896, CR, 1896, 122, 762-7, 30th Mar.
32 H.Becquerel, Seances Soc.Fr.Phys., 1896, 88, 6th Mar., comment by M.de ChardonneT7
33 See e.g. C.Raveau, Les faits nouvellement acquis stir lee rayons de Roentgen, Rev.Gen.des Sci., 1896, 7, 251, 15th Mar.
34 H.Becquerel, Sur quelques proprietes nouvelles des radiations invisibles emises par divers corps phos-phorescents, CR, 1896, 122, 559-64, 9th Mar.
35 I.e. Hertzian waves. 36 G.C.Schmidt, Ueber die von den Thorverbindungen and
einigen anderen Substanzen ausgehende Strahlung, Ann. d.Phys., 1898, 65, 141-51; Schmidt concluded that Ehorium rays were reflected and refracted.
37 E.Rutherford, Uranium Radiation and the Electrical Conduction Produced by It, Phil.Mag., 1899, 47, 109-63, Jan.; Papers, 170-1; H.Becquerel, Note sur quelques proprietes du rayonnement de l'uranium et des corps radio-actifs, CR, 1899, 128, 771-7, 773, Mar.
38 L.Troost, Sur ITemploi blonde hexagonale artificielle pour remplacer lee ampoules de Crookes, CR, 1896, 122, 564-6, 9th Mar.; id., 694, 23rd Mar.
39 n7BecquereTTSur lee radiations Tvisibles emises par lee sels d'uranium, CR, 1896, 122, 689-94, 23rd Mar.
40 H.Becquerel, Seancesoc.Fr.Phys. 1896, 105, 20th Mar., Ch.-Ed. Guillaume, TEid., On Stokes' Law.
41 Op.cit., 23rd Mar.1896. 42 CR, 1596, 122, 762-7. 43 117Moissan,PF6paration et proprietes de l'uranium,
CR, 1896, 122, 1088-93, 18th May; id., Le Four rlectrique, Paris, 1897.
44 CR, 1596, 122, 1086-8, 18th May. 45 Missan, Description d'un nouveau four electrique,
CR, 1892, 115, 1031-3. 46 Mir la preparation de l'uranium a haute temperature,
CR, 1893, 116, 347. 47 Etude du carbure de l'uranium, CR, 1896, 122,
274-80, 10th Feb.
48 Becquerel, Sur diverses proprietes des rayons uraniques, CR, 1896, 123, 855-8, 23rd Nov.
49 TEld., 8567-- 50 amour la loi de la decharge dans l'air de l'uranium
electrise, CR, 1897, 124, 800-3.
Notes for Chapter 2, p.48-118) 290
51 CR, 1896, 122, 689-94. 52 CR, 1896, I77, 762-7, 30th Mar. 53 Z7M.StewarT7Experiments on Beoquerel Rays, Physical
Review, 1898, 6, 239-51. 54 J.J.Thomson, Tile Rantgen Rays, Nature, 1896, 22, 581-3,
23rd Apr. 55 Rayons cathodiques, rayons X et radiations analogues,
Seances Soc.Fr.Phys., 1896, 121, 8th Apr. 56 G.G.Stokes, On the Nature of the Rbntgen Rays, Proc.
Camb.Phil.Soc., 1896, 9, 215-6, 9th Nov.; id., Wilde Lecture, 2nd Jul.1897, Papers, 4, 256-77, Mb: the irregular impacts of 'cathode ray' particles produce a series of thin ether pulses, which constitute the X-rays; the irregularity of the sequence of pulses implies that the molecules of the glass of a prism cannot vibrate in harmony, thus the X-rays are not refracted; the thinness of the pulses implies absence of diffraction; both these properties of the pulses imply their penetrating nature.
57 See 0.M.Stewart, Becquerel Rays, A Resume, P sical Review, 1900, 11, 155-75, 175; R.H.Stuewer, W am H. Bragg's Corpuscular Theory of X-Rays and X-Rays, Brit.J.Hist.Sci., 1971, 5, 258-81, on corpuscular X-ray theories.
58 C.Henry, CR, 1896, 122, 312-4, 10th Feb. 59 S.P.Thompson, On Hyperphosphorescence, Phil.Mag., 1896,
42, 103-7, dated 6th Jun. 60 L.Badash, Radioactivity before the Curies, Amer.J.Phys.,
1965, 33, 128-35; A.S.Russell, Madame Curie Memorial Lecture, J.Chem.Soc., 1935, 654-63; J.S. and H.G.Thompson, Silvanus Phillips Thompson his Life and Letters, London, 1920, 185-9.
61 G.G.Stokes, Mem.& Correa., 2, 495-6, letter from Thompson dafgY-2Uth Feb.189.b.
62 Ibid., letters from Stokes dated 29th Feb. and 2nd Mar.1896.
63 Op.cit., Phil.Mag., Jul.1897. 64 G.G7gTokes, Mem.& Correa., 2, 498, letter from Stokes
dated 28th May. 65 Letter from W.Crookes to S.P.Thompson, dated 2nd Jun.
1896, Imperial College Archives. 66 W.Crookes, Rep.Brit.Ass., 1898, 23. 67 The Evolution of Mater, New York, 1907, 22-3. 68 as Johanniskifferlicht, Ann.d.Phys., 1896, 59, 773-81. 69 H.Muraoka and M.Kasuya, 7ohanniskeerliEHt und die
Wirkung der Dampfe von festen und fliissigen Korpern auf photographischen Platten, Ann.d.Phys., 1898, 64, 186-92, received 24th Nov.1897.
70 Ueber Luminescenz, Ann.d.Phys., 1897, 61, 313-29. 71 A.F.McKissick, Becquerel Rays, ElectriETan, 1897,
38, 313. 72 Verh.phyp.Ges.Berlin, 1896, 15, 101. 73 Ann.dka771TeibI., 1897, 21, 366. 74 G.Le Bon, L'uranium, le radium et les emissions
metalliques, Rev.Sc., 1900, 13, 548-52; id., The Evolution of Rater, 19-25, T9-28.
75 La radioaciTvite de la matiere et l'energie susceptible de se dovelopper A la surface des corps, Rev.Sc., 1901, 16, 161-70, 167.
Notes for Chapter 2, p.48-118) 291
76 See E.Picard, Gustave Le Bon et son Oeuvre, Paris, 1909. 77 D.Martindale, The Nature aig '.hypes of Sociological
Theo , London7-1961, 309717; R.A.Nye, The Origins of row Psychology. Gustave Le Bon and thii-Crisis of Mii-Democracy in the ThirZ-Republre,-ranM,97.
78 T.17 Son, La luia-fere n311757 CR, 1896, 122, 188-90, 27th Jan.; the name 'dark rays' had been used earlier by W.de W.Abney as synonymous with infra-red rays, it simply meant rays not visible to the human eye (Spectrum Analysis etc., RI Lib.Sci., (1882), 3, 207-15).
79 CR, 1896, 122, 75767 80 M, 1896, I22, 463-5. 81 M, 1897, T, 857-9. 82 M, 1897, Imo, 984-8. 83 M, 1896, 122, 233, 386, 462, 522, 1057, Feb. to May 1896;
'aimilar notes in Rev.Sc., 1896, Jan. to May, and 1897, Mar. to May; CR, 1697, 124, 755-8, 892-5; Sur lee propriotes de certaines radiations du spectre. Reponse aux objections de M.Becquerel, CR, 1897, 124, 1148-51.
84 CR, 1897, 124, 892-5. 85 Temarques W-firopos d'une Note recente de M.G.Le Bon, CR,
1900, 130, 1072. 86 G.Le Bon, Intra-Atomic Energy, R2p.Smithsonian Inst.,
1904, 263-93; id., L'kvolution a la Me.tiere, par s, 1905; id., La Naissance et Logvanouissement de Ia Matiore, Wiris, 190d. On universal radioactivity see MUT"- Chapter 5, Section 3.
87 P.Curie, 22.cit., 1900; repeated by H.Becquerel, Recherches sur une propriete nouvelle etc., 1903, 5-6.
88 M.RutherforZTRaEro-activity, Cambridge, 1904, section 2; 1905, 4-5.
89 R.Colson, Wile des differentes formes de l'energie dans la photographie au travers de corps opaques, CR, 1896, 122, 598-600; id., Action du zinc sur la plaque photo-graphique, CR1-1896, 123, 49-51; id., La Plaque Photo-graphique, Taris, 1897:-
90 R.Colson, 22.cit., CR, 1896, 123, 49-51. 91 W.J.Russell; Proc.ROT.Soc., 1877, 61, 424-33, received
13th May, he cis Colson. For his work in another area see J.R.Brown and J.L.Thornton, William James Russell (1830-1909) and investigations on London fog, Ann.Sci., 1955, 11, 331-6.
92 E.Rutherford, A Radioactive Substance emitted from Thorium Compounds, Phil.Mag., 1900, 49, 1-14; Papers, 226.
93 Russell, p.cit., 1897, 424. 94 Ibid., 425:- --- 95 Ibid., 427. Note the use of the terms 'active' and
'activity'; Marie Curie's use of the expressions in 1898 was thus not the first as has been supposed (A.Romer, Radiochemistry and the Discovery of Isotopes, New York, 1970, 64). C.T.R7WiIain too, Proc7Vamb.Phil. Soc., 1897, 9, 337, wrote of 'active' uranium salTat-
96 W77.Russell, 22.cit., 432-3. 97 Ibid., 433. 98 W.Crookes, Presidential Address, ap.Brit.Ass.,
1898, 3-38, 26. 99 C.T.R.Wilson, On the Action of Uranium rays on the
Condensation of Water Vapour, Proc.Cainb.Phil.Soc., 1897, 9, 333-8, 25th Oct.
Notes for Chapter 2, P.48-118) 292
100 Proc.Camb.Phil.Soc., 1897, 9, 372, 22nd Nov. 101 1777Russell, further experiments on the action exerted
by certain metals and other bodies on a photographic plate, Proc.Roy.Soc., 1898, 63, 102-12.
102 Id., On the Action of Certain Metals and Organic Bodies on a Photographic Plate, 222.Brit.Ass., 1898, 834-5 (Abstract).
103 Id., On hydrogen peroxide as the active agent in prod- ucing pictures on a photographic plate in the dark, Proc.Roy.Soc., 1899, 64, 409-19; id., On the Action of Wood on a Photographic Plate in the Dark, Chem.N., 1904, 90, 104-6.
104 G.L.Keenan, Substances which Affect Photographic Plates in the Dark, Chemical Reviews, 1926, 3, 95-111, 108.
105 G.C.Schmidt, Ueber die vom Thorium and den Thorver- bindungen ausgehende Strahlung, Verh.P:ys.Ges.Berlin 1898, 17, 14-16, 4th Feb., id., Ueber die vonen ThorveiTindungen and einigen anderen Substanzen ausgehende Strahlung, Ann.d.Phys., 1898, 65, 141-51.
106 Ibid., Verh.Phys.Ges.B-Olin, 16. 107 771TioquTigl, Sur diverses proprietes des rayons
uraniques, CR, 1896, 123, 855-8, 23rd Nov.; id., Recherches sur les rayons uraniques, CR, 18977 124, 438-44; id., Sur la loi de la decharge dans l'aIr de l'uranium electrise, CR, 1897, 124, 800-3, 12th Apr.
108 E.Rutherford, The Velocity and Rate of Recombination of the Ions of Gases exposed to Röntgen Radiation, Phil.Mag., 1897, 44, 422-40, 440; Pa ers, 148.
109 H.Becqa5rel, CR, 1896, 55-8, 23rd Nov. 110 Id., CR, 1897, 124, 438-44, 443, 1st Mar. 111 U7Elster, Jahre -7d.Ver.f.Wiss.,Braunschweig, 1897, 10,
149-53, 10th Dec.189TT-J.EnTer and H.Geitel, Ann.d. P s.,Beibl., 1897, 21, 455.
112en .d.R.Acad.d.Scienze fis.e mat., 1897, 36, 178f. 113 kelvin, Papers, 6, 1; most oT'ITT) relevant papers
are collected in this volume. 114 Cited in Kelvin, J.C.Beattie, M.S.de Smolan, On
Apparent and Real Diselectrification of Solid Dielect-rics Produced by Röntgen Rays and by Flame, Edin.Roy. Soc.Proc., 1897, 21, 397-403; Kelvin, Paperi7-67 65.
115 RavIETJ.C.Beatfrg, M.S.de Smolan, Edin.Roy.Soc.Proc., 1897, 22, 131-3, 1st Mar.; Kelvin, Pa ers-,--6, 95-77 -
116 Id., Edin.Roy.Soc.Proc., 1897, 21, 1 - , Wth Apr.; re1viETPa ers7-6, 84-95.
117 J.C.Beattiet PhiT.Mag., 1897, 44, 102-7; Note by Kelvin, ibid., 107-8; read-1-6 Edin.Roy.Soc. 7th Jun.1897.
118 Citing J.J.Thomson and McClelland, Proc.Camb.Phil.Soc., Mar.1896, and E.Rutherford, Phil.Mag., ATi.71897; see Kelvin, Papers 6, 72-3, (1siMia-TTT ibid., 184, (17th Jun.
119 See J.J.Thomson, The Relation between the Atom and the Charge of Electricity carried by it, 1895, 537: 'contact electricity' due to oxide coatings; also Kelvin, Contact Electricity of Metals, Papers, (1898), 6, 110-47, 130, 138 etc.: true metallic contact electricity due to the affinity of differing metals.
Notes for Chapter 2, p.48-118) 293
120 Kelvin and M.Maclean, On the Electrification of Air, Phil Mag., 1894, 38, 225-35; Kelvin, Papers, 6, 6-16, 6.
121 'N(7) -Chapter 1, Section 4, p.43. 122 Kelvin, Contact Electricity of Metals, RI Lib.Sci.,
(1897), 5, 50-83; Papers, 6, 110-47. 123 Id., Pa 4Fs, 6, 144-5. 124 ee apter T, Section 4, p.39: chemical H atom consists
of two Boscovichean atoms. 125 Phil.Mag., 1899, 48, 97-106. 126 Ibid., 97. 127 E.V.Appleton, 'The Young Rutherford' in The Collected
Papers of Lord Rutherford of Nelson, ed. J.Chadwick, 3 vole., London, 1962, 1, 17.T71171 of this work is hereafter referred to as E.RutherfUrd, Papers.
128 See Chapter 1. 129 Trans.New Zealand Institute, 1894, 27, 481-513; Papers,
25-55. 130 Ibid., Pa ere, 34. 131 Weabove, Chapter 1, Sections 3-4, p.31-2. 132 Rutherford, Pa ers, 25. 133 See Chapter , ection 2, p.133-4, 141-4. 134 Rutherford, Papers, 51. 135 Trans.New Zealand Institute, 1895, 28, 182-204; Papers,
55-79. 136 Rutherford, Pa ere, 69-70. 137 See below, C ap er 2, Section 3, p.95-6. 138 Refers most frequently to J.J.Thomson, Recent Researches
in Electricity and Magnetism, 1893.
139 Ibid., 35:
140 Chapter 1, Sections 3-4, p.31-2. 141 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge,
1942; repr. London, 1962, 62. 142 A.S.Eve, Rutherford.Being the Life and Letters of the
Rt.Hon.Lord Rutherford,O.M., dgER67, 1939, 151 ITtTE57 to Mary Newton Oct.-1895.
143 N.Feather, Lord Rutherford, Glasgow, 1940; repr. London, 1973, 28-9.
144 R.Sviedrys, The Rise of Physical Science at Victorian Cambridge, Hist.Stud.Phys.Sci., 1970, 2, 127-51, 143; A History oTTHEUirrendish Laboratory,I871-1910, London, 9IU.
145 See the letter from Thomson, Cambridge, to Rutherford, London, dated 24th Sep.1895, in A.S.Eve, Rutherford. Etc., 13; also letter from Rutherford, Cambridge, to Mary Newton, N.Z., Oct.1895, ibid., 16.
146 A.S.Eve, Rutherford.Etc., 22-7. 147 Phil.Trans.A., 1697,19, 1-24; Rutherford, Papers,
80-10q7 -- 148 A.S.Eve, Rutherford.Etc., 34; letter to Mary Newton. 149 J.J.Thomson, Recent Researches etc., 53-207. 150 Ibid., 189. 151 Ibid. 152 Ibid., 119-27. 153 Ibid., 128. 154 TM., 45-7, 189-90. 155 Ibid., 193. 156 Ibid.
Notes for Chapter 2, p.48-118) 294
157 Ibid., 45-6, 195-6. 158 TX Thomson, The Connection between Chemical Combin-
ation and the Discharge of Electricity through Gases, Rep.Brit.Ass., 1894, 482-93.
159 Ibid., 489-92. 160 Ibid., 486. 161 Ibid., 487; also id., On the effect of electrification
an chemical action on a steam jet etc., Phil.Mag., 1893, 36, 313-27.
162 Proc.RZY.Soc., 1895, 58, 244-57, received 17th Jun. 163 IETT.filag.77895, 40, 311-44. 164 Heber-die electrolTtische Leitung verdannter Gase,
Ann.d.Phys., 1897, 61, 737-47, and references therein. 165 See above, Chapter IT Section 4, p.39-44. 166 A.S.Eve, Rutherford.Etc., 27. 167 See N.Feather, X-ra s and the electric conductivity of
gases, Alembic u eTi;InTf2, Edinburgh, 1958, 16,-T1f. 168 Longitudinal Electric Waves, and Pecintgen's X Rays,
Proc.Camb.Phil.Soc., 1896, 9, 49-61, 61; Kelvin, Papers, 6, 6571IsTg-geveral independent announcements of this, at about this date.
169 J.J.Thomson, On the Discharge of Electricity produced by the R8ntgen Rays, and the Effects produced by these Rays on Dielectrics through which they pass, Proc.Roy. Soc., 1896, 59, 274-6, received 7th Feb., assisted by J.A.McClellsEa.
170 Ibid., 275. 171 77.7ic.Camb.Phil.Soc., 1896, 9, 126-40, 9th Mar. 172 Ibid.7-170. 173 ITTa., 131-2. 174 Chapter 1, Section 4, p.42-6. 175 J.J.Thomson and J.A.McClelland, 22.cit., 132. 176 Of the order 0.001 cm./sec. per vofWm. 177 22.cit., Mar.1896, 128. 178 J.J7TEomson and E.Rutherford, On the Passage of Elec-
tricity through Gases Exposed to Montgen Rays, Phil.Mag., 1896, 42, 392-407; read to British ssociation, Sep.I896; Rutherford, Papers, 105-18.
179 See N.Feather, Lord Rutherford, Chapter 2, 'Cambridge, the First Period, 1895-1898', 41. For his brief account N.Feather consulted the correspondence of Rutherford, used by A.S.Eve, also Rutherford's laboratory notebooks.
180 1896, 53, 581-3. 181 Ibid.,-583. 182 J.J.Thomson, The Rantgen Rays, Nature, 1896, 54, 302-6;
305-6; Rede Lecture, delivered 10th Sun. 183 Ibid., 304. 184 Ibid., 305. 185 Letter to Mary Newton, dated 18th Jun.1896, A.S.Eve,
Rutherford.Etc., 36. 186 Read to BriiIih Association, Sep.1896; Rutherford,
Papers, 105-18. 187 Similar to those of Kelvin, who is not cited; Kelvin,
Papers, (1895), 6, 35, 51-2. 188 Thomson and Rutherford, op.cit., Rutherford, Papers, 106. 189 Ibid., 107. 190 TEid., 106. 191 Ibid. 192 Ibid., 117.
Notes for Chapter 2, p.48-118) 295
193 Ibid., 107-8. 194 The-intermittency of the X-ray discharge affected the
results, ibid., 109-10. 195 Rep.Brit.1713737, 1894, 482-93, 491-2. 196 ThomTEEailia-Rutherford, op.cit., 1896, 114. 197 See above Chapter 1, SecTfori, p.47. 198 Phil.Mag., 1897, 43, 241-55, Apr. issue, dated 28th Dec.
1896; Rutherford, Papers, 119-31. 199 Ibid., 128. 200 TM., 127. 201 Ibid., 119-22. 202 UtPirrin, Mecanisme de de-charge des corps olectrises
par lee rayons de Ontgen, Seances Soc.Fr.Phys., 1896, 254-61, 261; id., Rayons cathodiques et rayons de Roentgen, Annales de Chimie et de Physique, 1897, 11, 496-554, Thesis Jun.1897.
203 THil.Mag., 1897, 44, 422-40; Papers, 132-48. 204 MIT.Mag., 1898, 7.L 120-54. 205 E.Rutherford, 22.-61t., Papers, 144-8. 206 Ibid., 148. 207 Nacre, 1896, 53, 581. 208 Nature, 1896, 5T, 304. 209 TeTTETtlow, Chapter 2, Section 4, p.114-5. 210 See above, Chapter 2, Section 1, p.63-5. 211 E.Rutherford, Papers, 148. 212 Proc.Camb.Phil.Soc., 1898, 9, 401-16; E.Rutherford,
Pa ers, 149-62. 213 rbid., 149. 214 This was accepted by J.J.Thomson in 1893, Recent
Researches etc., 54. 215 0.Lodge, Modern Views etc., 1889, 301-2. 216 E.Rutherford, Pa ers, 159. 217 Phil.n.E., 189 „ 109-63; Papers, 167-215. 218 mid., 214-5. -- 219 J.J.Thomson, Proc.Camb.Phil.Soc., 1898, 9, 393-7,
24th Jan. ---- - 220 See below, Chapter 2, Section 3, p.104-5. 221 J.J.Thomson, 22.cit., 1898, 397. 222 Ibid. 223 gig-below, Chapter 2, Section 3, p.100-1. 224 E.Rutherford, Uranium Radiation etc., Papers, 180. 225 Ibid., 214-5. 226 Ibid., 214. 227 TETU., 215. 228 Papers, 167-215; Phil.Mag., Jan.1899, dated 1st Sep.1898. 229 Ibid., 167, 170-1. 230 TWoquerel, CR, 1899, 128, 771-7, 772, 27th Mar. 231 E.Rutherford, Papers, 177=6. 232 Ibid., 185-6. 233 UTI7-1896, 122, 762-7, 765, 30th Mar. 234 Proc.Camb.f.al.Soc., 1896, 9, 126-40, 139-40, 9th Mar. 235 ConductioriOEM7tricity through Gases, Cambridge,
1903, 278. Rutherford's own resulfg-6Y-1898 (Pa ers, 178) showed that the alpha/beta ratio increase w h the thickness of the Ur layer; Thomson noted (loc.cit.) that if alpha rays were produced at the surfaci-Ey-Feta, then their ratio should be constant. New evidence of independence was produced by F.Soddy in 1902.
Notes for Chapter 2, p.48-118) 296
236 Rutherford, Uranium Radiation etc., Papers, 180-1. 237 Ibid., 180. 238 Ibid., 178. 239 Ibis. 240 Ibid. 241 t177-1898, 126, 1101-3. 242 17ge e.g. M=Hesse, Forces and Fields, London,
1961, 2-3, 6. 243 J.C.Maxwell, Electricity and Magnetism, 1892, 2, 470. 244 H.R.Post, Atomism 1900, P sics Education, 196U, 3,
1-13, 5; discusses views of L. I.tzmann, E.Mach, - W.Ostwald.
245 Seven papers in CR, 1880-2; P.Curie, Oeuvres, 6-32. 246 M.Curie, Pierre Curie, trans., New York, 1923;
repr., 1963, 20. 247 C.Friedel, Sur la pyroelectricite dans la topaze, la
blends et le quartz, Neues Jahrb.Mineral., 1879, 585-6. 248 A.-C.Becquerel, De quTTTEFisTorigEomenes electriques
produits par la pression et le clivage des cristaux, Annales de Chimie et de Physique, 1827, 36, 265-71.
249 M.Curie, Pierre Curie, N.Y., 21. 250 P. and J.Cur e, Dilatation electrique du quartz,
Journal de P si ue, 1889; P.Curie, Oeuvres, 35-55. 251 P. and J.Tur e, R, 1881; P.Curie, Oeuvres, 18-21. 252 Ibid., 19. 253 157iahem crossed swords with P.Curie (Oeuvres, 33-4) in
1887 over the origin of piezo-electricity. In 1893 Kelvin corresponded with P.Curie (7 letters, Curie papers, dossier 32, BN) who provided him with a piezo-electric electroscope.
254 P.Curie, Sur les questions d'ordre: Repetitions, Oeuvres, (1884), 56-77; id., Sur la symetrie, Oeuvres, (1b84), 78-113.
255 E.g. Kelvin, Papers? 1 281, stated that 'Hall's recent great discovery' (186U) of the e.m.f. produced by a steady current in a constant magnetic field 'proves the rotatory quality to exist for electrical conduction through metals in the magnetic field'; but P.Curie was first to consider the symmetry of the Hall effect, Oeuvres, (1894), 137.
256 M.Curie, Pierre Curie, N.Y., 24-8, gives an account of this.
257 Oeuvres, (1894), 118-41. 258 Ibid., 141. 259 E.g. J.J.Thomson, Applications of Dynamics etc., 1888,
ch.4, 32. 260 L.Rougier, En Marge de Curie de Carnot et d'Einstein,
Paris, 1920, discusses 'Le prEiciTT-E sym6triet, ch.l. The principle is much used in modern electron theories of the chemical atom.
261 Oeuvres, (1895), 232-334. 262 Determined over a more limited temperature range by
others, P.Curie, Oeuvres, 280-1; of present importance in electron theories of magnetism, this is now known as the 'Curie law'.
263 A and R are constants, different for each substance; T = temperature, H = magnetic field, I = magnetic intensity; D = gas density, P = gas pressure.
Notes for Chapter 2, p.48-118) 297
264 P.Curie, Oeuvres, 331-2. 265 The I = f(H) and D = f(P) curves at constant T were
dissimilar, ibid., 334. 266 Ibid., 333. ---- 267 157-Curie, Madame Curie, trans., London, 1938; repr.
The Reprint Societ77EUndon, 1942, is a biography valuable for the non-scientific aspects of Marie Curie's life, much personal correspondence is published here. R.Reid, Marie Curie, London, 1974, provides a much improved account on similar lines. See also M.Curie, Pierre Curie, N.Y., 'Autobiographical Notes', 77-118.
268 'MentioElEtTes bien' and 'AB' respectively, Curie papers, dossier 29, BN. The Licence was of about the present masters or first degree standard.
269 Ibid 270 E.g., M.Curie, Proprietes magnetiques des aciers
trempes, CR, 1897, 125, 1165-9. 271 P.Curie, Oeuvres, 277Y. 272 J.Hurwic,-MER-gSklodowska-Curie en tant que chimiste,
Etudes d'Histoire de la Science et de la Technologie, Warsaw, 1966, 197-0'2.
273 See E.Curie, Madame Curie, Appendix: lists of Marie Curie's prizeT37als, decorations, honorary titles.
274 M. Curie, Pierre Curie, N.Y., 96-7. 275 Discussed in letti7F-fiom E.Rutherford, Manchester, to
W.H.Bragg, dated 20th Dec.1911, CUL. Also R.Reid, Marie Curie, ch.17.
276 See M.Curie, La Radiologie et la Guerre, Paris, 1921; E.Curie, Madame Curie, ch.21, N:1779.
277 Irene Curie, lat1171Fene Joliot-Curie. 278 The film 'Madame Curie', M.G.M., U.S.A., 1943, re-shown
occasionally to the present time by the British Broadcasting Company exemplifies one popular aspect.
279 I am indebted to L.Badash, who informed me of this claim. 280 M.Curie, Opening Lecture, Cours du physique gen6rale
professe a la Sorbonne, Oeuvres, 322-35, 334-5. 281 Rev.Gen.des Sci., 1899, 10, 41i.; Oeuvres, 60-76. 282 See mow, Chapter 5, SeTiion 1, p.227-31, 245-6. 283 M.Curie, Pierre Curie, N.Y., 44-5, 89; her daughter
Irene waiUOTE-in Sep.1897. 284 CR, 1897, 124, 800-3. 285 A.Romer, Radiochemistry, 6. 286 M.Curie, Pierre Curie, N.Y., 45, 89. 287 Ibid., 34. 288 77Terrin, Rayons cathodiques etc., Seances Soc.Fr.Phys.,
1896, 121-9; G.Sagnac, Journal de Physique, 5, 193f.
289 M.Curie, Pierre Curie, N.Y., 40. 290 Seances Soc.Fr.Phys., 1896, 105, 20th Mar. 291 Laboratory notebooks of the Curies, 1897-9, comprise
dossier 1, Curie papers, BN,but unfortunately these are radioactive, are undergoing treatment with some other items in the collection and are not available. They are described as: Uranium I, mainly Marie Curie's hand, pp.159, 1897-8; Uranium II, P. and M.Curie's hands, pp.143, 1898; Uranium III-Polonium, pp.126, P. and M.Curie's hands, 1898-9. Fortunately their
Notes for Chapter 2, p.48-118) 298
291 contd.) previous owner , I.Joliot-Curie, has given a brief account of their contents in M.Curie, Pierre Curie, Paris, 1955, 103-20. See above also A.Romer, REITTOchemistry, 6-8, 64-75, who uses this source and gives translations of published papers. A fourth laboratory notebook of M.Curie, 1899-1902, is at the Wellcome Historical Institute and was made available to me.
292 J.Curie, Recherches sur le pouvoir inducteur specifique et la conductibilite des corps cristallises, Annales de Chimie et de P si ue, 1889, 17, 385-434.
293 M.Cur e, Oeuvres, 60-76,-71, Jan. 294 I.Joliot-Curiel in M.Curie, Pierre Curie, Paris, 103-7. 295 M.Curie, CR, 1898, 126, 1101-3, 12th. 296 G.C.Schmidt, CR, lags, 126, 1264. 297 E.Wiedemann aFa G.C.Schiaat, Ueber Lichtemission
organischer Substanzen etc., Ann.d.Phys., 1895, 56, 18-26; id., Ueber Luminescenz von festen Wirpern und festen TUsungen, ibid., 201-54, 241-8; see above, Chapter 1, SectioriT p.30; on Wiedemann's earlier ether-envelope theory in spectroscopy see McGuoken, Nineteenth-Century Spectroscopy, 179-81.
298 G.C.Schmidt, Ueber die vom Thorium und den Thor-verbindungen ausgehende Strahlung, Verh.Phys.Ges. Berlin, 1898, 17, 14-16, 4th Feb.; see also Cat, Ueber die Jeziehung zwischen Fluorescenz und Actinoelek-tricitat, Ann.d.Phys., 1898, 64, 708-24.
299 J.Elster and H.Geitel, Ann.d.Phys.,Beibl., 1897, 21, 455, reviewed by G.C.Schmidt.
300 Ann.d.Phys., 1889, 60, 507f. 301 G75:chmidt, Ueber Tire von den Thorverbindungen etc.,
Ann.d.Phys., 1898, 65, 141-51. 302 G.C.Schmidt, op.cit., Verh.Phys.Ges.Berlin, 1898, 17, 16. 303 See above, p.aU. 304 CR, 1898, 126, 1101-3, 12th Apr.; Oeuvres, 43-5. 305 E.N.Harvey,Ei!toryof Luminescence, 284; J.Elster and
H.Geitel, Ani.T.aPhSis., 1890, 39, 321-31; S.Bidwell, Diselectri
b
ffe-aTion by Phosphorus, Nature, 1896, 55, 6; G.C.Schmidt, Ueber die Emanation des Phosphors, 7Hys.Z., 1902, 3, 475-81; F.Harms, phys.Z., 1902, 4, 111-3; J.J.Thomson, Conduction of Electricity thrau h Gases, Cambridge, 1903, 324; E. Rutherford, Rado-ac vity, Cambridge, 1905, 529-30.
306 These elements gave 1 to 10% of the uranium reading, which was 24 x 10' amps. All other substances, except phosphorus, gave less than 1% of this current: M.Curie, Oeuvres, 44.
307 See Section 2 above, p.90-1. 308 This consists in mixing a solution of uranium nitrate
with one of copper phosphate in phosphoric acid then warming gently; crystals of chalcolite, copper uranyl phosphate, slowly separate. Although not mentioned in the note of 12th Apr., preliminary success in chemically concentrating the active ingredient may also have con-tributed to the evidence by this time.
309 E.Rutherford, Papers, 178. 310 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul.
Notes for Chapter 2, p.48-118) 299
311 W.Crookes, Genesis of the Elements, 1887, 410-11; also W.N.Hartley, Opening Address to Brit.Ass. Chemistry Section, on spectroscopy, Nature, 1903, 68, 472-81, 481.
312 M.Curie, Oeuvres, (1898), 45, 12th Apr. 313 3 new elements were proposed in 1897, no less than 9 in
1898 (3 radioactive), and 2 in 1899, C.Baskerville, The Elements: Verified and Unverified, Chem.N., 1904, 89, 109-10, 121-3, 135-7, 150-1, 162-3, 170-1, 186-7, 194-5, 210.
314 G.Sagnac, Sur is mecanisme de la decharge des conducteurs frappes par lee rayons X, CR, 1898, 126, 36-40, 3rd Jan.; id., Transformation des rayons X par-Transmission, ibid., 4T7-70; id., Emission de rayons secondaires par l's17- sous l'iiTluenoe des rayons X, ibid., 521-3; id., Caracteres de la transformation des rayons X par la matiere, ibid., 887-90, 21st Mar.
315 J.Perrin,-13-6harge par les rayons de Röntgen. Role des surfaces frappoes, CR, 1897, 121, 455-8; L.Benoist and D.Hurmuzescu, CR, 1U76, 122, 779f., had expressed a similar view. --
316 G.Sagnac, Sur la transformation des rayons X par les differents corps simples,Seances Soc.Fr.Phys., 1899 1*, 6th Jan.
317 See Section 2 above, p.89. 318 M.Curie, Oeuvres, (1898), 45, 12th Apr. 319 Ibid. 320 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul.;
P.Curie, Oeuvres, 335-8. Active substances are here called 'radioactive' for the first time.
321 A.Romer, Radiochemistry, 80-105, gives a brief account of controversies concerning active bismuth, polonium and radiotellurium during 1899-1906 and provides trans-lations of some papers.
322 P.Curie, M.Curie, G.Bemont, Sur une nouvelle substance fortement radio-active, contenue dans la pechblende, CR, 1898, 127, 1215-7, 26th Dec.; P.Curie, Oeuvres, 779-42.
323 Ibid., 340. 324 The authors thanked M.Suess, correspondent de l'Institut
de France, Professeur a l'Universite de Vienne, for his request to the Austrian government, who donated the waste material freely, ibid., 342. All of the increasing amounts of material subsequently used by the Curies came from Joachimsthal, M.Curie, Pierre Curie, N.Y., 91.
325 Sur le spectre d'une substance radio-actiViTCR, 1898, 127, 1218; appended to the paper of Curies ana-Bemont.
326 P.Curie (with M.Curie and Bemont), Oeuvres, 341. 327 Atomic weights determined by Marie for increas-
ingly concentrated radium were approximately as follows: M.Curie, Sur le poids atomique du metal dans le chlor-ure de baryum radifere, CR, 1899, 129, 760-2, atomic weight of Ba = 138, Ba-Ra = 140 to-175; id., Sur le poids atomique du baryum radifere, CR, 1700, 131, 382-4, Ba-Ra = 174; id., Sur le poids atomique du radium, CR, 1902, 135, 161-3, Ra = 225, the modern value.
328 P.Curie7Twith M.Curie and G.Bemont), Oeuvres, 340. 329 Rev.Gen.des Sci., 1899, 10, 41f., Jan.; M.Curie,
Oeuvres,-0-75.
Notes for Chapter 2, p.48-118) 300
330 Ibid., Oeuvres, 73. 331 Ibid., 71. 332 Ibid., 75. 333 Ibid. 334 Ibid., 72. 335 Ibid., 75. 336 Ibid., 76. 337 M.Curie remarked on this in a later footnote added
during or after Dec.1898, Oeuvres, 76. 338 Re .Brit.Ass., 1898, 3-38. 339d., 26. 340 Ibid., 27. 341 J.Elster and H.Geitel, Versuche aber Hyperphosphor-
escenz, Ann.d.Phys.,Beibl., 1897, 21, 455; J.Elster, Jahresb.17Ver.f.Wiss.,Braunschweig7-1897, 10, 149-53, 10th Dec.187'.
342 Verh.lys.Ges.Berlin, 1898, 17, 14-16. 343 1898, 8, W7U. 344 J.Elsf.E. and H.Geitel, Ann.d.Phys., 1898, 66, 735-40. 345 Ibid., 736. 346 77RUtherford, Pa ers, 169-215, dated 1st Sep.1898. 347 Sur la source e nergie dans lee corps radio-actifs,
CR, 1899, 128, 176-8, 16th Jan., presented by H.Moissan. 348 1898, 739. 349 Tbia77 740. 350 L.Badash, Radioactivity before the Curies, Amer.J.Phys.,
1965, 33, 128-35, 130, 134. 351 Nature, 1896, 53, 581; ibid., 54, 304. 352 See below, p.1.7-5. 353 Proc.Camb.Phil.Soc., 1897, 9, 372. 354 G.G.S=E62-45s, Mem.& Corres., "ff, 471. 355 W.Crookes, La7ratory Notebooks, vol.16, p.54-6,
3rd to 10th Aug.1897, RI. 356 Verh.Phys.Ges.Berlin, 1898, 17, 14-16. 357 Proc.Cam
o . 5HIf7-87F7T 1898, 9, 393-7, 397.
358 LetteTWoET7P.Thompson to G.G.Stokes, dated 28th Feb. 1896; Stokes, Mem.& Corres., 2, 495.
359 Stokes to Thompson, dated 29th Feb.1896; ibid., 495-6. 360 Thompson had already been anticipated by Becquerel,
see this Chapter, Section 1, p.59-60. • 361 Typescript letter from Stokes, correspondence of
S.P.Thompson, Imperial College Archives; the inverted commas are omitted from Stokes, Mem.& Corres., 2, 495-6.
362 G.G.Stokes, Papers, 4, 256-77. 363 Ibid., 273-4. 364 ST61ies, Mem.& Corres., 1, 299. 365 Ibid., 294-7. 366 Saes, Mem.& Corres., 2, 478-83. 367 See Section above, p.'69. 368 E.Rutherford, Papers, 215; pub. Jan.1899. 369 R.B.Owens, Thorium Radiation, Phil.Mag., 1899, 48,
360-87, 361, pub. Oct. 370 J.Elster and H.Geitel, Weitere Versuche an Becquerel-
strahlen, Ann.d.Phys., 1899, 69, 83-90, received 5th Aug.; p.83-7-g1so pub. as Ueber Becquerelstrahlen, Jahresb.d.Ver.f.Wiss.,Braunschweig, 1899, 11, 183, 271-6, IgtliTan.KGiesel, known as a cheast,
Notes for Chapter 2, p.48-118) 301
370 contd.) joined in the discussion of the paper in Brunswick on 19th Jan., ibid., 183; his later public-ations on active substances and their rays are of importance.
371 Letter from J.Elster to E.Rutherford, dated 10th Feb. 1899, CUL.
372 Elster and Geitel, op.cit., Ann.d.Phys., 1899, 88. 373 E.Rutherford, Uranium TgaiatTUE etc., Papers, 215. 374 Having corresponded with Elster and Geitel n 1899
(letters from J.Elster to E.Rutherford, dated 10th Feb., 27th Jun.1899, CUL) and mentioned radioactivity, one would expect him to look out for their publications. Also, Elster in his letter of 10th Feb. promised to send Rutherford their paper on the subject.
302 NOTES FOR CHAPTER 3 (pages 119-178)
1 Jahrosb.d.Ver.f.Wiss.,Braunschweig, 1896, 10, 68, 73-7. 2 Ibid., 1899, 1T, 163; see above, Chapter 2, Section 4, n.370.
3 F.Giesel, Einiges fiber das Verhalten des radioactiven Baryts und tiller Polonium, Ann.d.Phys., 1899, 69, 91-4-
4 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69,-8'3-90, 87. 5 F.Giesel, 6 See E.de Hadn, Ueber eine radioactive Substanz, Ann.d. Phys., 1899, 68, 902.
7 Letter from J.Elster and H.Geitel, in Elster's hand, to E.Rutherford, dated 27th Jun.1899, CUL.
8 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69, 83-90; p.88-90, Ueber den EinflaTiis eines magnetie-CEen Feldes auf die durch die Becquerelstrahlen Bewirkte Leitfahig-keit der Luft, communicated to Deut. Phys. Gee., 5th May 1899.
9 Id., Ann.d.Phys., 1889, 38, 27-39; ibid., 1899, 69, F7-907-U8.
10 I.Joliot-Curie, in M.Curie, Pierre Curie, Paris, 1955, 110.
11 H.Becquerel, Influence d'un champ magnotique sur le rayonnement des corps radio-actifs, CR, 1899, 129, 996-1001, 11th Dec.
12 F.Giesel, Ueber die Ablenkbarkeit der Becquerelstrahlen im magnetische Felde, Ann.d.Phys., 1899, 69, 834-6, received 31st Oct.
13 L.Badash, An Elster and Geitel Failure: Magnetic Deflection of Beta Rays, Centaurus, 1966, 11, 236-40, has calculated the field required to defleFf the rays from radium and concludes that their magnet was too weak to give a noticeable effect in the phosphorescence experiment. A.Romer, Radiochemistry, 11, seems to con-sider that their positive air-conduction results were in fact due to deviation of the rays; the comment made here, that Elster and Geitel did not think in ionic terms, is debatable.
14 S.Meyer and E.von Schweidler, Tiber das Verhalten von Radium und Polonium im magnetischen Felde, Phys.Z., 1899, 1, 90-1, received 10th Nov., from Boltzmann's lab.
15 Ann.d.Phya., 1899, 69, 83-90, 90. 16 S.Meyer and E.von Schweidler, 22.cit., 91. 17 S.Meyer and E.von Schweidler, Ube-Faas Verhalten etc.,
P s.Z., 1899, 1, 113-4, received 18th Nov. Elster and e eT, ibid., D399, 1, 153, made it clear that this
was Giese 'sdiscovery not their own; Meyer's paper did not.
18 See above, Chapter 1, Section 4, p.45-6. 19 W.Sutherland, Cathode, Lenard and Röntgen Rays, Phil.Mag.,
1899, 47, 269-84; J.J.Thomson, Note on Sutherland's paper,-Ibid., 415-6.
20 J.J.Thomson, Phil.Mag., 1899, 48, 547-67, Dec. issue. 21 Ibid., 566-7. 22 J.J.Thomson, The Magnetic Properties of Systems of
Corpuscles etc., Phil.Mag., 1903, 6, 673-93, 689.
Notes for Chapter 3, p.119-178) 303
23 J.J.Thomeon, 22.cit., 1899, 565. 24 Rayleigh, J.J.Thbmson, 132-3. 25 Letter from 3.J.Thomson to E.Rutherford, dated 21st Dec.
1899, CUL. 26 Letter from Rutherford to Thomson, dated 9th Jan.1900,
CUL. See Section 2 below, p.131f. for discussion of 'emanation'.
27 E.Rutherford, Pa ers, (1898), 180. 28 Electrisation negative des rayons secondaires produits
au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 37-62„ 362.
29 E.Rutherford, Energy of Rbntgen and Becquerel Rays etc., Pa ers, 260-95, 293, received Jun.1900.
30 295. 31 Ibid., 292-3; my stress. 32 H.Becquerel, Seances Soc.Fr.Phys., 1899, 71*-72*,
15th Dec. Mainly repeating 'Influence d'un champ magnetique etc.', CR, 1899, 129, 996-1001, 11th Dec.
33 H.Becquerel, Note our quelques proprietes du rayonnement de Puranium et des corps radio-actifs, CR, 1899, 128, 771-7, 27th Mar.
34 Id., Sur le rayonnement des corps radio-actifs, CR, 1899, 179, 1205-7, 26th Dec.
35 td., Contribution a l'etude du rayonnement du radium, Uff, 1900, 130, 206-11, 29th Jan.
36 PTCurie, Ac lion du champ magnetique sur lea rayons de Becquerel, CR, 1900, 130, 73-6, 8th Jan.; Oeuvres, 349-52. M.Curie, Sur la p6netration des rayons de Becquerel non deviables par le champ magnetique, CR, 1900, 130, 76-9; Oeuvres, 85-8.
37 :aTETT:: CR, 29th Jan.1900 38 R = radius of curvature of path of a particle, produced
by magnetic field H; v = velocity of particle, m = its mass, e = its charge.
39 9.2.cit., n.35, 209. 40 H.Becquerel, CR, 1900, 130, 372-6, 12th Feb. 41 Letter from 147Urookes foiff.G.Stokes, dated 16th Dec.1900;
Stokes, Mem.& Corres., 2, 484. 42 J.J.Thomson, Nature, 1876, 54, 302. 43 Id., Cathode Rays, Phil.Ma., 1897, 44, 293-316, 310. 44 -nem.N., 1900, 81, L45-6, 30th Mar.;-Trans. from
Rev.Gn.des Sci.. 15th Mar.1900. 45 70707-617-539-40, 5th Apr. 46 P. and V.Curie, Sur la charge electrique des rayons
doviables du radium, CR, 1900, 130, 647-50, 5th Mar.; P.Curie, Oeuvres, 353-7.
47 Ibid., 356. 48 tql7-1900, 130, 809-15, 26th Mar.; E.Dorn, CR, 1900, 130,
TT26, 23rd Apr. wrote to claim priority for the qualit-ative electrostatic deviation, in February, of the rays from Giesel's active barium compound.
49 P. and M.Curie, CR, 1899, 129, 714-6, 6th Nov.; prior to the experimental deflection of the rays.
50 E.Dorn, Ueber die von den radioactiven Substanzen aus-gesandte Emanation, Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, Jun.1900.
51 PT and M.Curie, op.cit., 716. 52 CR, 1899, 129, 7]- 6,-----6-11 Nov., appended to paper of Curies.
Notes for Chapter 3, p.119-178) 304
53 CR, 1899, 128, 771-7, 773. Refraction and polarisation were also now rejected, for different reasons.
54 1.1RR2ER, 169-215, 180-1, dated 1st Sep.1898; he cited only Schmidt on thorium.
55 Ibid, 56 E.Rutherford and R.B.Owens, Thorium and Uranium Radiation,
Trans.Em.Soo.Canada, 1899, 2, 9-12, read 26th May; 1utherfor47-Pa ers, 216-9, The authors state that thorium nitrate gave a -a_rly constant radiation; we note that this contradicts the stated results of 1898. E.Rutherford, Notebook 3, CUL, contains experimental results of Owens and Rutherford.
57 Ibid., 218; E.Rutherford, Radio-activity, 1905, 238. TaTeter and H.Geitel, Phy2.2., 1899, 1, 11-14, (received 19th Aug.) in examining the conductivity of the ordinary air in the laboratory noted that this was markedly increased by a draught from the room containing Ra and Po samples, without at this stage stating any conclusions; they were soon to take up Rutherford's view.
58 E.Rutherford, Le2212, 218, May 1899. 59 R,B.Owens, Phil.Mag., 1899, 48, 360-87, Oct. issue,
probably wrT117m—Vifore Jul. 60 E.Rutherford, Phil.ns.. 1900, 49, 1-14, Jan. issue,
dated 13th Sep.1899; Papers, 2231. 61 Ibid., 220. 62 E.Rutherford, Some Remarks on Radioactivity, Phil.Mag.,
1903, 5, 481-5; Viers, 578. 63 N.Feather, Lord Rutherford, 69-73, and A.Romor,
The Restless Atom, 43-52, give brief accounts of the researches ofTaherford and others at about this time.
64 Letter from J.J.Thomson to E.Rutherford, dated 23rd Jul. 1899, CUL. The thorium oxide layer is designated AB.
65 Ibid.; question-mark omitted sic. 66 TT :Owens, Thorium Radiation, 1599, 366, 67 Ibid., 372-3. 68 Letter to Mary Newton, dated 2nd Dec.1899, A.S.Eve,
Rutherford.Etc., 69. 69 E.Rutherford, A Radioactive Substance emitted from
Thorium Compounds, Papers, 221. 70 Ibid., 222. 71 Ibid., 225. 72 min, Papers, (1894-7), 6, 17f., had found that gases
could retain conductivity when bubbled through water; Rutherford made no mention of this; his experiments went much further.
73 E.Rutherford, Rp.cit., 224. 74 Id., Papers, (1902), 432. 75 17RutheaER, A Radioactive Substance etc., Papers,
227-9. 76 E.Rutherford, Uranium Radiation etc., Papers, (1898-9),
214-5. 77 Id., A Radioactive Substance etc., Papers, (1899-1900),
728. 78 1900, 62, 31-2, 10th May. Described more fully, and
quantfT5.tively in 'Indications relatives ift la constitution do la matiere etc.', Rapports , Cong.Int.de Physique, 1900, 3, 138-51, Aug.
Notes for Chapter 3, p.119-178) 305
79 Letter from Thomson to Rutherford, dated 23rd Jul. 1899, CUL.
80 J.J.Thomson, 22.cit., Nature, 10th May 1900. 81 A.S.Eve, RutherfOW.Et67767, letter dated Sep.1899. 82 Pa ere, 230. 83 Rutherford in 1905, Radio-activity, 239, mentioned
only the method he used in 1t399. 84 E.Rutherford, Papers, (1899-1900), 230. 85 Ibid., 226. 86 Taa., 230. See below, Section 4, p.176. 87 Ibid., 231. 88 5' above Chapter 2, Section 2, p.81; E.Rutherford,
Papleirs, 106. 89 Ruterford, Pa ere, 231. 90 Id., Phil.Mee., 1900, 49, 161-92; Papers, 232-59;
Tited 22nd Nov.1899. -- 91 Papers, 255-7, Nov.1899. 92 .11, d., 256. 93 6W-above, Chapter 2, Section 2, p.71; Papers, 27-31. 94 E.Rutherford, Papers, (1897), 132f. 95 J.J.Thomson, Conduction of Electricity through Gases,
1903, 296. 96 E.Rutherford, Radioactivity Produced in Substances
etc., Pa ere, 258. 97 Ibid., 98 17-Rutherford (and R.B.Owens), Pa ers (1899), 219. 99 P. and M. Curie, CR, 1899, 129, ; see above, p.129-30. 100 In fact it was Becquerel who had used the term phos-
phorescence, in his appended remarks; the Curies wrote 'rayons secondaires'.
101 E.Rutherford, Pa ere, 238. 102 Letter from J. ."homson to E.Rutherford, dated 21st Dec.
1899, CUL. 103 Letter, id., dated 22nd Nov.1898. 104 J.Zeleny, On the ratio of the velocities of the two ions
produced in gases by Röntgen radiation etc., Phil.Mag., 1898, 46, 120-54.
105 Ibid., 134-5. 106 Letter from J.Zelony, Univ. Minnesota, to E.Rutherford,
dated 25th Mar.1900, CUL; quoted in part, with a different interpretation, in N.Feather, Lord Rutherford, 73.
107 See above, Chapter 2, Section 4, p.117. 108 Ueber Luminescenz von festen Korpern and festen
Losungen, Ann.d.PhT, 1895, 56, 201-54, 241-50. 109 Ann.d.Phys., 69, 220-35, Sep. issue; from Giittingen
where Behrendsen (b.1850) was Professor at the Gymnasium. 110 Behrendsen, ibid., 234, refers to their paper read at
Braunschweig-Tri-Jan., not the reprint in Ann.d.Phys., 1899, 69, 83-90, Sep. issue.
111 BehrenTgen, 22.cit., 233. 112 Only the German scientists had in fact said this;
Marie Curie had speculated on the evolution of the elements, but to this Behrendsen made no reference.
113 Ibid., 235. 114 TEM. 115 T1F71.Z., 1900, 1, 476-8, Aug. issue.
Notes for Chapter 3, p.119-178) 306
116 E.Rutherford, Uranium Radiation etc., Papers, (1898-9), 167-215, 215.
117 M.Curie, Les Rayons de Becquerel etc., Oeuvres, (1899), 60-76, 71.
118 G.G.Stokeo, Mem.& Corres., 1, 293-4, letter to Becquerel dated 16th Aug.1399; see above, Chapter 2, Section 4, P.115.
119 Stokes, Ibid., 294-7, letter dated 25th Aug.1899. 120 See e.g. E.Rutherford, Radio-activity, 1905, 210, 249,
391, and his references. 121 E.Rutherford, Pa era, (1900), 230, Jan. 122 Ann.d.Phys., 1900, 2, 335-7, dated May, pub. Jun. 123 T.-Elster and H.Geitel, Uber Becquerelstrahlen, Verh.
Deut.Phys.Ges., 1900, 5-8, 5th Jan. meeting. 124 Read Jun. 1-970, see below, Section 4, p.172-3. 125 CR, 1899, 128, 771-7. 126 Md., 777. 127 Ibid. 128 G.G.Stokes, Mem.& Corres., 1, 297-9, letter from Stokes
to Becquerel Tdied 4th Sep.1899. 129 H.Becquerel, CR, 1899, 129, 716, appended to the
Curies' paper. 130 H.Becquerel, Deviation du rayonnement du radium dans
un champ electrique, CR, 1900, 130, 809-15, 815; Curie Ra sample of unspecified T6Tivity.
131 Id., Sur le rayonnement de l'uranium etc., Rapports, anig.Int.de Plysique, 1900, 3, 47-78, 78.
132 ma. 133 7:75cquerell 1901, 63, 396-8. 134 Ibid., 398. 135 Tee-below, Chapter 4, Section 1, p.186-8. 136 Nature, 1900, 61, 443. 137 See e.g. 'S.W.', The Principle of Radium, Nature, 1903,
68, 496-7, who makes a similar point. 138 See above, Chapter 2, Section 4, p.113, n.355. 139 See Chapter 5, Section 1, p.230, on the emission of
heat from radium. 140 CR, 1899, 129, 823-5. 141 a.cit.„ Nature, Mar.1900, 142 P.CaTIe, Oeuvres, 353-7; CR, 1900, 130, 647-50; see
above, Section 1, p.127. 143 See above, Section 1, p.126-8. 144 Rev.Sc., 1900, 14, 65-71; M.Curie, Oeuvres, 95-105. 145 P7 aid M.Curie, Les nouvelles substances radioactives
et lee rayons qu'elles omettent, EakvortE,Cona.Int.de P si ue, 1900, 3, 79f., Aug.; P.Curie, Oeuvres, 33
3
s409
146 Ibid., 409. 147 FTUarie and G.Sagnac, kectrisation negative des rayons
secondaires produits au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 358-62; E.Dorn, Abh.der Naturf.Ges.zu Halle, 1900, 22, 40-2.
148 See above, Chapter 2, sTaTion 4, p.112. 149 22.cit.; M.Curie, Oeuvres, 95-105. 150 mx7Fre, ibid., 104. 151 Ibid. 152 Nature, 1900, 62, 31-2, 3rd May issue.
Notes for Chapter 3, p.119-178) 307
153 See above, Chapter 2, Section 4, p.117. 154 Mt. Curie, 92.cit., Oeuvres, 104-5. 155 See below, Chapter 4, Section 1, 156 See H.Becquerel, Sur le rayonnement des corps radio-
actifs, CR, 1899, 129, 1205-7. 157 See below, Chaptei27 Section 1, p.189. 158 Phil.Trans.A., 1901, 196, 25-59, received 15th Jun.1900;
E.RutherforU, Papers, 260-95. 159 Ibid., 292. He mentioned Becquerel and Giesel, not
7:754yer and Schweidler, Phys.Z., 1899, 1, 90-1, Nov., who first pointed out this di?ference.
160 H.Beoquerel, Note our le rayonnement de l'uranium, CR, 1900, 130, 1583-5, in fact announced a magnetic deflection, also in June; he was unsure of the uranium's purity.
161 Rutherford (and McClung), op.cit., Papers, 292. 162 ibid., 260. 163 Letter from E.Rutherford to J.J.Thomson, dated 9th Jan.
1900, CUL. 164 E, Rutherford, 22.cit., 268-70. 165 Ibid., 273-4; assumptions soon rejected - see below,
this Section, n.I80. 166 ibid., 285. 167 See above, Chapter 1, Section 4, p.36-7. 168 E.Rutherford, 22.eit., Pte, 287. 169 Kelvin, RI Lib.8617, (1 J), 3, 227-56, 227; atomic or
molecular diEneters ranged from 10-' to 10-6 cm. 170 E.Rutherford, op.cit., Papers, 294. 171 A.S.Eve, Rutherfo7Z-Etc., 172 E.Rutherford, Papers, 295. 173 Ibid., 294. 174 (7:7:Thomson, Radium, Nature, 1903, 67, 601-2;
E.Rutherford, 22.cit., Papers, 287, 294; id., Radio- _._.qI121-±Y, 1905, 457.
175 -Elhirford, 22.oit., 2222E2, 294. 176 Ibid. 177 Ibid., 295; he had used the term 'atom' in 1898
rfaTers, 214-5). 178 For example, J.P.Cooke, The New Chemis_tEz, London,
11th ed., 1903, 72-7, wiTHreTirence fd-Keivin, states that 1 litre of any gas under standard conditions contains 61 x 10' molecules, and 1 litre of hydrogen weighs 0.09 gm. One can from this deduce the weight of a hydrogen atom to be 1046 gm., hence Ur = 2 x 10-/4' gm.
179 E.Rutherford, op.cit., 0.._.apers, 293. 180 Ibid., 295. RaTrierford, Papers, (1903), 607, had to
accept Townsend's commenin a letter from Oxford mis-dated 14th Jan.1900 (written in 1901), that his value for the ionisation energy was 'far too large' by a factor of at least twelve. Any difference this might have made to Rutherford's arguments soon vanished as the Curies reported radium specimens,of activities sufficiently high to compensate. The discrepancy was in the early assumption that all the radiated energy produced conductivity in the gas; it was later'impossible to estimate' how much was dissipated as heat(RUtherford, Radio-activity., 1905, 58-9).
Notes for Chapter 3, p.119-178) 308
181 ibid., 294. 182 Ibid., 295. 183 X7g:Eve, Rutherford.Eto., 73. 184 Ibid. 185 reigon, 1900. 186 62, 525-6. Also, see the Proutian interpretations of
7Illard's work, below p.168, n.213. W.Kaufmann, Electrician, 1901, 48, 95-7, was to express ideas similar to those of Fitzgerald.
187 Chem.N., 1900, 81, 304-5; trans. from Ber.deut.chem.Ges. 188 7717tTica, Chem N., 1900, 82, 166-7; from Miliiik711"-
Zeitung; he hadFeld such Views for twelve years or more.
189 E.E.Pournier d'Albe, The Life of Sir William Crookes, London, 1923, 372.
190 F.P.Venable, The Nature of Valence, J.Amer.Chem.Soc., 1899, 21, 192-200, 220-31, p.197. -
191 CR, 1899, 129, 593-5. 192 A.Debierne, CSR, 1900, 130, 906-8. 193 H.W.Kirby, T Discovery of Actinium, Isis, 1971, 62,
290-308, questions the identity of theEETerials described in Debierne's two papers and credits F.Giesel with the discovery in 1902 of 'emanium', the element now known as actinium.
194 P.Curie, Oeuvres, (1899), 345, Nov. 195 A.Debierne, p.cit., n.192, Apr.1900. 196 E.Rutherford (and ,.Soddy), The Radioactivity of Thorium
Compounde.I, Papers, (1902), 376-402, 378, Jan. 197 Ann.d.Pplys., 1899, 69, 91-4; briefly in Phy.2.Z., 1899,
. 198 The Curies appear to have observed this by July 1899,
before Giesel's publication, see I.Joliot-Curie in M.Curie, Pierre Curie, Paris, p.120; but they accord priority to GiesCi1 TCongr6s 1900, P.Curie, Oeuvres, 388. The effect was later explained by an accumulation of 'emanation'.
199 F.Giesel, Einiges abor Radium-Baryum etc., Verh.Dout. Phvs.Gos., 1900, 2, 9-10, dated Dec.1899.
200 A.DebTeTne, CR, 1700, 131, 333-5, 30th Jul.; trans. Chem.N., 1907 82, 85.
201 751777 Chem.N. 202 Ibid., TN:- 203 FiTT:deuTTchem.Ges., 1900, 33, 1237-40; Chem.N., 1900, 82,
2576, dat:TiTiffay. 204 Ibid., 25. 205 J76T:deut,chem.Ges., 1900, 33, 1665-8, received 28th May. 206 F.Giesel, ibid., 1668; he sIso pointed out that his own
and the Curies' polonium differed both in radiation type and rate of decay, to which the age of the specimens might be relevant. Marie Curie herself wavered towards the belief that polonium was merely induced bismuth, in 1902.
207 K.A.Hofmann and E.Strauae, Radioactives Bloi and radio-active seltene Erden, Ber.deut i chem.Ges., 1900, 33, 3126-31. Of interest are modern transformation series in which no less than four natural, active, true lead isotopes feature; their half-lives are about 27 min., 36 min., 11 hr., and 22 yr. It seems that Hofmann may have had any or all of these, followed by their active
Notes for Chapter 3, p.119-178) 309
207 oontd.) decay products, Bi, Po, Tl, in his preparations from different minerals; even if he had avoided traces of other active elements.
208 P.Giesel l Ueber radioactive Stoffe, Ber.deut,chem.Ges., 1900, 33, 3569-71; Chem.N., 1901, 837-122-3.
209 Hofmann and StrausiTriiia., 1901, 7, 8-11, received 28th Dec.1900.
210 F.Giesel, ibid., 3772, thought this work unreliable. 211 Hofmann and A.Korn, ibid., 1901, 34, 407-9. 212 SOances Soc.Fr.TIcET.;7700, 59*, bth Jul. 213 In relatICTI T45 hIS G.Sagnac had written to J.Larmor
(letter dated llth May, 1899, Royal Society Library) concerning types of vacuum tube. He suggested that Villard's idea that Whydrog6ne est indispensable a la formation dee ions cathodiques' would be unnecessary if all bodies were 'forme dune meme matiero simple qui serait la matiare radiante de Crookes'. Villard himself believed in the unity of radiant matter but explained this by the universal presence of the penetrating and chemically reducing element hydrogen, presumably as an impurity; see e.g. P.Villard, La formation des rayons cathodiques, Rev.Gon.des Sci., 1899, 10, 301-8; id., Les Rayons CathodUiaes, Rapports,Cong.int.de Physique, 1900, 3, 115-37 / 136-7.
214 See Stokes, Memac Correa., 2, 484. 215 J.C.McLennan, PhIl.Mag., 19u2, 3, 195-203. 216 Hofmann and Strauss, Leber das radioactive Blei.2.
Mitteilung, Ber.deut.chem.Ges., 1901, 34, 907-13, 913. 217 Another feature was the dffiriirence between salts, id.,
3.Mitteilung, Ibid., 3033-9. 218 F.Gieeel, Uebe7-717dleactive Stoffe, ibid., 3772-6; the
modern theory Ewes only admixtures and does not admit induction; active lead is a 'transformation product' of radium.
219 K.A.Hofmann and Strauss, Ueber radioactive Stoffe, ibid., 3970-3, received 27th Nov.1901.
220 P,Giesel, On radio-active lead, Chem.N., 1902, 85, 89-90; from Ber.deut.chem.Ges., 7517, 35, 102f., Jan.
221 K.A.Hofmann 770, 1f1, radioacTIve Stoffe.l. Ueber radioactive° Blei, ibid., 1902, 35, 1453-7, Apr.; self-recovery shows the activity is not the induced kind.
222 Id., Chem.N., 1903, 87, 241-3; from Ber.deut.chem.Gos., 703,77; 7040f.
223 Proc.22y.Soc., 1900, 66, 409-22, read 10th May; reprinted WITFoai-iETTIrpretation in A.Romer, Discovery of Radio-activity, 70-84.
224 Crookes in Romer, 22.cit., 82. 225 G.G.Stokes, Memac.-Co7i-e-s., 2, 490. 226 Ibid., 490-2. 227 7/7nFookes, Notebook 16, pp.270, 305f., RI. 228 Ibid., Notebook 17, 102f. 229 Ibid., 106. 230 Crookes in Romer, opecit., 74. 231 Ibid., 71. 232 MU., 77. Becquerel was soon to deny this, with important
riequences, see below, p.171; and Chapter 4, Section 1, p.187, n.28.
Notes for Chapter 3, p.119-178) 310
233 Letter from W.Crookes to P.Curie, dated 13th Jul.1900, BR.
234 References were provided, namely Ber.deut.chem.Ges., 1900, 33, 1237-40 and ibid., 16657U7 Sae move, p.166-7.
235 TypescHpt translation-GT-letter from P.Curie to W.Crookes, dated 17th Jul.1900, appended at end of Crookes' Notebook 17, RI.
236 Letter from Crookes to P.Curie, dated 19th Jul.1900, BN. 237 W.Crookes, Radio-activity and the Electron Theory,
Chem.N., 1902, 85, 109-12, 109. 238 Chem.7.1., 1901, UT, 179-81, 187-9; he referred to
737Riaunerts similar work; the latter, Chem.N., 1901, 84, 219, claimed priority; these researches did not extend beyond the field of inorganic chemistry.
239 Baskerville, ibid., 179. 240 CR, 1900, 130, 1583-5. 241 H.Becquerel, Sur is rayonnement de l'uranium, ibid.,
1900, 131, 137-8, 16th Jul. 242 BecqueI, Sur le rayonnement de l'uranium etc.,
11122ports,241264.Int.do Pbrsi ue, 1900, 3, 47-78, 74. 243 P. aT7M.(:urieTres nouvo lea substances radioactives
etc., ibid., 79f.; P.Curie, Oeuvres, 374-409. 244 ibid.,qm 245 YETa., 407. 246 See above, Section 3, p.155-6. 247 P.Curie, 22.cit., Oeuvres, 384. 248 Ibid., 379-80, 404-b. 249 Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, read
7aii.1900. 250 Ibid., 1. 251 YETU., 11-12. 252 Ibid., 13. 253 TUTU., 15. 254 77gnning, Ueber radioactive Substanzen, Ann.d.Phys.,
1902, 7, 562-75. 255 Ibid., 569. 256 Letter from J.J.Thomson to E.Rutherford, dated 15th
Feb.1901; A.S.Eve, Rutherford.Etc., 76. 257 E.Rutherford, Pa ers, 261. 258 E,Dorn, Phys.Z., 1 1, 2, 218, received 24th Dec.1900. 259 Rutherford, PEyq.Z., 19a, 2, 429-31; Papers, 296-300. 260 Ibid., Papers, 230. 261 Rutherfor op.cit., (1901), 300. 262 A.S.Eve, Rutherford.Etc., 77. 263 Ibid., 78. Letter dET-Jd 12th Apr.1901. 264 Above, Section 2, p.145. 265 Letter from J.J.Thomson to E.Rutherford, dated 25th
Apr., CUL. 266 E.Rutherford, Papers, (1901), 325, 358-9, Dec.; see
below, Chapter 4, Section 2, p.208, 223-4. 267 Id., Papers, 230. 268 TTss Brooks' first published research, performed with
Rutherford's help and on one of his subjects, concerned 'Damping of the Oscillation in the Discharge of a Leyden-jar', Phil.Mag., 1901, 2, 92-108; she was B.A. Tutor in MathWffialics, Royal Victoria College for Women, Montreal, at the time.
Notes for Chapter 3, p.119-178) 311
269 Trans.jya.Soc.Canada, 1901, 7, 21-5; Rutherford, 15656713, 301-5,
270 Nature, 1901, 64, 157-8; Papers, 306-8. 271 Ibid., 305, 3087 272 5-675-e.g. S.Glasstone, Sourcebook on Atomic Energy,
London, 1950, 125. F.Soddy, Radio-activity, Electrician, 1904, 52, 681, deduced from the same results a doubled aTFinic weight of 160, at a time when theory demanded a value oloee to that of radium.
273 E.Rutherford, Papers, (100, 545. 274 Rutherford,227-etT77 Papers, (1901), 305, 308, May. 275 Rutherford, op.-617., Nature, Papers, 308. 276 See below, CH-gpter 5, Section 1, p.233-6. 277 H.Geitel, Ueber die Elektrizitatszerstreuung in abge-
schlossenen Luftmengen, 1900, 2, 116-9. 278 J.Elster and H.Geitel, Weiiire Versuche etc., ibid.,
1901, 2, 560-3. 279 Id., Leber eine fernere Analogie in dem elektrischen
Wrhalten der naturlichen and der durch Becquerel-strahlen abnormleitend gemachten Luft, ibid., 590-3.
280 H.Geitel, Ueber die durch atmospharischirrift induzierte Radioaktivitat, ibid., 1901, 3, 76-9.
281 Id., Archives dee Sciences, S02, 13, 113-28, 122; 271-Fie the followIE7g Section.
282 Ibid., 124.
312 NOTES FOR CHAPTER 4
(pages 179-225)
1 H.Geitel, Archives des Sciences, 1902, 13, 113-28, dated Dec.190i.
2 Ibid., 127. 3 Ibid., 126-7. 4 Ibid., 117. 5 Mister and H.Geitel, Vereuche fiber induzierte Radio-
activitAt der atmosphilrischen Luft durch positive Potentiale, Phys.Z., 1902, 4, 97.
6 Id., On the radio-active emanation in the atmospheric air, Chem.N., 1903, 88, 29-32, 52-4, received by P s:277-6-Th Jun.
7 er .Deut.Ges.Natf., 1902, 73, 83-99, 98; read ME 76571901.--
8 CR, 1901, 132, 548-51; P.Curie, Oeuvres, 410-13. 9 UR, 1901, 137, 768-9; P.Curie, Oeuvres, 414-6;
read 25th /5.7.1901. 10 Paers, 301-5; with Miss H.T.Brooke. See above,
apter 3, Section 4, p.176. 11 P.Curie, 2p.cit., Papers, 414-6. 12 Ibid., 41 13 T.77 at the time of the Congress in Aug.1900, P.Curie,
Oeuvres, 407-8. 14 1'. (,curie, Oeuvres, 412, 5th Mar.1901. 15 Notebook, pp.125 plus pp.18 in reverse direction, written
by P. and M.Curie; records experiments on induced activity from 5th Dec.1900 to mid-1902; also various radiation and chemical studies from May 1899 to late 1902; held at Wellcome Historical Institute.
16 E.g., ibid., p.72; mouvement propre: 10th Jan., 90 gm. in 40 sec.;11th Jan., 90 in 36; 12th Jan. worsening to 90 in 11.
17 Ibid., p.101, 10th Jul.1901, action of the extreme cold of liquid oxygen upon radium and thorium: radium's activity fell from 2000 gm. in 20 sec. to 200 in 12; thorium gave initial readings of 50 gm. in 30, then 23, then 37 sec. But mouvement propre was 50 in 23, consequently 'c'est mt. propre pas action'.
18 CR, 1901, 133, 276-9, 29th Jul.; P.Curie, Oeuvres, 420-3. 19 Tad., 421. 20 CR, 25th Mar.1901; P.Curie, Oeuvres, 416. 21 Mem.N., 1901, 84, 88-9. Perhaps P.de Heen's publication
TO-Tu-Li.1901 on 'La radioactivit4 de la matiere et l'energie susceptible de se developper e. la surface des corps' (Rev.Sc., 1901, 16, 161-70) should be mentioned here, forme gave something of a mechanism for radio-active induction through gases: molecules irradiated by an active source themselves emitted rays or 'jets d'ether', which excited radiations in other molecules, and so on. No doubt P.Curie thought this work as weak as that of G.Le Bon which de Heen cited and which Curie had already criticised. The latter published no further comment on the researches of either of these obscure scientists, reserving his considerations for others now better known.
Notes for Chapter 4, p.179-225) 313
22 P.Curie and A.Debierne, Sur la radioactivito induite provoquee par los sells de radium, CR, 1901, 133, 931-4, 2nd Dec.; P.Curie, Oeuvres, 424-7.
23 Letter from F.Giosei to P. and M.Curie dated 23rd Mar. 1902, BN; see below, Chapter 5, Section 1, p.232.
24 P.Curie, Oeuvres, 421. 25 E.g. Sur la radio-activite secondaire, CR, 1901, 132,
734-9, 25th Mar. 26 CR, 1901, 133, 977-80, 9th Dec. 27 On the Conductivity of Gases under the Becquerel Rays,
Phil.Trans.&., 1901, 196, 507-27, 525. M. Curie, Sur la pbnetration dos rayons etc., CR, 1900, 130, 76-9, 8th Jan., had earlier likened these rays to 'projectiles'.
28 The statement flatly contradicted the comments published by Crookes on Ur nitrate in 1900; see above, Chapter 3, Section 4, n.232.
29 CR, 1900, 130, 1583-5, 1585. 30 -H.Becquerei;-Sur la radio-activitO de l'uranium, 978. 31 P. and M.Curie, CR, 1902, 134, 85-7, 13th Jan; P.Curie,
Oeuvres, 428-30; A.Romer, Discovery of Radioactivity, 117-23, gives translations of this paper an of Becquerel's.
32 P.Curie, Oeuvres, (1900), 356. 33 M.Curie, Oeuvres, 104-5; see above, Chapter 3, Section 3,
p.156-7. 34 Chem.N., 1902, 85, 169-72, read at RI on 7th Mar. 35 Rev.Sc., 1901, 15, 449-61, read Feb., pub. Apr. 36 Ibid., 460-1. 37 767above, Chapter 2, Section 4, p.110-1. 38 G.G.Stokes, Mem.& Corres., 2, 478-81; both hypotheses
required an external supply. 39 See o.g. Phil.Trans.A., 1901, 196, 507. 40 Letter dated 16th Dec.1900 in reply to Stokes' of 15th;
Stokes, Mem.& Correa., 2, 481-5. 41 Crookes, loc.cit. in Stokes, Mem.& Correa. The cited
paper is W-P7VIllard, Sur la perm6EFI1ITe de la silice fondue pour l'hydroOne, CR, 1900, 130, 1752-3.
42 P.Villard, Les Rayons CaliRidiques, tiapportili,22E,E.Int.de Ph ai ue, 1900, 3, 115-37, 136-7, Aug.
43 Crook©s in Stokes, loc.cit., 489-90, letter dated 15th Jun,1901.
44 W.Crookes, Notebooks, 17, and 18, RI. 45 Ibid., e.g. 17, 308-69; 18, 149-85. 46 -076R.IT., 1907 85, 109-12, read to the Royal Society
MeE. 47 1901, 83, 130, from Bristol; Martin gained his B.Sc. in
that year. He studied at University College Bristol and several German universities before becoming Lecturer at University College Nottingham in 1907 and at Birkbeek College, London in 1910. He later held a variety of industrial posts and published prolifically on chemistry -pure, industrial, and popular.
48 Ibid., 141, 22nd Mar.; E.Booth, ibid., 262-3, discussed Ts further.
49 Chem.N., 1902, 85, 205-6, dated 26th Mar., Berlin University.
Notes for Chapter 4, p.179-225) 314
50 L.Boltzmann, Lectures on Gas TheoriL, (1898), 377, dopioted the chemical-. YOna"-Tis an overlap of supposed sensitive regions of blank material atoms. VI.Sutherland, The Cause of the Structure of Spectra, Phil.Mm., 1901, 2, 245-74, 269, illustrated his spherical material atom as furnished with a few electrons, some of whose orbits collided with the atomic surface. See Chapter 1 above and Chapter 5, Section 2 below for further discussions of theories of atomic structure.
51 Chem.N., 1911, 103, 169. 52 Leipzig, 1902; pref. dated Easter. 53 E.g. see W.Kaufmann, The Development of the Electron
Idea, Electrician, 1901, 48, 95-7; see the preceding Section, p.163T-YOr Fitzgerald's similar speculations in 1900.
54 J.Stark, 22.cit., 93-4. 55 Ibid., 34. 56 Tail., 35. 57 Letter dated 5th Jan.1902, A.S.Eve, Rutherford.Eto., 80-1. 58 Letter to Mary Newton, dated 25th Jan.1896, ibid., 23-6,26. 59 Letter from F.Soddy to E.Rutherford, dated 17TE-Dec.1903,
CUL, concerning among other items Becquerel's book of 1903. 60 N.Foather, Lord Rutherford, 1940, 78-90, describes some
of the poin1 A.Romer, The Transformation Theory of Radioactivity, Isis, 1958, 49, 3-12; id., The Restless Atom. The Awakening of Nuclear Physics, NeW-York, 1960, 591, outlines ithe stages in clear and simplified form. T.J.Trenn, Rutherford and Soddy: from a search for radioactive constituents to the disintegration theory of radioactivity, Rote, 1971, 1, 51-70; id., The rise and early development of the disintegration theory of radio-activity, Dies., Univ. Wisconsin, 1972, gives more det-ailed but sometimes less clear descriptions specifically limited to the Rutherford-Soddy experimental collaboration of Sep.1901 to May 1903.
61 E.Rutherford, Pa ere, 305, 308; see above, Chapter 3, Section 4, P-1
62 T.J.Trenn, Dips., 14-15, 60, states that there is no evidence for the assumption that the collaboration began before Sep.1901.
63 A.S.Eve, Rutherford.Etc., 77. 64 M.HoworthTPIWEIFFResearch on the Atom ... The Life
St2ry of Frederick Aaddz, London, 1958, 79-81; the Yaw mss. are in th-e-nddleian Library, Oxford, where they were placed by M.Howorth. Trenn, Dies., 60, has found Rutherford's brief notes on the 465Tings of the Physical Society from 1898 to 1907 in the McGill University Archives, and confirms that both parties refer to the same meeting.
65 'The Indivisibility of the Atom', pp. 23, typescript, Soddy-Howorth Collection, Bodleian Library.
66 M.Howorth, Pioneer Research.Soddy, 81; id., Atomic Transmutation. The Greatest Iiiadvery Ever Made, London, 17577617----
67 J.J.Thomson, On Bodies Smaller than Atoms, pop.Sci. Monthly, 1901, 59, 323-35, Aug.; similar to R1Lecture o 19th Apr.19017
68 M.Howorth, Pioneer Research.Soddy, 65; Soddy's own comment.
Notes for Chapter 4, p.179-225) 315
69 Ibid., 64; six lectures are preserved. 70 Tura., 85. 71 E.Rutherford and F.Soddy, J.Chem.Soc., 1902, 81, 321-50;
Rutherford, aTers, 376-407. 72 E.Rutherford an .Soddy, An Investigation of the Radio-
active Emanation produced by Thorium Compounds.I , Proc. Chem.Soc., 1902, 18, 2-5; Chom.N., 1902, 85, 55-6.
73 E70treFford (and-P.Soddy)77a.at., Papers, 381. 74 Ibid., 385-7. 75 1:.g. at that time G.Martin asked 'Is Argon an Elementary
Substance?', Chom.N., 1902, 85, 9, 3rd Jan., but only to suggest it m3.ghh be a mixare of several inert gases. H.E.Armetrong, who attacked W.Ramsay's conclusions regarding these gases, was in this case an exception.
76 E.Rutherford (and F.Soddy), 22.eit., Papers, 395-6. 77 Ibid., 395-6. 78 YTT3., 388-9. 79 170., 392-4. 80 nu., 390. 81 TEM., 391. 82 TE,frOworth, Atomic Transmutation, 44; id., Pioneer
Research.Soddy, 82-3. 83 E.RutherfordCand F.Soddy), ap.eit., Papers, 396. The
other possibility was that thiiiraR induced activity upon one of the atmospheric inert gases; concerning which see below.
84 E.Rutherford, Early days in radio-activity, J.Franklin Inotitute, 1924, 198, 281-9, 285.
85 T-Fullord, Papers_, 396. 86 Ibid., 391. 87 Tailierford had seen J.J.Thomson's paper to that Congress
by March 1901; letter from Rutherford to Thomson, dated 26th Mar.1901, A.S.Eve, Rutherford.Etc., 77.
88 J.Amer.Chem.Soc., 1901, 2:3, 761f., presented 27th Aug.; aem.N., 1961784, 179-817 187-9, p.181. 11th Oct.
89 TETEerford, 2p.at., Pa ers, 379-80. 90 Letter from Ur-o-61-gs to z erford, dated 18th Dec.1901;
A.S.EVe, Rutherford.Etc., 79. 91 E.Rutherford, op.eit., The Radioactivity of Thorium
Compounds.I.Etc., Papers, 389. 92 Ibid., 396-7. 93 .11-5ra. 94 ibid., 397. 95 mu. 96 ibid., 398. 97 1.RUmer, Restless Atom, 61; T.J.Trenn, Dies., 94. 98 E.RuthernTITTEHff IF:Soddy), 22.cit., 398, 402; they
confirmed this dismissal with the German nitrate from which the impurity was absent, as noted in their 2nd publication of May 1902, Pa ers, 435.
99 Ibid., 398; the 'two directions' are probably the precipitation and washing methods of removing ThI from Th, see below.
100 See the preceding Section, p.184-5. 101 Op.cit., Rutherford, faaE2, 402. 102 ma 390. 103 Ibid., 399-400. 104 R.Rutherford (and F.Soddy), Papers, 447.
Niates for Chapter 4, p.179-225) 316
105 Id., The Radioactivity of Thorium Compounds.I.Etc., Pa ern, 399.
106 Barramer.Ehys.Sac., 1901, 2, 37-43; E.Rutherford, Ti-ir-67525-c30",-7351-9, &I-tea 15th Dec.
107 / .J.Allen, rhys.Z.. 1902, 3, 225-30, dated 20th Dec.1901; RaTher?ord, pppprs, 360-9.
108 E.Rutherford, Transmission off-Excited Radioactivity, DIT2TP, 329.
109 Td:,-Excited Radioactivity etc., Pa ers, 367. 110 LIR, 1901, 133, 977-80, 9th Dec. Ruvher±ord, whose paper
was dated I5Th Dec., received by Ehys.Z., 22nd Jan.1902 (Pa ors, 359) may just possibly have derived the recoil idea rom Becquerel; the Abstract of Rutherford's similar paper to the American Physical Society, dated 14th Dec.(Papere
' 330) does not mention it. Becquerel,
however, u-sedTHE) notion in a slightly different way. 111 E.Rutherford, ap..cit., Papers, 368. 112 E.Rutherford (and F.Soddy), the Radioactivity of
Thorium Compounds.I.Etc., IlusEE, 378; perhaps Soddy influenced the brief statement given there.
113 E.Rutherford, op.cit., Papers, 368. 114 Id., (with S.J.Afien), Papers? 509, dated Jun.1902. 115 13Toc.Rox.Soc., 1902, 6971-77:82, Dec.1901. 116 E.Rutherford, Excited -Radioactivity etc., Pa ers, 369. 117 E.Rutherford and F.Soddy, Chem.N., 1902, 8,-6. -6. 118 Ibid., 56. 119 771athorford (and F.Soddy), Radioactivity of Thorium
Compounds.I.Etc., Pates, 396. 120 E.Rutherford, Exciid Radioactivity etc.. Papers, 368. 121 Id., Phys.Z., 1902, 3, 254-7, Papers, 376-5. There are
apparently no surviving English versions of several of Rutherford's publications.
122 Made by early December, 1901; Rutherford, Papers, 327, 371.
123 ibid., 372-3. 124 E.Rutherford, Papers, 436. 125 CR, 1901, 133.-0778-0; see the previous Section, p.187. 126 rffiomson's letter of 2nd May 1902 to Rutherford (CUL) may
modify the view (A.Romer, Isis, 1958, 3) that the election of this candidate was not to be expected at the first attempt. The former regarded the election as certain, thought the result 'a great scandal', and accused the new Secretary of bias in favour of his 'fellow townsmen' of Belfast. Rutherford at 31 was not particularly young for those days; C.T.R.Wilson at that same age had been one of the fifteen out of sixty candidates to be selected in May 1900. Rutherford, however, along with J.S.Townsend had only a year to wait for the honour, whilst Pierre Curie was experiencing worse problems with the Acadomie des Sciences in Paris.
127 J.Chem.Soc., 1902, 81, 837-60, 15th May meeting; .Rutherford, Papers, 435-56.
128 Ibid., 436. 129 Ma., 438-40. 130 UAW., 440. 131 Mid., 441-2; the emanation was as usual prevented from
interfering by means of a draught of air, ibid., 436.
Notes for Chapter 4, p.179-225) 317
132 Ibid., 449. They did not make it clear that this was E3T—a case of direct induced activity, and in one place referred to excited activity as a 'secondary radiation' (ibid., 450). A year earlier Rutherford (Pa ers, 305, 308) wrote of the production from radium emanation of 'a positively charged substance which ... becomes a source of secondary radio-activity'. The terminology was sometimes ambiguous but was explicitly clarified shortly afterwards as discussed below.
133 Ibid., 451. 134 7737., 447. 135 151a., 442-4, 448. 136 mod., 444-5. 137 Ibid., 455. 138 Ibid., 440-1. 139 TM. 140 !Era., 379. 141 See above, Chapter 3, Section 4, p.167-9. 142 K.A.Hofmann and F.Zerban, Ueber radioactive° Thor,
Ber.deut.chem.Gos., 1902, 35, 531-3, received 23rd Jan. 143 Ylj.561f-discliTa7ge in aboUT 4 mins. 144 Rutherford, 22.cit., 452. 145 Ibid. 146 mod., 454. 147 Ibid., 452. 148 =em.Soc., 1902, 81, 860-5, presented (not read)
15TE-May; also Chem.N., 1902, 86, 199-200. 149 Ibid., 863. Within weeks he hathe modern answer: the
7TEdual activity belongs neither to 1. nor 2. but to Ur itself; it declines immeasurably slowly. The path to that interpretation was not simple; see below.
150 Ibid., 864. 151 F.Giesel, On Radio-active Lead, Chem.N., 1902, 85,
89-90; from Ber.deut.chem.Ges., 18th 7-an. 152 F.Soddy, RadioactivitTU? Uranium, 864. 153 P.and M.Curio, Sur lea corps radioactifs, Jan.1902;
P.Curie, Oeuvres, 429. Rutherfores cautious description at this time was 'polonium (radioactive bismuth)', Deviable Rays etc., Pa ere, 470, dated 7th May 1902.
154 Ber.deut.chem.Ges., 02, 35, 2285-8, presented 9th Jun.; Chem.N., 1g57,-166, 52-3.
155 P. Soddy wrote from Montreal to E.Rutherford, who was on vacation, explaining the paper's contents and stressing its importance; letter dated 12th Jul.1902, CUL.
156 F.Soddy, Radioactivity of Uranium, 864- 157 Dated 29th Apr.1902, CUL. 158 Proc.Roy.Soc., 1902, 69, 413-22, discussed above, p.191-3. 159 ma., pr(567Roy.soc.,-799-413. 160 ibid., 410-3. 161 N.Feather, Lord Rutherford, 88; L.Badash, How the 'Newer
Alchemy' Was Received, Sci.Amer., 1966, 215, 88-95. 162 E.Rutherford (and F.SodiTYT, Radioactivity of Thorium
Compounds.II, Papers, 454-6. 163 E.Rutherford, The Existence of Bodies Smaller than
Atoms, Papers, 403-9, 409; read to the Royal Society of Canada, 2/th May 1902.
Notes for Chapter 4, p.179-225) 318
164 E.Rutherford, Radioaotivity of Thorium Compaunds.II, loo.cit., 452, 455.
165 PEI17E., 1902, 4, 569-85; Rutherford, Papers, 494-508. 166 WY:Trann, Diss.. 316-22, suggests a mid-June submission
since they did not cite Marckwald on Po, and the delay for some other papers at this time was around 5 to 6 months.
167 E.Rutherford, op.cit., Pa ers, 508; the phrase 'accomp-animent of a chemical change- used in the previous paper written in April (id., Papers, 455) evidently had no specific temporal meaning.
168 Ibid., 508. 169 THU. 170 Radioactive Change, Phil.Mag., 1903, 5, 576-91;
Rutherford, Pa ere, 3W608, 603;•proNlably submitted in about Mar. 1 .
171 Letter from P.Soddy to E.Rutherford, dated 26th Sep. 1903, CUL; any reply appears to be lost.
172 Ibid. 173 E.Rutherford, Phil.Mag., 1903, 5, 95-117, dated 29th Jul.
1902; Papers, 377:0, 529. 174 Ibid., 539-41. 175 IFItE, 358-9. 176 IcRatherford, Pa ors, 415-34, 421. 177 E.Rutherford (an .J.Allen), Papers, 509-27. 178 Ibid., 517. 179 ITRUtherford, Excited Radioactivity etc., Papers, 545-6. 180 Ibid., 546. 181 It is interesting that Rutherford at this point had
adopted a completely particulate or non-vibrational view of all radioactive radiations including, temporarily, the penetrating gamma rays which he thought were high velocity electrons; E.Rutherford, Penetrating Rays from Radio-active Substances, Nature, 1902, 66, 318-9, 6th Jul.; faa.2, 410-4, 413.
182 Rutherford, 22.cit., Pa ere, 546. 183 Ibid., 544-7; atom and molecule of an inert gas were
Fin to bo identical. 184 E.Rutherford, Phil.Mag., 1903, 5, 177-87, dated Nov.1902;
id., Papers, 1.ggl-57; T.J.Trenn, Dies., 209-31; A. Romer, Tistless Atom, 71-84.
185 TTUTEriford and F.Soddy, The Radioactivity of Uranium, Phil.Ma .1 1903, 5, 441-5; id., A Comparative Study etc. THU., 45-57; Rutherford, Papers, 561-75, 564, 575.
319 NOTES FOR CHAPTER 5
(pages 226-280)
1 M.Curie, Pierre Curie, N.Y., 57. 2 P.Curie, URT-Tg6277154, 420-3; Oeuvres, 431-4. 3 P.Curie, MI, 1902, 175, 857-9, I7TEYE.; Oeuvres, 435-8. 4 Seances Soc.Fr.phyp., 1902, 60*, 19th Dec. 5 Pieuriel7174-7903-,-- 136, 223-6; Oeuvres, 440-3. 6 Ibid., 4477 7 77RUtherford, Papers, 508. 8 Ibid., 498. 9 TTOTirie, 22.cit., Oeuvres, 442. 10 Id., 443. 11 E.Rutherford, 2E12E2, 507-8, Nov.1902; the previous
publication in Phil.M2a., Sep.1902, had made the point fairly clear however; and an abstract of the earlier paper of May 1902 in Rev.Gen.des Sci., 1902, 592, 30th Jun. ended 'La radioactivitb serai7Th manifestation d'un changement chimiqus sous-atomique'; whether Curie saw or understood this one cannot say.
12 E.Rutherford, Phil.Mea., 1903, 5, 481-5, dated 28th Feb., Apr. issue; Pa ere, 576-9.
13 P.Curie, Sur la r eparition de la radioactivite induite par le radium eur les corps solides, CR, 1903, 136, 364-6; Oeuvres, 444-7.
14 CR, 19077 T77 673-5. 16th Mar.; P.Curie, Oeuvres, 448-51. 15 A.Laborde,-fferre Curie dans son Laboratoire, Univ. do
Paris, 1956, 5773. 16 P.Curie, 22.cit., Oeuvres, 450. 17 E.Rutherford, Papers, 607, probably written Mar.; pub.
May 1903. 18 H.Becquerel, Recherches sur une propriet6 nouvelle etc.,
pref. dated Aug.1903. 333, seems to have thought that Curie had actually adopted a slow atomic transformation theory.
19 P.Curie (and J.Danne), Sur l' emanation du radium et son coefficient de diffusion dane lair, CR, 1903, 136, 1314-6, 2nd Jun.; Oeuvres, 452-5.
20 Proo.RI, 1903, 17, 389-402. 21 Letter from Soddy to Rutherford, dated 31st Mar.1903,CUL. 22 Trans. in Chem.N., 1903, 88, 85f., in several instalments;
contains no menTion of the condensation and diffusion experiments of P.Curie on the emanation.
23 See Chapter 3, Section 4, p.169. 24 Letter from F.Giesel to P. and M.Curie, dated 23rd Mar.
1902, from Braunschweig, pp.4, BN. 25 Ibid. 26 V.Giesel, Zeit.f.Elektrochemie, 1902, 8, 579-85, pub. Aug. 27 F.Giesel, Neues fiber Radium and radioaEtive Substanzen,
Jahresb.d.Ver.f.Wiss.,BraunschweiE, (1902), 13, 43-5, 43; 30th Oct. meeting, pub.1904.
28 Ibid., 45. 29 Ti7e7k)1, ok.cit., n.26. 30 E.Rutherford, Pa ere, (1904), 706. Also, early in 1903
Giesel criticise e chemical evidence for the trans- formation of thorium: this element, he thought, owed its activity to the 'permanently' active constituent which he had extracted from pitchblende, Chem.N., 1903, 87, 97-8.
Notes for Chapter 5, p.226-280) 320
31 E.Rutherford, Loos the Radio-aotivity of Radium depend upon its Concentration?,Nature 1904, 69, 222, dated 18th Dec .1904; Id., Papers, 6.18-9.
32 Radium, Nature,-T903.677601-2, 33 F.Giesel snot© equivoCgIly on the matter, as discussed
above; E.Dorn, discoverer of radium emanation, remained silent.
34 Letter from Thomson to Rutherford, dated 13th May 1902; A.S.Eve, Ruthorford.Etc., 82.
35 J.J.Thoms7,7767 755ffEiises of the Ions etc., 1899, 558-9 refers to this result of Wilson; see C.T.R.Wilson, On the condensation nuclei produced in gases by the action of Röntgen rays, uranium rays, ultra-violet light, and other agents, Phil.Trans.A., 1899, 192, 403-53.
36 C.T.R.Wilson, P166.Roy.goc., 1902, & 277-82. 37 Phil.ym., 1902, 3,-I95:2-0-3, dated Mic.1901; presented
by Thomson. 33 Proe.Camb.Phil.Soc., 1902, 11, 504. 39 ITATDDFCTIvity PY-auced by Action of Thorium Compounds,
papers, 259. 40 T:7.Thomson, Proc.Camb.Phil.Soc., 1902, 11, 505. 41 Letter from ThomsoirTiT RutheTTTIrd, dated-7nd May 1902, (JUL. 42 Ibid. 43 17:17-iutherford, 2.E.p.frs. 378, read Jan., pub. Apr.1902. 44 Ibid., 368-9, dated Dec.1901. 45 175TTer from Thomson to Rutherford, dated 13th May 1902;
A.S.Eve, Rutherford.Etc., 82. 46 Phil4ag., 1902, 4, 35Z-671 dated Jun., Sep. issue. 47 TEIL9-353. 48 'o. d. 357. 49 See above. Chapter 4, Section 1, p.179-80. 50 Thomson, op.cit.. 360. 51 Ibid., 364--57-- 52 17Etherford, papp_KR, 455, pub. Jun.1902. 53 E.Rutherford (iiidS-;J.Allen), Excited Radioactivity and
Ionization of the Atmosphere, Phil.Maa., Dec.1902; Papers, 509-27, 513.
54 II:Rutherford (and F.Soddy), Phil.Mag., Sep., Nov., 1902. Paers, 472-508.
55 1903, 106, 289-93. 56 Ibid., 291. 57 I:FIT..., 292. 58 TBIJ., 292-3. 59 Ibid., 293; my emphasis. 60 Described in a Note read on 19th Nov., Proc.Chem.Soc.,
1902, 219-20; E.Rutherford (and F.Soddy77-Papers, 528. First achieved in Oct. when a liquid air plant was installed, A.S.Eve, E.Rutherford.Etc., 89.
61 Letter from E.Rutherford to J.J.Thomson, dated 26th Dec. 1902, CUL; also reporting the as yet unpublished magnetic and electric deviation of alpha rays; Thomson's article published shortly afterwards in Harpers, 22.eit., describes them still as X-rays.
62 Phil.Mag., 1903, 5, 419-28, Apr. issue. 63 L. 64 Letter from Thomson to Rutherford, dated 14th Apr.1903,
A.S.Eve, Rutherford.Etc., 94; followed by Thomson's paper 'On the existence of a radio-active gas in the Cambridge tapwater', Proc.Camb.Phil.Soc., 1903, 12, 172-4, read
Notes for Chapter 5, p.226-280) 321
64 contd.) 4th May; also Nature, 1903, 68, 90-1. 65 J.J.Thomson, Radium, Nature, 1903, 677 601-2, Apr. 66 See below. Section 3, p.276-9. 67 The Disintegration Theory of Radioactivity, Times Lit.
Suna., 1903, p.201, 26th Jun. 68 T.-J.:Thomson, Radium, Nature, 1903, 67, 602. J.Stark
(Nature, 1903, 68, 230, 9th Jul.) then claimed the prior expression in 192 of this idea (see above Chapter 4, Section 1, p.195-6). Thomson's early statement on uranium was not brought up. The latter (Conduction of Electricity through Gases, 552) had accepted the later atomic-expulsion version of the disintegration theory by Aug.1903, citing Rutherford and Soddy's general statement made in Phil.Mag., May 1903.
69 Letter f176E-S'6d4y to Rutherford, dated 31st Mar.1903,CUL. 70 Ibid. 71 W7Ti'ookes, The Emanations of Radium, Chem.N., 1903, 87,
157-8, 158; read to the Royal Society, 19th Mar.; the scintillation effect was noticed independently by Elster and Geitel, and Becquerel, and given different inter-pretations: respectively, release of electrons, Chem.N., 1903, 88, 37; and crystal fracture, CR, 1903, 137762.g-34.
72 Times,-75th Mar.1903, p.10,d. 73 MiTE's suggestion of a possible atomic transformation
was not mentioned, 74 In a Minor Key, Punch, 1903, 124, 214, 25th Mar. 75 Times, 28th Mar.TUUTT p.14,f.--- 76 30th Mar.1903, p.12,f; dated 26th Mar., from
30 Ledbury Rd., Notting Hill, near Crookes' address; referred to by Soddy, see above.
77 Phil.M2c., 1893, 35, 389-92. 78 YETU., 392. 79 TN5otator'. Times, 13th Apr.1903, p.6,d. 80 W.Crookes, The Mystery of Radium, Times, 14th Apr.1903,
P,5,a- 81 Id., 7th Apr..1903, p.10,b: Chem.N, 1903, 87, 184. 82 TTCrookes and J. Dewar, Chem.N., 1903, 88,-75-6, read
28th May. These two chemists also collaborated e.g. in examining the 'London Water Supply', ibid, 40.
83 W.Crookes, Certain properties of the emanations of radium, Chem.N., 1903, 87, 241, 22nd May.
84 Crookes and Dewar, On tF6 effect of extreme cold etc., loo.olt.
85 UEWm7,77, 1903, 87, 277-81, 12th Jun.; delivered to Congress of Applied Chemistry in Berlin on 5th Jun.
86 Ibid., 279. 87 W.Crookes, The Stratifications of Hydrogen, 410-3; see
the previous Section, p.220. 88 Modern Views, 2E.eit., 280. 89 Ibid., 278. 90 7057., 281. 91 The Position of Radium in the Periodic Table as indicated
by its Spectrum, Chem.N., 1903, 87, 145-6; 2102.Z., 1903, 4, 285-7.
92 W.Crookes, Modern Views, loc.cit., 278. 93 0.Lodge, Pop.Sci.Monthly, 1903758, 289-303; delivered
12th Jun. at baroxd. 94 Ibid., 294-5.
Nbtos for Chapter 5, p.226-280) 322
95 Lodge made this second point most plainly in a Note to Nature, 1903, 68, 128-9, 11th Jun.
96 E.RIITE6Word, Tapers, 596-608. 97 0.Lodge, Modern Views etc., loo.cit., 299. 98 0.Lodge, On Electron°, ElectFIFian, 1903, 51, 286. 99 Lodge, Modern Views etc77-7778766tion 2 ViiTow contains
aocounts of the related views of Lodge and Thomson on the mechanism of radioactivity during 1903-4.
100 M.W.Travers, Life of Sir William Ramsay, ch.8, 133-54. 101 Ibid., ch.7, 100f., 170T. 102 77Titimsay and P. Soddy, Nature, 1903, 68, 246, 16th Jul.
First observed 8th Jul., Travers, RE.cit., 212-5. 103 Ramsay and Soddy, Chem.N., 1903, 100-1; communicated
to Royal Society, OTE Jul. 104 Letter from P.Curie to J.Dewar, dated 22nd Jul.1903, RI. 105 P.Curie, J.de Chimie Physique, 1903, 1, 409f.; Oeuvres,
456-90, 4189. 106 Production of helium from salts of radium (not from
its emanation) confirmed by about Nov.1903 by Curie and Dewar, CR, 1904, 138, 190f., Jan.; P.Curie, Oeuvres, 491=3.
107 P,Curie, Recherches rocentes, Oeuvres, 471-2. 108 Ibid., 489. 109 TFIJ., 463. 110 137.-Caric (and J.Danne), CR, 1904, 138, 683f., 14th Mar.;
Oeuvres, 494-7. 111 -(75iii57.Med after mid-Sep.1903, pub.1904, in Oeuvres. 112 M.Curie, ibid., Oeuvres, 219-21. 113 Ibid., 239. 114 75171. r 238; the adjective refers to 'hypothbse'. 115 See above Chapter 2, Section 3, p.98; M.Curie,
Oeuvres, 334-5. 116 Chem.N., 1903, 88, 40; at Bradford on 16th Jul. 117 TEa.V., 1903, 8d, 39-40; from Times, 20th Jul. 118 Proc.Roy,Soc., 1903, 72, 196-9, received 17th Jul. 119 Letter from Soddy to Rutherford, dated 7th Aug.1903,
CUL, concerning comments by a referee on E.Baly's paper on xenon.
120 Sir W. and Lady Huggins, 22.cit., Proc.RF.Soc., 1903, 72, 198-9, addition received-5Th Aug.; discussion continues in 'Further Observations etc.', ibid., 409-13, Oct.
121 Letter from Soddy to Rutherford, dated 28th Aug.1903, CUL; the cutting is now lost.
122 Punch, 1903, 125, 139, 26th Aug. 123 17th Jul.1903, 225-6. 124 Punch, 1903, 125, 133, 26th Aug. 125 ?(he U.S.A. see L.Badaeh, Dias., 174-82, ch.'Popular-
isation for the Public, 1900-1703'. 126 Lotter from P.Curie to W.Ramsay, dated 14th Feb.1904;
Ramsay, Letters and Papers, 13, p.85a, UC. 127 Letter from F. Soddy to E.Rutherford, dated 12th Dec.
1903, CUL. 128 Letter from Rutherford to his mother, dated 10th Aug.
1904; A.S.Eve, Rutherford.Etc., 118.
Notes for Chapter 5, p.226-280) 323
129 Soddy, loc.cit., 12th Dec., thought Ramsay might claim the entire TEWory as his own; J.J.Thomson wrote in similar vein to Rutherford about Ramsay (letter dated 4th Feb,1904, CUL). Further unpleasant priority disputes developed around mid-1903 involving e.g. Becquerel; Lodge thought he should be 'rapped over the knuckles for it' (letter to Rutherford, dated 11th Dec.1903, CUL). P.Curie's comments to Dewar about Rutherford and Dorn have been noted above; so too has Marie Curie's claim. A difference also arose, to be quickly settled, between Rutherford and Soddy concerning the publication of books.
130 P.Soddy, loc.cit., letter of 12th Dec.1903. 131 Letter dated 7fq Apr.1903, CUL. 132 Electrician, 1903, 51, 210-11, 22nd May; contains the
incorrect statement that excited activity could be produced directly by the rays.
133 Ibid., 314, 12th Jun. issue. 134 "TETE1-J.D.Everett, Analogue to the Action of Radium,
Nature, 1903, 67, 535-6, 9th Apr. 135 MTUUTHerford, Radioactive Processes, Proc.Physical
112121y, 1903, 18, 595-7, abstract and discussion; id., Pa ers, 614-7.
136 Letter from F.Soddy to E.Rutherford, dated 7th Aug. 1903, CUL.
137 J.V.Eyre, Henry Edward Armstrong, London, 1958, 125. See also W. rock, 4.1.E,A7EiTiron and the Teaching of Science, 1880-1930. Cam r dge, 173.
138 Letter from Soddy to Rutherford, dated 28th Aug.1903,CUL, 139 H.E.Armetrong and T.M.Lowry, Chem.N., 1903, 88, 89-91,
21st Aug.; read to Royal SociWiT IBth Jun. 140 ibid., 91. 141 TTOddy, loc.cit., letter of 28th Aug. 142 W.Crookes, Modern Views etc., Chem.N., 1903, 87, 281;
12th Jun. issue. 143 W.Brock, Lockyer and the Chemists etc., 93, 95. 144 H.E.Armstrong, Presidential Address, Rep.Brit.Ass.,
1885, 945-64, 961; id., Osmotic Pressure and Ionic Dissociation, Nature 1896, 55, 78-9. For his alter-native IresiduTIEYrinityl view of valency see C,A.Russell, Histo.,
s of Valenc , 205-13.
145 E.g. Report on p ys c a he rit.Ass., Nature, 1900, 62, 564.
146 Proc.E2y.Soc., 1902, 70, 99-109, 102. 147 H.E.Armstrong, Chem.N, 1902, 85, 86-8, 103-6, p.86. 148 In Chem.N.; mentioned by W.H.Brock, H.E.Armstrong etc.,36. 149 This marbe related to his adoption in 1903 of the view
that weak radioactivity might in fact be due to 'a minute amount of chemical change of an ordinary character ... a sort of Russell effect', H.E.Armstrong, The Assumed Radio-activity of Ordinary Materials, Nature, 1903, 67, 414, 5th Mar.
150 Summarised in Electrician, 1903, 51, 880; not contained in Rutherford, 2222E2,
151 L.Badash, Sci.Amer., 1966, 215, 88-95, 93; no sources given. There are valuable reports of the meeting in Electrician, 1903, 51, 880-1, 892-3.
Notes for Chapter 5, p.226-280) 324
152 In 1906 Armstrong again expressed sceptical views; see F.Soddy, The recent controversy on radium, Nature, 1906, 74, 516-8, 516,
153 Letterrom F.Soddy to E.Rutherford, dated 12th Dec. 1903, GUL.
154 Electrician, 1903, 51, 800, 4th Sep.; refers to letter in The Times.
155 L.Koenigeberger, Hermann von Helmholtz, (1906), London, 1965, 438.
156 F.Richarz and R.Schenck, Sitzber.Akad.Wiss.,Berlin, 1903, 1102-6; R.Schenck, ibid., 17547 77F7; mentioned by Rutherford, Radio--activity, 1905, 441.
157 But for Kelvin's views see Section 2 below, p.266. 158 Electrician, 1903, 51, 800. 159 11765ITEletEtrioian, 1903, 51, 835, 11th Sep. 160 W.Ackroyd, Experiments and-6bservations with Radium
Compounds, Chem.N., 1903, 88, 205-6, read at Brit.Ass. Chemistry (TT-Se-6-tion, Sep.1903; id., The Source of the Energy of Radium Compounds, Nature, 1904, 69, 295, 28th Jan.; id., On the Bearing of the Colour Phenomena presented by Radium Compounds, Chem.N., 1904, 90, 157, read at Brit.Ass. Chemistry (B) Section, Sep.1754. See also, C.Winkler, Radio-activity and Matter, Chem.N., 1904, 89, 289-91, who advocated a magnetic analogy for the energy source and accepted radioactive induction; he appears as a standard 'unconvinced chemist', see p.251 above.
161 M.Berthelot, CR, 1901, 133, 973-6; id., Essais etc., ibid., 659-64:-
162 M., CR, 1904, 138, 1553-5; stresses the effects of traoes of vapours of chemical substances.
163 See e.g. F.Giesel, Emanium, Chem.N., 1904, 90, 259-60, who exhibits some confusion; and W.Marckwald, Heber das Radiotellur, Ber.deut.chem.Ges., 1905, 38, 591-4, who does not.
164 Debierne, to radium et la radio-activity, Rev.Gon.des Sol., 1904, 15, 11-22, 60-71, 69-71, adoptiThle com-promise (like M.Curie) of Ra as a catalyst for atomic transformations; then, CR, 1905, 141, 383-5, the atomic disintegration theory.
165 E.Rutherford, Magnetic and Electric Deviation etc., Pa ere, 557=
166 E,Ru herford (and F.Soddy), Thorium II, Papers, 456, read May 1902.
167 Mentioned e.g. by E.Rutherford and A.G.Grier, Deviable Rays of Radioactive Substances, ibid., 457, dated 7th May 1902.
168 E.Rutherford, Magnetic and Electric Deviation etc., Pa ers, 557; Phil.Mag., Feb.1903, dated Nov.1902.
169 o . 170 1711772.22.p7og.Chem., 1904, 1, 30-54, 30-2. 171 Tic.ilgns., London, 1904; pref: dated Oct.1902. 172 See 'Professor Mendereeff on Argon', Nature, 1895, 51,
543, for his initial reaction. 173 Chemical Conception of the Ether, 44-51. 174 Ibid., 45. 175 Tr:F.:Brush, Chem.N., 1898, 78, 197-8; Science, 1898,
8, 485-94.
Notes for Chapter 5, p.226-280) 325
176 Chem.P., 1898, 78, 221-2. 177 Verh.ply2-Ges.B-6-ilin, 1898, 17, 135-7. 178 Opening address by President of Chemistry (B) Section
of Brit-Ass., Nature, 1903, 58, 472-81. 179 Ibid., 479. 180 Te-W-tAe preceding Section, p.246; M.Curie, Thesis, 2 ed.,
1904; id., Oeuvres, 239. 181 Notices of Books, Chem.N., 1904, 90, 326. 182 P.P.Bedson, op-cit., Anii.liep.prog:Uhem., 1904, 1, 32. 183 Chemical Conception o1 the Ethii; 6; D.MendeleeT, An
attempt to apply to the tryone of the principles of Newton's natural philosophy, RI Lib.Sci., (1889), 3, 540-59.
184 D.Mendeleef, The periodic law of the chemioal elements, J.Chem.Soc., 1889, 55, 634-56, 641-7.
185 Td77-MIWiacal Conce15Tion of the Ether, 14. 186 Tad-, 44-5, 47. 187 :e vinretained that notion at this time, Papers, (1905)
6, 223. 188 W.Ostwald, J.Chem.Soc., 1904, 85, 506-22; reprinted in
D.M.Knight, Classical Scientific Papers.Chemistry, London, 1968, 354-70.
189 Ibid., 356-7. 190 Ibid., 369. 191 -SW-e—e.g. S.Glaestone, Sourcebook on Atomic Ener .
London, 1950, 357-61, on liquid-drop models of nuclear fission in the 1930's.
192 He later appreciated the success of the nuclear atom, Ann.Rep.Prog.Chem., 1913, 10, 262-88, 271-2.
193 Sr(79 above, Chapter4, SectrOn 2, p.198-9. 194 In 19 parts in Electrician, 1903-4, 52, 7-10 etc.,
pub. Oct.1903 to Feb.190/f. 195 Ibid., 163. 196 1570-6-ddy, Radio-activity: an Elementary Treatise from
the Stand,oint of the Didategration Theory, London, 774, pref. datWU 5TE May.
197 Ibid., 55. 198 fbid., 164, 199 U7E.Vincent, Phil.Mag., 1902, 4, 103-15. 200 Soon to be moderated ( F.Soddy, Radioactivity, Ann.Rep.
pERg.Chem., 1904, 1, 244-80, 276) by one who a decade a or paced atomic disintegrations within the Periodic
Table by means of the Displacement Law. 201 F.Soddy, Radio-activtT, 178. 202 Pa ere, 73; see thiT011owing Section, p.272-4. 203 ..Soddy, Radio-activity ; 125, 176-8, citing only Lodge;
see belowTT5726I-.37f61; this physicist's suggestions. 204 Ibid., 178. 205 Ibid., 178-80. 206 fbid., 125. 207 "A-i-J,Walker on 'Time Conception of Minute Concentrations'
in 'General and Physical Chemistry', Ann.Rep.Prog.Chem., 1904, 1, 1-29, 25-6. Also C.A.Russell on 'The Oscillation Theory' in History of Valens , 254-6.
208 W.H.Bragg, On the Absorption ofc(Rays, and on the Class-ification of theotRays from Radium, Phil-Mag.1904, 8, 719-25; id. and R.Kleeman, On the Iorirgation Curves of RadiuiliT ibid., 726-38.
Notes for Chapter 5, p.226-280) 326
209 Letter from E, Soddy to W.H.Bragg in Adelaide, dated 12th Jan.1905, RI.
210 F. Soddy, Radio-aotivitz, 1904, 178. 211 See e.g. the FtiView 'by J.A.Fleming, The electronic
theory of electricity, RI Lib.Sci., (1902), 5, 551-69. 212 E.von Sohweldler, Dmt.-riit.poluTET•21-ude de la
Radiologie etc., 1905,- I, dated Jun.175. 213 E.g., L.BoliFiann, Lectures on Gas Theo , (1896, 1898). 214 E.Rutherford, Papers, (1908), 2710 , provides refer-
ences for 1905731-T5th electrical and scintillation methods were employed by him.
215 0.Lodge, On Electrons, Electrician, 1903, 51, 123-5, 125. 216 Nature, 1903, 67, 511, dated 28th Mar. 217 J,J.Thomson, Radium, Nature, 1903, 67, 601-2. 218 Ibid., 601. 219 0.Lodge, Nature, 1903, 68, 128-9, 11th Jun. 220 Reported in Electrician, 1903, 51, 417-9; see also
E.Rutherford, Pa e737616. 221 J.Larmor, Aether anti Matter, 227-32; 0.Lodge, On
Electrons, Electrician, 1903, 51, 286. 222 Lodge, On EITEIi5iiii,Toc.cit.,-125. 223 Lodge, Note on the pro-E11517occasional instability
of all matter, 128-9. 224 Ibid. 225 .I Jeans, Phil.Mag., 1901, 2, 421-55. 226 J.Heilbron,-15170., 137. 227 Phil.Mr3z., 1777 7, 445-55, paper read 5th Dec.1903
in Tokyo. 228 Ibid., 455. 229 Phil.Meo., 1904, 7, 237-65; see below, p.268-70. 230 IT5.--ft.58, Electrician, 1904, 52, 805; see also abstract,
ibid., 823. 231 laa., 805. 232 aaro-activity, 1905, 488; unchanged comment from 1904
ed.; see the following Section, p.272f. 233 Kelvin, Papers, 6, 206-9. 234 See e.g. Editorial note, 'Explanations of Radio-
activity', Electrician, 1903, 51, 892-3. 235 Perhaps referring io A.HeydweiTler, Phys.Z., 1902, 4,
81-2, and/or R.Geigel, Ueber Absorption von Gravitationsenergie lurch radioactive Substanz, Ann.d.Phys., 1903, 10, 429-35. But e.g. C.Forch, lEY:27Z., 1903, 4, 315.-9, 443-5, citing W.Kaufmann, Ann.d7Phy.z., 1903, 10, 894, had by autumn 1903 published experimen al refutations of the apparent weight-loss.
236 Kelvin, Pa ers, 6, 208. 237 Letter da e 23rd Aug.1903, RI. 238 Letter dated 22nd Aug.1903, in M.W.Travers, A Life of
Sir William Ramsay, 252. Travers here states that at a dinner which he attended in June 1903 Marie Curie attempted to 'convert' Kelvin to the disintegration theory. This however was before the announcement of the radium-helium transmutation in July. Also, a postcard from Soddy to Rutherford dated 22nd Jun.1903 (GuL) rep-orting Ramsay's visit to Marie Curie in Paris reads 'According to R., Curie thinks we are very "hardi" to put forward our hypothesis on such slight evidence. R. replied he thought there was a good deal of evidence'. Marie Curie may thus not have been a convert by that date.
Notes for Chapter 5, p.226-280) 327
239 At the Brit.Ass. 1903, Kelvin, papers, 6, 208-9. 240 Rutherford.Etc., 109; see also WI07on-differences
regarding geological-mineralogical time scales. 241 Pa err, 6, 216-22; Phil.Nas., 1904, 8, 528-34, Oct.
- scUee 242 Ibid., 216. 243 Re Vins Phil.Mag., 1902, 3, 257-83, written in 1901. 244 C.f. L.BaTimann, Lectures on Gas Theory, 3-4, 376-9,
who postulated overlapping moms to explain valency; mentioned above, Chapter 4, n.50.
245 Kelvin, Aepinus Atomized, loc.cit., 259; similar forces he conceived to act between atoms and the ether, Papers, 6, 237.
246 Ibid., 259. 247 Ibid.., 262. 248 RTIVin, Plan of a Combination etc., Papers, 6, 216. 249 Ibid. 250 TiiIVin v Pa ere, (1905), 6, 227-30. 251 Summarise by F.Soddy, THe recent controversy on radium,
Nature, 1906, 74, 516-8. Soo Rayleigh, J.J,Thomson, 141-2 for KelvIri's criticisms in 1906 of Thomson's radiation-loss disintegration theory.
252 An attempt to explain the Radioactivity of Radium, Papers, (1907), 6, 231-4.
253 kelvin, On the Motions of Ether etc., Papers, (1907), 6, 235-43, 235-6,
254 54R. Nagaoka, Kinetics of a system etc., 445. 255 Id., 454. 256 G.A.Schott, A dynamical System etc., Nature, 1904, 69,
437, from University College of Wales. J.Heilbron, Dies., 142-6, discusses the arguments of Schott and Nagaoka; those related largely to the above points but not to radioactivity,
257 H. Nagaoka, Kinetics of a system etc., 454. 258 Ibid., 454-5. 259 i.e. ultra-violet photoelectric action, the emission
of electrons from an irradiated metal. 260 Phil.Mag., 1903, 6, 673-93. 261 Y.hia., 673. 262 Rayleigh, J.j,Thomson, 140, letter dated 11th Apr.1904. 263 J.j.Thomson, Magri 611c Properties etc., 678-81. 264 E.g. J.Larmor, On the Theory of the Magnetic Influence
on Spectra; and on the Radiation from moving Ions, Phil.Mag., 1897, 44, 503-12, 512, had done so despite losses of the order 10-6.
265 Thomson, Magnetic Properties etc., 682-5. 266 Ibid., 689. 267 TEIT.Mag., 1904, 7, 237-65, Mar. issue. 268 Ibid.7-255. 269 MU., 265. 270 T517. 271 Letter from Thomson to Rutherford, dated 18th Feb.1904,
CUL, describing the above ideas shortly before their publication.
272 W.Kaufmann's results of 1901-2 were reported e.g. by E.Rutherford, Radio-activity, 1905, 127; 1904, section 76; by7„,7Trtrutt, The Becquerel Rays and the Praperties of Radium, London, 1904, 69; and by
Notes for Chapter 5, p.226-280) 328
272 contd) Thomson himself, Conduction of Electricity throuEh Gases, 1903, 532-5.
273 17.757he0Fia-T,Radio-octivit 1904, 325, section 200. 274 Ibid„ 333, se-aion 'u3. 275 Ibid., 326, section 200. 276 YETU., 305, section 194. The point was noted by Soddy
and Rutherford (Papers, 564, 599) without great emphasis early in 1903, and had possibly been anticipated by mid-1902 (121=2, 508).
277 Phil.Trans.A., 1904, 204, 169-219, ms. received 20th Aug.;-Pa-b-rs, 671-722.
278 ibid., 1'. 279 ibid. 280 TM., 712-3. 281 E.Rutherford, Radio-activity, 1905, 450. 282 Ibid., 456, 283 The apparent effect of temperature on the decay of
RaC was the only exception; E.Rutherford, Bakerian Lecture, Papers, 713; Radio-activity, 1905, 390-1.
284 Ibid., 1903, 487-8; 1904, section 206. 285 2. Rutherford, Radioactive Transformations, (London,
1906), repr. Yale U.P., 1919; lectures delivered Mar.1905. pref. dated Jun.1906.
286 Ibid., 267. 287 17697., 268; there is no reference to Soddy. See
Rutherford, Radio-activit , 1905, 446, for his own previous discussion regar ing the 'average life' of 'metabolons'.
288 E.Rutherford, Radioactive Transformations, 268. 289 Rutherford considered statistical aspects of radio-
activity by 1908 (Ta.p_sE2, 2, 58, 69, 94, 106-8). However, in 1909 he reported results which made it seem 'probable that the atoms of emanation undergo a progressive change in properties before disintegration' (ibid., 168-9); some months later Thomson defended the theory of continuous atomic change by attributing a suitable distribution to the atoms themselves; these he supposed were of differing intrinsic strength when first formed (Rayleigh, J.J.Thomson, 142). But in 1910 Rutherford rejected the results on progressive change in the emanation (Pa ens, 2, 214-20) and intensified his statistical stud. es; e.g. the number of alphas he detected per minute fluctuated wildly and randomly between zero and twenty, in accord with probability laws. But regarding the cause of instability and dis-integration, whether in nuclear or electronic (1912; ibid., 286-7) or quantum terms (1927: Papers, 3, 178-9, 183), the physicist still admitted ignorance.
290 E.Rutherford, Phil.Mag., 1907, 13, 110-17, dated 1st Nov.1906; Papers, 910-16,91-6.
291 Ibid., 916. 292 E.Rutherford, Radioactive Transformations, 276. 293 Radioactive Change, Papers, 596-608. 294 Ibid., 608, See Chapter 4, Section 1, p.195, for
77.7fark's similar suggestion in mid-1902. 295 1903, 68, 222, 9th Jul.
Notes for Chapter 5, p.226-280) 329
296 G.H.Darviin, ibid., 496. 297 J.Joly, ibid, 7526. Rutherford, Pa ere, (1907), 926,
later chid to have made calculations in 1902, un-published, on the gao-thermal effects of active mineraln.
298 The problem that radioactive rays from the sun should be detectable on earth was raised by W.B.Hardy, 'Radium and the Cosmical Time Scale', ibid., 548, and disposed of by R.J.Strutt, 'Radium and the Sun's Heat', ibid., 572, 15th Oct. The lack of radium lines in the TaTir spectrum was partly eased by the abundance of helium, see e.g. E.Rutherford, Radio-activity, 1905, 492; 1904, section 207.
299 Radio-activit , 1905, 491-6; 1904, section 205. 300 !bid., 1 5, 459-66, and references there cited. 301 17,757therford, Pa era, (1907), 917-31, 930; earlier
discussions, cting Ramsay and Soddy, appear in Rutherford, Radio-activit , 1905, 485-6, 554-8; also • Pa ere, 774-5, bop. n 1907 Rutherford left McGill to - ace A.Schuster's Chair at Manchester.
302 A.Sohuster, Rep.Brit.Aps., 1903, 538. 303 E.g. G.Le Bon, P,de Heen, G.Martin, D.Mendoleef,
H. Wilde. During 1903-4 some connection was also seen, e.g. by Jean Becquerel, between radioactivity and the N-rays, which Blondlot imagined to be emitted by various materials.
304 P.Curie, Oeuvres, 378; Congr6s, 1900. 305 Radio-active Substances, Chem.N., 1903, 88, 99. 306 R.J.Strutt, Nature, 1903,-7,769-70, 19T Feb. See
also J.J.Thomson, ibid., 391, who cited McLennan and Burton; and E.Ruther5ird, ibid., 511-2, 2nd Apr. citing Rutherford and H.L.Cooke.
307 Noted by Rutherford, ibid., also in Radio-activity, 1904, section 220.
308 E.Rutherford, Radio-activity, 1905, 487; 1904, section 206.
309 J.J.Thomson, Proc.Camb.Phil.Soc. 7 1904, 12, 391-7. 310 Ibid., 397. ---- 311 TIETI.MaE., Mar.1904 issue; see the preceding Section,
p.270. 312 N.R.Campbell, Phil.Mafc., 1905, 9, 531-44; id., 545-9;
id., 1906, 11, 202-2 . See also J.J.Thomson, On the emission of negative corpuscles by the alkali metals, Phil.Ma., 1905, 10, 584-90.
313 E.Rutherford, Papers, 708, 775; Radio-activity, 1904, section 220.
314 Rutherford, Radio-activity, 1905, 539-42, section 286. 315 Rutherford, Radioactive Transformations, 217-8. 316 Summarised bYTTETIT6Word, Radio-activitq., 1905, 552-3. 317 See e.g. J.C.McLennan, On the Radio-activity of
Potassium and other Alkali Metals, Nature, 1908, 78, 29-30; N.R.Campbell's defence, ibid., 55; then E.Rutherford, Radioactive Substances and their Radia-tions, Cambridge, 1913, 58U1:9, 596.
Notes for Chapter 5, p.226-280) 330
318 E.Rutherford, Radio-aotivkIE, 1905, 483; 1904, section 201.
319 Ibid. 320 L. Rutherford, Baker-Lan Lecture, Pa ers, (1904), 712. 321 W.H.Bragg, Phil,Mag., 1904, 8, 7 -25,, 719-21, cited
Rutherford's Bakerian 322 Letter from Bragg to Rutherford, dated 18th Dec.1904,CUL. 323 E.Rutherford, Radio-activity, 1905, 484, this edn. only. 324 Ibid., my emphases. 325 lbid., 1905, 77; again, my stress. 326 Men- ioned obliquely, ibid., 78. 327 J.J.Thomson, Phi1.n.A777906, 11, 769-81; discussed
e.g. by 0.Lodge, Electrons etc., 1906, 146-51, 162, 192-4,
328 J.J.Thomson, Cathode Rays, 1897, 312; he estimated its rate of production at one three-millionth gm. per year.
329 Thomson, RI Lib.Sci„ (1907), 6, 232-47. 330 The general notion projected into the future. Rutherford
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331 0.Lodge, Radio-Activity, Electrician, 1904, 53, 216-8.
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364 ABBREVIATIONS
Abh.der Naturf.Ges.zu Halle Abhaiialungen der naturforschenden Gesellschaft zu Halle.
Amer.J.Phys. American Journal of Physics.
Ann.d.Phys. Annalen der Physik (und Chemie), Leipzig. Also known as Wied.Ann., Wiedemann's Annalen. Beibl.= BeiblUtter.
Ann.1122.212E.Chem. Annual Reports on the Progress of Chemistry, Chemical Society of London.
Ann. Sci. Annals of Science.
Arch.Hist.Exact Sci. AFFave for History of Exact Sciences.
Ber.deut.chem.Ges. —Fgrichte der deutschen chemischen Gesellschaft, Berlin.
BN Bibliotheque Nationale, Paris.
Brit.J.Hist.Sci. British Journal for the History of Science.
Bull.Amer.Phys.Soc. Bulletin of the American Physical Society.
Chem. N. The Chemical News.
CR Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, Paris.
CUL Cambridge University Library.
Edin.Roy.Soc.Proc. Proceedings of the Royal Society of Edinburgh.
Hist.Stud.Phys.Sci. Tiritorical Studies in the Physical Sciences.
J.Amer.Chem.Soc. Journal of the American Chemical Society.
365
J.Chem.Soc. Journal/Transactions of the Chemical Society of London.
J.Franklin Inst. Journal of the Franklin Institute.
Jahresb.d.Ver.f.Wiss.,Braunschweig riE7isbericht des Vereins fur Naturwissenschaft zu Braunschweig.
Jap.Stud.Hist.Sci. Japanese Studies in the History of Science.
Manchester Memoirs Memoirs of the Manchester Literary and Philosophical Society.
Nature Nature, London.
Phil.Mag. London, Edinburgh and Dublin Philosophical Magazine, and Journal of Science.
Phil.Trans.A.
Phys.Z.
Pop.Sci.Monthly
Philosophical Transactions of the London Royal Society, series A.
Physikalische Zeitschrift, Leipzig.
Popular Science Monthly, New York.
Proc.Camb.Phil.Soc. Proceedings of the Cambridge Philosophical Society.
Proc.Chem.Soc. Proceedings of the London.
Chemical Society of
Proc.RI
Proceedings of the Great Britain.
Royal Institution of
Proc.Roy.Soc. Proceedings of the London Royal Society.
Rapports,Cong.Int.de Physique Rapports presenters au Congres international de Physique, Paris.
Rep.Brit.Ass. Report of the British Association for the Advancement of Science.
366
Rep. Smithsonian In3t. Annual Report of the Smithsonian Institution.
Rev.Gen.des Sci. Revue Generale des Sciences Pures et Appliquees.
Rev. Sc.
RI
RI Lib.Sci.
Revue Scientifique, also Revue Rose.
Royal Institution of Great Britain.
The Royal Institution Library of Science. Physical Sciences, 10 vols., ed. W.L.Bragg and G.Porter, London, 1970. Mainly reprinted from Proceedings of the Royal Institution, 1851-1939.
Sci.Amer. Scientific American.
Seances Soc.Fr.P s. ociete Franpaise de Physique. Seances,
1873-1901.
Sitzber.Akad.Wiss.,Berlin Sitzungsberichte der preussischen Akademie der Wissenschaften, Berlin.
Trans.Camb.Phil.Soc. Transactions of the Cambridge Philosophical Society.
Trans.Roy.Soc.Canada Transactions of the Royal Society of Canada.
UC University College, London.
Verh.Deut.Ges.Natf. Verhandlungen der Gesellschaft Deutscher Naturforscher and Aertze, Leipzig.
Verh.Deut.Phys.Ges. Verhandlungen der Deutschen Physikalischen Gesellschaft, 1899f.
Ver.Phys.Ges.Berlin Verhandlungen der Physikalischen Gesellschaft in Berlin, 1882-98.
Zeit.f.Elektrochemie Zeitschrift fur Elektrochemie, Halle.