modern theory of physical phenomena, radio-activity, ions, electrons_a_righi

8/13/2019 Modern Theory of Physical Phenomena, Radio-Activity, Ions, Electrons_A_Righi

1/196


2/196


3/196


4/196


5/196


6/196


7/196

MODERN THEORYOF

PHYSICAL PHENOMENA


8/196


9/196

MODERN THEORYOF

PHYSICAL PHENOMENARADIO-ACTIVITY, IONS, ELECTRONS

BYAUGUSTO RIGHI

PROFESSOR OF PHYSICS IN THE UNIVERSITY OF BOLOGNA

AUTHORIZED TRANSLATIONBY

AUGUSTUS TROWBRIDGEPROFESSOR OF MATHEMATICAL PHYSICS IN THE

UNIVERSITY OF WISCONSIN

THE MACMILLAN COMPANYLONDON : MACMILLAN & CO., LTD.

1904All rights reserved


10/196

COPYRIGHT, 1904,BY THE MACMILLAN COMPANY.

St up and electrotyped. Published December, 1904.

NortoooBJ. 8. Cashing & Co. Berwick & Smith Co.

Norwood, Mass., U.S.A.


11/196

PREFACEIT is with a certain trepidation that I

have entertained the proposal of an Englishtranslation of an entirely unpretentious book,which is, in fact, only an extension of achapter added to Telegrafia senza Filo (Telegraphy without Wires).The fact that this book has gone throughtwo Italian editions in a very short timecertainly shows that my work has not beenuseless in my own country.- But to issue anedition in the language of those illustriousmen to whom, above all, we owe the ad-mirable theory which forms the subject ofthis little book is quite a different thing.

That which has weighed with me in mydecision was the thought that this Englishtranslation might at least serve to show withwhat great favour the development of theideas concerning the first cause of physical


12/196

VI PREFACE

phenomena has been received in Italy, andalso to show how fully the works of thosephilosophers to whom this development isdue are appreciated in this country.

AUGUSTO RIGHI.BOLOGNA, ITALY,

August, 1904.


13/196

TRANSLATOR'S PREFACEFOR more than twenty-five years Profes-

sor Righi has been an indefatigable investi-gator and constant contributor to electricalscience, and has attained a foremost rankamong Italian scientists. While he is notone of the relatively small group of investi-gators to whom we owe the extremely im-portant theory outlined in this book, he is,nevertheless, admirably qualified to discussand explain the theory, both by reason ofhis deep insight into electrical phenomenaand because of his ability to explain intri-cate physical processes without the aid ofmathematical formulas.As Professor Righi states in his prefaceto the first edition, the Italian original was

written more with the object of interestingthe greatest possible number of readers inthis new and important branch of physics


14/196

viii TRANSLATOR'S PREFACEthan as a book of reference for physicists.With this end in view the subject-matterwas presented in an elementary form, andthe book met with a very marked success.

I have made this translation believing thatthere are at least as many English as Italianreaders to whom an elementary treatment ofthe electron theory as it stands at presentwill be acceptable.

Professor Righi has read the proofs of thetranslation, thus insuring its accuracy, andhas kindly provided me with a special prefacein English.

AUGUSTUS TROWBRIDGE.MADISON, WISCONSIN,

September, 1904.


15/196

CONTENTSFAGB

INTRODUCTION xiCHAPTER

I. ELECTROLYTIC IONS AND ELECTRONS . . iII. THE ELECTRONS AND THE PHENOMENA OF

LIGHT nIII. NATURE OF THE CATHODE RAYS ... 28IV. THE IONS IN GASES AND IN SOLIDS . . 40V. RADIO-ACTIVITY 54VI. MASS, VELOCITY, AND ELECTRIC CHARGE OF

THE IONS AND OF THE ELECTRONS . . IO8VII. THE ELECTRONS AND THE CONSTITUTION OF

MATTER 141BIBLIOGRAPHY . 153

ix


16/196


17/196

INTRODUCTIONA NEW and interesting branch of sciencehas been formed, partly as a result of the

numerous recent experimental researches onthe electric discharge, and partly as a conse-quence of the discovery of radio-activity andnew phenomena in magneto optics. At thesame time these researches have caused atheory to spring up which harmonizes allthe facts, and which has profoundly modifiedthe dominant ideas concerning the imme-diate cause of electrical phenomena, and ofphysical phenomena in general.When the old hypothesis of the electricfluid was abandoned, chiefly because of thedisinclination to admit action at a dis-tance, it seemed for a time as though Fara-day's ideas, formulated later by Maxwell,must lead to a new concept regarding thecause of electrical phenomena, since, accord-ing to Faraday, the ether, and not the so-


18/196

xil INTRODUCTIONcalled electrified bodies, is the seat of thephenomena. But the impossibility of find-ing a satisfactory mechanical representationof the supposed elastic deformations of theether, to which were attributed in Maxwell'stheory the apparent forces at a distance, andthe necessity of admitting at any rate theexistence of an entity distinct from the etherand from matter, made it soon apparent that,even under the new order of ideas, theconception of the nature of electricity stillremained obscure.At present a new evolution is being ac-

complished, since, without knowing anythingmore about the first cause, an atomic struc-ture is attributed to electricity. This newconception, suggested by the studies men-tioned farther on, already shows promise ofbecoming as fruitful as the analogous onewhich has long been admitted regarding theconstitution of matter, inasmuch as it per-mits us to place in reciprocal relation, ofteneven quantitative, phenomena which seemutterly different and independent of eachother.


19/196

INTRODUCTION Xlll

What the electrons or electric atoms reallyare remains a mystery ; but in spite of thisthe new theory may perhaps acquire not alittle importance in the future even fromthe philosophic point of view, since it pointsout a new mode of considering the structureof ponderable matter and tends to bring backto a single origin all the phenomena of thephysical world.

It is indeed true that, with the modernpositivist and utilitarian tendencies, manypeople do not appreciate this advantage, andprefer to consider a theory principally as aconvenient means to arrange and coordinatefacts, or as a guide for the investigation ofnew phenomena. But if hitherto men haveconfided too much in the power of humaningenuity and have too easily believed them-selves to be on the point of discovering theultimate cause of things, we perhaps fall intothe contrary excess.

In this book I shall set forth the principalfacts which have led to the electron theory,and I shall endeavour to make this theoryclear at least along its general lines.


20/196


21/196

THE MODERN THEORY OFPHYSICAL PHENOMENA

CHAPTER IELECTROLYTIC IONS AND ELECTRONS 1

THE hypothesis of electrolytic dissociationis generally admitted in order to explainelectrolysis in accord with the well-knownlaws of Faraday which this phenomenonobeys. Each molecule of an electrolyte maybreak up into two ions; that is, into twoatoms or atomic groups having equal chargesof opposite sign. Thus when a solid, suchas chloride of sodium, or common salt, isdissolved in water, some of its moleculesundergo dissociation; that is, these mole-cules cease to exist as such, and their ionsbecome separated and free. Owing to the

1 The numbers in parentheses inserted in the text refer tothe bibliography at the end of the book.


22/196

2 ELECTROLYTIC IONS AND ELECTRONSinvisible molecular and atomic motions,whose energy constitutes the heat containedin a body, these ions wander through theliquid, without, however, any given directionof motion predominating. In the resultingmutual collisions it happens at one instantthat a molecule breaks up into ions, atanother instant that individual ions recom-bine into molecules. There are, so to speak,incessant unions and separations, in spite ofwhich the number of dissociated moleculesremains sensibly independent of the time.When two electrodes connected with thepoles of a battery are immersed in the solu-tion, the ions of the two kinds for ex-ample, the positive ions of sodium and thenegative ions of chlorine no longer wanderat haphazard in any direction ; but, obeyingthe electric force, the first approach the nega-tive electrode, or cathode, the second thepositive electrode, or anode. On arrivingat the electrodes, the ions give up theircharges and become neutral atoms, whichremain free, at least if no special chemicalaction takes place between these atoms and


23/196

ELECTROLYTIC IONS AND ELECTRONS 3the surrounding bodies (this, by the way, isprecisely what would happen in the case ofthe sodium). The electric current in theliquid consists in this transport of electricitybrought about by the ions.

Electrolysis obeys the two laws whichwere enunciated by Faraday. The first ofthese laws, which asserts the proportionalityexisting between the quantity of electricitytraversing the liquid and the quantity ofmatter deposited on the electrodes, is em-bodied in the statement that all the ionsexisting in the liquid possess charges equalin absolute value. Thus, in the case ofchloride of sodium, the ions of the metalhave positive charges all equal among them-selves, and equal, but of opposite sign, to thecharge of any one of the ions of chlorine.

In order to satisfy Faraday's second law,according to which a proportionality existsbetween the relative chemical equivalents ofdifferent electrolytes and the amount of eachdecomposed when the same quantity of elec-tricity is transmitted through them all, aswould be the case when all were placed in


24/196

4 ELECTROLYTIC IONS AND ELECTRONSseries in a single circuit, it is necessary toadmit that all univalent atoms possess acharge equal in absolute value to that of thesodium or of the chlorine ion : that all atomswhich possess twice this charge behave asbivalent, etc. The following example willmake this point clear. If a current is passedthrough a solution of the chloride of copperthe molecules of which contain two atoms ofcopper (univalent) and two of chlorine, andalso through a solution of the other chloride,the molecules of which contain one atom ofcopper (bivalent) and two of chlorine, therewill collect on the cathode of the first solu-tion a quantity of copper double that on thesecond cathode, although, naturally, the quan-tity of positive electricity transported throughthe two liquids will have been the same.

In 1 88 1 the illustrious Helmholtz pointedout that the laws of electrolysis suggest theidea that the electric charge pertaining toany valency of an ion may be a fixed quan-tity having a separate existence ; and, sincea material atom is a fixed and determinateportion of a certain kind of matter and is


25/196

ELECTROLYTIC IONS AND ELECTRONS 5considered indivisible, it is thus natural toconsider this electric charge as fixed and in-divisible, all the more because a quantity ofelectricity smaller than this is never encoun-tered. The charge of the ion (univalent)may therefore be called an atom of electric-ity, or better, as Mr. Stoney has proposed,an electron (electric ion).

In fact, as early as 1871, considerably inadvance of Helmholtz, Weber conceived theidea of the atomic structure of electricity.This famous physicist and mathematicianproposed the well-known theory, accordingto which electric phenomena are due to par-ticles or atoms of positive and negative elec-tricity acting

on each other at a distance,with forces depending, not only upon thedistance itself, but also on the velocity of theparticles and on their accelerations ; that is,on the manner in which these velocities vary.Naturally this theory in which action at adistance was still admitted has nothing incommon with that now in favour except thefundamental concept of the electric atom ;and although Weber, searching in his theory


26/196

6 ELECTROLYTIC IONS AND ELECTRONSfor the cause of the forces which govern theatomic structure of bodies, advanced thehypothesis that to every ponderable atomthere is united an electric atom (i), the re-lation now admitted between ions and elec-trons seems to have been better perceivedby Helmholtz.We do not believe that the atomic hy-pothesis concerning the nature of electricityimposes on us the necessity of considering itas matter, since we are still free to supposethan an electron may be simply a speciallocalized condition of the universal ether.We may, on the contrary, from now on addthat instead of considering electricity asmatter, we are led to the exactly oppositehypothesis that the atoms of various bodiesare systems of electrons.When the ions arrive on the electrodesand become neutral atoms, the electrons en-ter into the circuit to constitute the electriccurrent. Now it seems natural to supposethat these electrons, instead of merging, soto speak, into a homogeneous whole (theold electric fluid), preserve their individual-


27/196

ELECTROLYTIC IONS AND ELECTRONS 7ity; this is all the more natural because, ifthey are to pass from one atom to another,it is most probable that they must exist mo-mentarily isolated ; thus the electric currentin conductors would be nothing else than amotion of free electrons across interatomicspace. It remains undetermined whetherthe current consists in the motion of posi-tive electrons in one direction and negativein the opposite direction, or in the motion ina given direction of one of the two kinds ofelectrons, say the negative; but preferenceis given to the latter opinion, because, whilethere is reason to hold that the negativeelectrons may exist in a free state, this is nottrue of the positive electrons. Only theformer, as it appears, suffer displacement,separate themselves from ponderable matteror unite with one another, and vibrate inlight sources, as we shall soon see. There-fore, while a negative ion in being depositedon the anode gives up its electron, a positiveion arriving at the cathode does not give upthe positive electron, but takes away a nega-tive electron from the cathode itself.


28/196

8 ELECTROLYTIC IONS AND ELECTRONSHere, then, is the old theory of the electric

fluid in a certain sense called back to life,but profoundly modified. It is no longera question of a continuous fluid, but ofspecial atoms (the electrons), which, however,as has already been observed, are not neces-sarily to be considered as material in theordinary sense of the word.

Besides, and this is of more importance,we do not attribute to the atoms of electricitythat mysterious faculty of acting at a dis-tance, with which the old fluid was supposedto be endowed, but instead we suppose thatthe reciprocal forces between the electronshave their origin in the special elasticdeformations of the ether, identical withthose called for in Maxwell's theory to takeaccount of the electric forces betweenconductors.To explain the phenomena of electrolysis

it is sufficient to admit, as is always done,the hypothesis of electrolytic dissociation ;but this hypothesis is not well adapted toexplain the propagation of electricity ingases and certain other phenomena. How-


29/196

ELECTROLYTIC IONS AND ELECTRONS 9ever, with the admission of electric dissocia-tion, that is, the separation of the negativeelectrons from the neutral atoms, we assigna reason both for electrolysis and for theseother phenomena as well.

In order that a negative electron mayseparate itself from a neutral atom, energymust be expended to overcome the attrac-tion by which the electron is held to thepositive ion, which is what remains of theatom when the negative electron is takenfrom it, precisely as it is necessary to fur-nish heat energy to separate the moleculesof a liquid from one another in evaporation,or as it is necessary to do mechanical workin lifting a weight from the earth.The energy necessary to ionize or dissoci-ate an atom naturally varies according to itschemical nature. Experiment indicates thatthis energy is a minimum for the so-calledelectropositive bodies, such as the metals,and gradually becomes greater as we pro-ceed toward the more electronegative bodies,which, moreover, may even take on newnegative electrons. This energy depends


30/196

10 ELECTROLYTIC IONS AND ELECTRONSalso upon the nature and on the conditionof the atoms surrounding the one whichis about to break up into an electron anda positive ion; it is extremely small forbodies in aqueous solution.

This being so, electrolytic dissociation, orthe separation of a molecule into two ions,for example, sodium chloride into a positiveion of sodium and a negative ion of chlorine,

should be considered to be a consequenceof the dissociation of the metallic atom.This atom breaks up into a positive ion ofsodium and into a negative electron, whichis seized by the chlorine atom, transformingthe latter into a negative ion. Once weadopt this mode of considering electrolyticdissociation, it, with all its very importantconsequences, enters into the more generalelectron theory.


31/196

CHAPTER IITHE ELECTRONS AND THE PHENOMENAOF LIGHTWHILE the hypothesis of the electrons

springs in such a natural manner from elec-trolytic phenomena, it is in an entirely differ-ent field of physics, namely, that of optics,that it finds an unexpected and brilliant con-firmation.

It is a fact, now recognized by all, thatlight is a vibratory phenomenon, and may nolonger be considered to be due to the emis-sion of discrete corpuscles by luminousbodies, as Newton supposed. In support ofthis many beautiful and classic experimentsexist, with which the names of Young, ofFresnel, and of Foucault are connected.And when we speak of light, we necessarilyinclude radiant heat, because since the cele-brated researches of Melloni there can existno doubt concerning the identity of the


32/196

12 THE PHENOMENA OF LIGHTnature of these phenomena, which appear tobe so different.

But the undulatory theory demands amedium capable of propagating the waves;from this arises the necessity of admittingthe existence of the ether ; that is, of a sub-stance distributed everywhere throughoutinterplanetary and interstellar space andthroughout interatomic space as well. Thehypothesis of the ether forces itself on us inan irresistible manner, and almost seems toacquire the character of reality and certitude,when we consider the perfection with whichthe undulatory hypothesis takes account ofall optical phenomena even quantitativelyand in the minutest detail.

Following the example of Fresnel, lightvibrations were considered for a long whileto be true mechanical vibrations of theethereal and material particles, but later itwas recognized, especially in consequence ofthe work of Maxwell, that light waves couldbe considered as electromagnetic waves ;thus two distinct classes of physical phe-nomena were united. The electromagnetic


33/196


34/196

14 THE PHENOMENA OF LIGHTvibrations, and if both the electric and mag-netic forces generated by their motion betaken into account, one arrives at an electro-magnetic theory of light capable of explain-ing even those phenomena which eluded thetheory based simply on the formulae ofMaxwell or of Hertz.

Let us here consider a most interestingphenomenon discovered by Zeeman, a formerpupil of Lorentz, because it is one of thoseby which the relative independence of thenegative electrons and their characteristicfreedom of motion is most clearly demon-strated.

It is a well-known fact that a luminousgas

emits radiations of definite periods ofvibration, and not those corresponding to acontinuous series of intermediate periods.As a result the spectrum of the light emittedby the gas reduces to a limited number ofnarrow lines, which are the images of theslit through which the light is passed inorder to analyze it with the prism. For ex-ample, the spectrum of the light emitted bysodium in the gaseous state consists of two


35/196

THE PHENOMENA OF LIGHT 15yellow lines very near each other, which withlow-power spectroscopes appear blended intoa single line. Now Zeeman showed that ifa gas is placed in an intense magnetic field,say between the poles of a powerful electro-magnet, each simple line of its spectrum isin general broken up into a group of newlines.

There are two important cases to be con-sidered : First, that in which the luminousray which we are considering is parallel tothe lines of magnetic force ; second, that inwhich the ray is perpendicular to the lines offorce. The general case is naturally a triflecomplicated, and for its treatment I refer thereader to original articles on this subject (2).Let us suppose that we have a luminousgas between the two opposite magnetic poles ;for example, vapour of cadmium obtained bypassing electric sparks between two wires ofthat metal. If we examine the light whichis propagated in the direction of the lines offorce (Case I), that is, from one pole towardthe other, we easily ascertain that, while thegreen line of the spectrum of cadmium ap-


36/196

i6 THE PHENOMENA OF LIGHTpears sharp and simple as in A (Fig. i) be-fore the magnetic field is set up, the instantthe field is created the line A vanishes, andinstead of it there appear two new lines, Band C, one on either side of the position A\

B A' c

FIG. i.

which the single line at first occupied, and atequal distances from it.

If we examine with the spectroscope a rayof light in the equatorial direction (Case II),that is, perpendicular to the direction of themagnetic field, the single line, A (Fig. 2), isreplaced by three lines, C, A', B> of whichthe one in the middle, equidistant from


37/196

THE PHENOMENA OF LIGHTthe other two, occupies the position of theoriginal line.

In the case of other spectrum lines, eitherthe phenomena are identical with thosewhich are exhibited by the green cadmium

A

B Ar c

FIG. 2.

line, or slightly more complicated effectsare obtained. Thus, for example, of thetwo sodium lines, the one usually called D\is transformed in the second case into fourlines, A, B, C, D (Fig. 3), while the line D^changes into a group of six lines (Fig. 4),A, B, C, D, E, F.

Complete explanation of these phenomena,


38/196

18 THE PHENOMENA OF LIGHTat any rate in the least complicated cases, isfurnished by Lorentz's theory; but for ourpurpose it will be sufficient to treat thesingle case, to which Figure I refers, of lightemitted by the vapour of cadmium in thedirection of the lines of force.

A B C D

FIG. 3.

Let us consider an electrified particlewhich, attracted toward a position of equi-librium, O (Fig. 5), vibrates about this pointwith circular motion, describing a circumfer-ence of radius OA. The vibrating electri-fied particle generates light waves. Supposewe study the light which is propagated in


39/196

THE PHENOMENA OF LIGHTthe direction perpendicular to the plane ofthe circumference. If a magnetic force acts

.A B C D E F

I


40/196

20 THE PHENOMENA OF LIGHTand will act, as the case may be, eitherfrom A toward O, or in the opposite direc-tion. The effect of this new force, whichaugments or diminishes the force whichmaintains the particle in its orbit, is to varyits vibratory period, that is, the time re-quired for the particle to describe the cir-cumference, just as a change in theintensity of the force of gravity has theeffect of varying the period of oscillationof a pendulum.From the effect produced by a magneticfield on a circular vibration we may easilypass to that produced on any vibrationwhatsoever by a consideration like the fol-lowing :

Light vibrations are perfectly well under-stood. They follow the same laws as do thesmall oscillations of a pendulum, and are, ingeneral, elliptical ; in special cases they maybe rectilinear or circular; they are alwaystransverse, that is, they lie in the planeperpendicular to the light ray. Now it maybe shown that every elliptical vibration iskinematically equivalent to the resultant of


41/196

THE PHENOMENA OF LIGHT 21two circular vibrations of opposite directionsof rotation, the one right-handed (motion inthe same sense as thatof the hands of awatch), the other left-handed ; and in ad-dition the circularvibration which has thesame direction of rota-tion as the ellipse EF(Fig. 6), namely AB, Ehas a diameter equalto half the sum of the axes of the ellipse,while the other circular vibration, CD, hasa diameter equal to half the difference ofthe same axes.1 If we do not wish to have

1 Employing the usual symbols, the elliptical vibration,referred to its axes, may be represented by means of its rec-tangular components

x=asinO, y bcQsQ.This is evidently equivalent to the resultant of two circular vi-brations, one of which is right-handed like the given ellipse,and has the components

FIG. 6.

while the other is left-handed and has the components


42/196

22 THE PHENOMENA OF LIGHTrecourse to a mathematical demonstration,we may convince ourselves of the proof ofthis statement by employing a special pieceof apparatus which serves, among other pur-

FIG. 7.

poses, that of effecting the composition of twopendular vibrations of circular character (3).Two pendulums (Fig. 7) are suspendedfrom two fixed points, which for the sake ofsimplicity are not represented in the figure,


43/196

THE PHENOMENA OF LIGHT 23placed one above the other on the samevertical line. One of the pendulums con-sists simply of a wire carrying at its lowerend a heavy ring and a funnel, A, filled withsand ; the lower part of the other consists ofa platform BC, situated below A, which car-ries on one of its sides a second funnel, Z?,also filled with sand. The length of thefirst pendulum may be varied at will, butfor the experiment with which we have todeal this length should be such that thetwo pendulums have the same period ofvibration. A simple electric device controlsthe openings of the funnels and thus pre-vents or allows the flow of sand.At first let us suppose the pendulum BCto be fixed while we impart a circular motion

to A ; it is easy to ascertain whether or notthis motion be circular from the trace left bythe sand on the platform BC. Let us thenimpart a circular motion to the pendulumBC as well, but in the direction opposite tothat of the first pendulum; we may easilyascertain if we succeed in doing this byobserving the trace left by the sand from the


44/196

24 THE PHENOMENA OF LIGHTfunnel D on the plane of support. If, nowthat the two pendulums are vibrating, weallow the sand to run out of the funnel A,it will form an elliptical trace on the plat-form BC. This ellipse becomes a straightline if the two component circular vibrationshave equal diameters. We can thus con-vince ourselves of the truth of the statementmade, and in addition we learn that, whenthe two component circular vibrations of op-posite rotation have equal amplitudes, theresulting vibration is rectilinear.

Returning now to the case of the particlevibrating in a magnetic field, it in generalexecutes an elliptical vibration, for which wemay conceive the two equivalent circularvibrations to be substituted. But these lastare of opposite sign of rotation; if one ofthem is accelerated by the magnetic field,the other must be retarded. As soon astheir periods cease to be equal, they can nolonger cause a single spectrum line, butcause instead two new lines situated oneither side of the single primitive line. Thisexplanation, furnished by Lorentz's theory


45/196

THE PHENOMENA OF LIGHT 25to explain the experiment of Zeeman, wasproved correct by this able experimenter ina series of new experiments, which showedthat the two new lines were in fact due tocircular vibrations, one right-handed and theother left-handed.

Suitable qualitative and quantitative ex-periments made it possible to deduce twovery interesting results from the Zeemanphenomenon. On investigating which ofthe two new lines was due, for a givendirection of the magnetic field, to the right-handed vibrations and which to the left-handed, the sign of the charge of thevibrating particles could be determined, andit was recognized that, in order to make theobserved facts accord with their explanation,it was necessary to admit that these particlespossessed a negative rather than a positivecharge. In the second place, it was possibleto obtain an approximate evaluation of theratio of the electric charge of the vibratingparticle to its mass. The result to whichthis led was, that this ratio is more than athousand times greater than that which re-


46/196

26 THE PHENOMENA OF LIGHTlates to the atom of hydrogen in electrolysis,and hence still greater than that pertainingto the atoms of other substances.

This result may be interpreted in severalways, the more important of which are thefollowing: either the vibrating particles areions, and the charge of each is more thanone thousand times as great as that whichpertains to each valency in electrolysis; orthe vibrating particles have a charge equalto that of the electrolytic ions, and theirmass is less than one-thousandth as great asthat of an ion of hydrogen. The secondinterpretation is naturally the one accepted,and the vibrating particles are considered tobe free electrons. These therefore possess,or at least there is united with them, a smallmaterial mass; but we shall see that thissame mass probably has an electromagneticcause. At any rate, this result is corrobo-rated by those which are reached in otherways, as will be shown later.

Lorentz's theory receives, therefore, a splen--did confirmation through the experiments ofZeeman ; and hence it may be retained, that


47/196


48/196

CHAPTER IIINATURE OF THE CATHODE RAYS (4)

THE phenomena on which we shall nowtouch show the negative electrons in the actof undergoing very rapid motions of transla-tion instead of the vibratory motions whichwe have considered in the preceding chapter.Hence they present themselves under condi-tions favourable for a closer study, and thus,by modifying their motion in various ways,new and interesting effects may be produced.But for the sake of clearness it will be wellfirst to state the principal characteristics ofelectric discharges in rarefied gases.

Let us consider a glass tube, AC (Fig. 8),through the end walls of which are fused twoplatinum wires terminating in aluminium elec-trodes, A, C. If the air pressure in the tubeis somewhat less than that of the atmosphere,

for example, eight or ten millimeters ofmercury, and if an electric discharge is

28


49/196

NATURE OF THE CATHODE RAYS 29

caused to pass from one electrode to the other,instead of the well-known loud and brilliantspark which is formed in the open air, a char-


50/196

30 NATURE OF THE CATHODE RAYSacteristic luminous phenomenon is obtained,in which two regions are distinguishable : thepositive luminous column, a sort of ill-defined,rose-coloured spark having a blurred contourwhich reaches from the anode up to within ashort distance of the cathode ; and the nega-tive column, or negative glow, violet in colourand contiguous with the cathode. Betweenthese two luminous regions there is an inter-val called the Faraday dark space.

If now the pressure of the air is diminished,the luminous character changes. We will con-cern ourselves with the negative light with-out considering further the positive columnwhich, with increasing rarefaction, graduallydiminishes both in size and in luminous in-tensity, often subdividing into distinct regionsseparated by relatively dark intervals (striateddischarge). At first the negative light ex-tends over the entire cathode, as at C, incase

initiallyit only covered the extremity ;but later, with still further rarefaction, it ex-

tends all around to greater and greater dis-tances, detaching itself at the same time fromthe electrode, as at C . In the meantime a


51/196

NATURE OF THE CATHODE RAYS 31new luminous stratum forms in contact withthe electrode, and thus the negative light isdivided into two parts; namely, the firstnegative column, adhering to the cathode,and the second negative column, or nega-tive glow, separated from each other by arelatively dark region which, to distinguishit from the Faraday dark space, is called thecathode dark space. Continuing the rare-faction still further, the two luminous nega-tive columns extend farther and farther out,becoming continually less bright and sharpin outline (C f, Fig. 8). The interval whichseparates them also becomes greater; and,when the highest rarefaction is obtained, thatis to say when the air pressure is reduced toless than one-thousandth of a millimeter ofmercury, almost every trace of luminosity inthe gas disappears.

But before this stage is reached a newphenomenon appears. At first the portionof the walls of the tube about the cathode,and later that in front of it, becomes lumi-nous, diffusing a brilliant light, usuallygreen, due to a species of phosphorescence,


52/196

32 NATURE OF THE CATHODE RAYSor perhaps better of fluorescence, since this isthe name given to the emission of light byfluorspar and certain other substances, whichdoes not last appreciably after the causewhich produced it has ceased. The causeof this phenomenon is to be sought in thecathode, because, if an obstacle is placedbetween it and the wall, a very sharp shadowis thrown, as if the fluorescence were excitedby invisible radiations sent out from thecathode. We will now turn our attentionto these radiations, which are called CathodeRays.They are propagated in straight lines and

leave the cathode in a direction at rightangles to its surface ; hence if this has theform of a concave mirror, the cathode raysconverge practically to the centre of curva-ture. When concentrated in this mannertheir singular properties become more evi-dent, as Sir William Crookes has shown ina very brilliant and suggestive manner withthe aid of cleverly devised apparatus.The principal properties of the cathoderays are the following : they excite phospho-


53/196

NATURE OF THE CATHODE RAYS 33rescence not only in glass, as we have seen,but in a large number of other bodies, includ-ing those which phosphoresce under the ac-tion of light. The cathode rays heat bodieswhich they strike and tend to move them asif the impact were mechanical. It is pos-sible, however, that this mechanical actionmay be, at least in a large part, a simple con-sequence of the preceding effect. Finally,bodies struck by cathode rays become sourcesof new radiations ; namely, the famous X-raysdiscovered by Professor Rontgen. In orderto explain all these phenomena, Crookesbrought forward his hypothesis of radiantmatter.As early as 1816 the celebrated Faraday (5)

pointed out the possibility of a fourth stateof matter, as a consequence of a hypotheticaltransformation which transcends evaporationby as much as evaporation transcends thefluid state ; or, he expressed his thought stillbetter by saying that he looked forward withthe greatest impatience to the discovery ofa new state of the chemical elements. Hesuggested further, and this has an especial


54/196

34 NATURE OF THE CATHODE RAYSinterest with reference to the theory withwhich we are now occupied, that the decom-position of the metals, their recompositionand the realization of the formerly absurdidea of transmutation, were problems whichchemistry one day must solve.

According to Crookes, when the electricdischarge takes place in a highly rarefiedgas, very minute negatively electrified mate-rial particles are projected from the cathodeand, forming a fourth state of matter tran-scending the gaseous state, produce the ob-served effects as a result of their collisions ;moreover, the trajectories of these particlesconstitute the cathode rays. It was laterthought that the particles were the actualatoms of the gas residue which on accountof its extreme rarefaction presented suchnew properties as were brought to light bythe rotation of the vanes of Crookes' famousradiometer.

But some people, among whom was theillustrious Hertz, preferred to consider thecathode rays as an undulatory phenomenonsimilar to light, having its origin at the sur-


55/196

NATURE OF THE CATHODE RAYS 35face of the cathode and its seat in the ether.However, this opinion had very soon to begiven up on account of subsequent experi-ments. Thus, while many physicists, notablyJ. J. Thomson (6) in England, to whom weowe so much of the present electron theory,and Q. Maiorana (7) in Italy, were findingout that the velocity of the cathode rays isnoticeably less than that of light, J. Perrin (8)was making it evident that the cathoderays produce a transport of negative electric-ity. This last effect may be obtainedeven when the rays have passed through athin metallic plate, as was later shown byLenard (9).

Perrin's experiment may be made with a dis-charge tube similar to that shown in Figure 9.The cathode consists of an aluminium disk,and the anode ABDE is a cylindrical boxwith circular openings at the centre ofthe bases. This box is in connection withthe earth, and contains the conductor F,which is connected to an electroscope. Theconductor F usually has the form of a hol-low cylinder with an opening turned toward


56/196

NATURE OF THE CATHODE RAYSthat in the base, DE, of the anode. A nega-tive charge collects on the conductor /''whenthe discharge enters the tube. Evidentlythis can only be due to a transport of chargeeffected by the cathode rays. Moreover, atthe approach of a magnet, the action ofwhich, as we shall see, is to make the cathoderays assume a curved path, the rays cease to

FIG. 9.

enter the cylinder AD and cause a luminousspot to appear on the base DE, which forthis purpose is usually coated with a phos-phorescent substance. Now just as soon asthe conductor F ceases to receive the rays,it, in turn, ceases to receive electricity.

Naturally the discovery of these facts fur-nished the strongest support for Crookes'theory. But countless recent experimentsdue to many physicists have led to a slight


57/196

NATURE OF THE CATHODE RAYS 37modification, and a better statement of theoriginal hypothesis and to the admission thatthe particles, which in their rapid motionconstitute the cathode rays, can be nothingbut the negative electrons themselves. Thisopinion, at present held by all, rests princi-pally on the following facts which have beenaccurately verified, and with which we shallconcern ourselves in detail a little later. Inthe first place, the cathode rays always haveidentical properties whatever may be therarefied gas in which they are formed, andwhatever may be the nature of the cathode;in the second place, the moving negativeparticles all possess that same small massless than one-thousandth that of a hydrogenatom which is encountered, as we have seen,in the study of the Zeeman effect, and whichis deduced from the results of various otherphenomena as well.

Cathode rays may also be produced with-out having recourse to the electric discharge.Thus, for example, a body exposed to theaction of light, or better of ultraviolet rays,emits electrons. Unless the surrounding


58/196

38 NATURE OF THE CATHODE RAYSgas is extremely rarefied they unite withneutral atoms and form negative ions; butif the gas is almost entirely removed, theelectrons remain free, and on leaving thebody form true cathode rays (10), which ingeneral possess a velocity less than thatwhich they have in discharge tubes. Thisvelocity decreases as the negative potentialof the illuminated body is diminished.

Also in the case of the cathode rays themass of the electron was not separately de-termined, but rather the ratio between theelectric charge of any electron and its mass.Such a determination as this is based on theeffects produced on the cathode rays by elec-tric or magnetic forces, and these effects arein good accord with the accepted hypothesis.In fact, it is clear that when an electric forceacts on negative particles in motion, theyshould deviate from their ordinary rectilinearpath; and since an electrified particle inmotion should behave in a manner analo-gous to a current, or more accurately to anelement of current, it follows that the par-ticle itself should deviate from its ordinary


59/196

NATURE OF THE CATHODE RAYS 39path when it is exposed to the action of amagnetic field. But we shall consider suchphenomena and the measurements relatingto them a little farther on.


60/196

CHAPTER IVTHE IONS IN GASES AND IN SOLIDS

IN electrolytes the electrons are joined tothe neutral atoms to form free ions, and themotion of these ions is what constitutes theelectric current. At present the opinion isheld that the same thing happens in gases ;namely, that when a gas possesses electricconductivity, it owes it to the presence ofions, and to their motion under the actionof electric forces. The hypothesis of theionization of gases, which, for a long time,was held by very few, is now generallyadmitted in consequence of the numerousexperiments made in recent years.We are, then, of the opinion that a gascontains free ions. These are ordinarilypresent in such a small number that theresulting conductivity is very small. Butthere are circumstances in which, by the

40


61/196

THE IONS IN GASES AND IN SOLIDS 41action of appropriate external energy, thegas is ionized ; that is to say, many of itsatoms are broken up into positive ions andnegative electrons. If the gas is not suf-ficiently rarefied, the electrons unite withthe neutral atoms and form negative ions.Moreover, certain facts seem to indicate thatatoms or neutral molecules can unite withions to form groups which, while having theusual charge of the ions, possess massesmuch greater than those which can pertainto a simple ion.The most natural explanation of theknown facts, and in particular of thoseabout to be mentioned, is that the electricalconductivity of gases is due to the presenceof electrified particles which are free to movebetween its molecules.An ionized gas loses its conductivitywhen passed through minute interstices, saythrough a mass of glass wool, or throughlong and fine metallic tubes, or is made tobubble through a conducting liquid (n),which, however, should not contain anyradio-active substance. The same result is


62/196

42 THE IONS IN GASES AND IN SOLIDSobtained if the gas is made to pass betweentwo oppositely electrified conductors in sucha way that it may serve as a conductor forthe current. In the first case, the phenome-non is explained by the attraction exerted onthe ions by the bodies near which they pass ;in the second case, the two conductorsattract and hold the ions which carry acharge opposite in sign to their own and soremove them from the gas.The manner in which an ionized gas be-haves when it is carrying an electric cur-

rent is also in perfect accord with theaccepted hypothesis. Let us suppose thatwe have, for example, two parallel metallicdisks, one of which communicates with theinsulated pole of a battery, and the otherwith an electrometer. If we ionize the airbetween the disks by passing Rontgen raysthrough it, and if we vary the value of thepotential furnished by the battery, we findthat the gas fails to follow the well-knownlaw of Ohm, which holds for constant elec-tric currents, and according to which theintensity of the current in a conductor in-


63/196

THE IONS IN GASES AND IN SOLIDS 43creases in proportion to the difference ofpotential between its ends. In fact, theintensity of the current measured by thecharge, which is acquired in a given time bythe disk in communication with the elec-trometer, increases considerably less rapidlythan the potential. The intensity evenfinally assumes a limiting value which doesnot increase as the potential of the battery israised. When the current has attained thisvalue, called the saturation value, all theions generated in a given time by the Ront-gen rays (or in general produced by what-ever source of ionization is employed) areutilized in transmitting the current in thissame time. An increase in potential is ofno effect, as there are not a greater numberof ions to be disposed of.

Moreover, a curious phenomenon metwith by the writer (12), and which wasconfirmed and

rightly interpreted by J. J.Thomson and E. Rutherford, is obviouslyexplained by the accepted theory. The phe-nomenon is the following: if the distancebetween the two metallic disks considered


64/196

44 THE IONS IN GASES AND IN SOLIDSabove is varied, the intensity of the currentwhich traverses the ionized air between themvaries as well, but in a manner contrary tothat which one would suppose. In fact, theintensity of the current increases, withincertain limits, with an increase of the dis-tance. This is easily explained when wereflect that with an increase in the distancebetween the plates there is an increase inthe amount of air in which the phenomenontakes place, and in consequence also in thenumber of ions, which, by their motion, con-stitute the saturation current.The ions in gases move around between

the molecules, frequently colliding withthem. New ions may form by the breakingup of neutral molecules, and ions of oppositesign may recombine into molecules. Thislast action, namely, the disappearance ofions, is continually taking place, and it isbecause of this that the number of ions doesnot increase beyond a certain limit underthe action of an ionizing cause.

If the ions are generated in a single regionof the gas, they diffuse into the remaining


65/196

THE IONS IN GASES AND IN SOLIDS 45portion. In gases under ordinary pressurethe velocity of diffusion is usually extremelysmall because of frequent collisions ; but ifan electric field acts, the velocity of diffusionbecomes large ; the first time that a measure-ment of this kind was made (13) the velocitywas found to be several decimeters persecond.

Ultraviolet rays, cathode rays, Rontgenrays, rays emitted by radio-active substances,heating to a relatively high temperature, areall causes of ionization. This is greater orless according to circumstances, and is limited,as has already been mentioned, by a continualrecomposition of atoms and neutral mole-cules.

But there exists still another cause ofionization, to which in reality some of theabove causes reduce ; this is the collision ofthe ions (and, in fact, of the electrons as well,since probably some of these exist, at leastmomentarily, in the free state in gas underordinary pressure) with the atoms and mole-cules. When an ion possesses a sufficientlyhigh velocity, it can furnish the energy neces-


66/196

46 THE IONS IN GASES AND IN SOLIDSsary to transform an atom into a positiveion and a negative electron, and hence alsoto transform a molecule into two ions ofopposite sign. Let us briefly take up thesevarious means of ionizing a gas.

Light radiations, and especially the ultra-violet, may ionize a gas in two differentways. If they strike a solid or liquid body,they produce an emission of negative elec-trons which results in the rapid discharge ofthe body, if it was negatively electrified, andeven the formation on it of a positive charge,as has been demonstrated by the writer (14).Ordinarily the experiment is performed withmetals, because the effect is rather weakwith liquids, and solid insulators are not sowell adapted to quantitative determinations.As a source of active radiations, the invisibleultraviolet rays emitted by an arc light orby an electric spark are employed, althoughcertain bodies, such as the alkaline metalsand amalgamated zinc, give a marked effecteven with visible radiations. Now if theelectric field determined by the negativecharge of the body is sufficiently intense,


67/196

THE IONS IN GASES AND IN SOLIDS 47the negative electrons which are emitted mayacquire a velocity great enough to ionize theneutral atoms by impact.

But even directly the more refrangibleultraviolet radiations given off by the electricspark cause ionization of the gas throughwhich they pass, as was demonstrated byLenard (15), who allowed the radiations froma spark formed between aluminium wiresto fall on electrified bodies. These becamedischarged with about the same rapiditywhether they were charged positively ornegatively and whatever was the nature andcondition of their surfaces. All this couldnot be attributed to a surface action, butrather to an action on the mass of the airtraversed by the radiations; that is, to theionization which they produced. An experi-ment which may be repeated also with othersources of ionization confirmed this explana-tion. It consists in blowing the air from theplace where it is ionized to another place,where, as a result of the conductivity whichit retains for a certain time, it brings aboutthe discharge of electrified bodies. In order


68/196

48 THE IONS IN GASES AND IN SOLIDSto cause the effect to cease it is sufficientto intercept the radiations.

It appears that only the more rapid ultra-violet vibrations produce direct ionizationof gases in any marked degree. Indeed, theexperiments described above do not succeedexcept when the path traversed in the airby the radiations is reduced to but a fewcentimeters, and it is a well-known fact thatthe most refrangible ultraviolet radiationsare very rapidly absorbed by air at ordinarypressure.The cathode rays which are, as we haveseen, nothing but negative electrons inmotion, ionize a gas, as will be explainedbefore long in some detail.With regard to the Rontgen rays, whichapparently are the manifestation of etherwaves generated by the sudden variationsin velocity of the electrons, the ionizationof

gases produced by them seems to be dueto a sudden electric impulse produced in theelectrons of the gaseous atoms.

Finally, a rise in temperature, which isequivalent to an increase in atomic velocity


69/196

THE IONS IN GASES AND IN SOLIDS 49and apparently to an increase in the velocityof the negative electrons as well, naturallytends to free the latter from their bond withthe positive part of the atom. A red-hotmetallic wire ionizes the gas in contact withit, and gases in flames always appear to bestrongly ionized.

In order that the molecules of the gas maybecome ionized by the impact of those ionswhich already exist in it, it is generally neces-sary to expose the gas to the action of suffi-ciently intense electrical forces. In tooweak a field, the ions, while they follow theelectric force, do not acquire a sufficientvelocity between one collision and the next,and the effect of the collisions is to keep thisvelocity constantly at a low value, since natu-rally a part of the energy of motion of theions is given up to the molecules which havebeen struck. Under such circumstances thepaths described by the ions cannot differmuch from the lines of electric force, or, inother words, the ions must continually movevery nearly in the direction of the force whichurges them. The so-called phenomena of


70/196

5


71/196

THE IONS IN GASES AND IN SOLIDS 51by impact the gas molecules at some distancefrom the cathode. This gives rise to thesecond negative column, or negative glow,which is thus a region of the gas where ion-ization takes place. The electric force drivesthe positive ions created in this mannertoward the cathode, close to which theypossess the velocity required to ionize thegas molecules. This causes the formationof the first column of negative light.The electrons produced in this regionmove away from the cathode, and in this way

the two regions of ionization furnish eachother the necessary ions or electrons. Thecathode dark space is thus simply the regiontraversed by the electrons constituting thecathode rays, and especially by the positiveions which move toward the cathode, beforethey have acquired the velocity necessary toproduce ionization.We will not concern ourselves furtherwith the negative electrons after they havearrived at the second negative column ; how-ever, it is of interest to us to know what be-comes of the positive ions after they arrive


72/196

52 THE IONS IN GASES AND IN SOLIDSat the cathode. Some of them naturallybecome neutralized by the negative elec-trons ; but others, on account of their velocity,which varies in direction as a result of colli-sions, may bend around the cathode, or evenpass through it, if it is provided with open-ings or canals, or if it is made of wire gauze.Beyond the cathode the positive ions thenconstitute the positive or anode rays, analo-gous to the cathode rays, and which are oftencalled canal rays, the name given them byGoldstein.An electric or a magnetic field causes a

deviation of the positive rays, but in thedirection opposite to that which would beobserved with cathode rays; it is preciselyon account of this that the conclusion isreached, that these rays consist of positivelyelectrified particles in motion. However, thedeviation is noticeably smaller for the posi-tive

raysthan for the cathode

raysunder

similar conditions. From measurements ofthe deviation it may be shown that the par-ticles in motion do not possess a minute massas in the case of the cathode rays, but a mass


73/196

THE IONS IN GASES AND IN SOLIDS 53comparable with that of atoms or of electro-lytic ions. In this case, therefore, we haveto deal not with positive electrons, but withions and probably with groups of greatermass as well.

If we admit electric conductivity to be aphenomenon of convection in gases as wellas in liquids, the analogous hypothesis forsolid conductors, already stated in Chapter I,becomes all the more natural. And, inas-much as it appears that only the negativeelectrons, and not the positive, can exist iso-lated, it is held that the electric current in aconductor consists, at least principally, in themotion of negative electrons. The experi-mental evidence that metals do not offer aninsurmountable obstacle to the motion of theelectrons is furnished by their perviousnessto the cathode rays. Without entering intodetails we may add that this mode of consid-ering the current permits us to explain fairlywell various observed facts, as, for example,the proportionality between the thermal andelectric conductivity of various bodies, and toexplain such phenomena as those relative to


74/196

54 THE IONS IN GASES AND IN SOLIDSthe optical properties of metals. Thus theelectron theory not only finds no contradic-tion in phenomena of this sort, but alsoshows itself able to furnish a simple explana-tion of them.


75/196

CHAPTER VRADIO-ACTIVITY

THE discovery of the so-called X-rays byProfessor Rontgen early in 1896 gave rise tonumerous experiments relating to the possi-ble existence of other radiations capable ofaffecting photographic films and of passingthrough opaque bodies. Effects of this sortwere described by Le Bon ; but we will notconsider these, as it was recognized that theeffects attributed by this experimenter to anew radiation which he called black light proceeded, at least in nearly every case, fromcauses which have no intimate relation withthe subject which we are now about to treat.

Certain experiments which have, on theother hand, a relationship with radio-activitywere made by Henry (17) with phosphores-cent sulphide of zinc ; by Niewenglowski (18)with calcium sulphide ; and by Becquerel (19)

55


76/196

56 RADIO-ACTIVITYwith double sulphate of uranium and potas-sium. The result of these experiments isthat these substances emit rays capable ofpassing through opaque bodies and actingon a photographic film, when phosphores-cence is excited in them by exposure tolight or to X-rays.When Becquerel began his experimenta-tion, he had a special object in view. It wasalready known that the X-rays had theirorigin at that part of the wall which is ren-dered luminescent where the cathode raysstrike; it was consequently natural to sup-pose that phosphorescence and the emissionof X-rays were related phenomena, thoughlater it was recognized that such is not thecase. H. Becquerel therefore wished to de-termine whether bodies rendered phospho-rescent by the action, not of cathode rays,but of light, would emit X-rays. And sincethese rays can affect a photographic filmeven when it is surrounded by opaque ob-jects, the French physicist placed variousbodies above a film, protected in this man-ner from the light, and exposed the whole


77/196

RADIO-ACTIVITY 57to sunlight.

After several unsuccessful at-tempts, he obtained a marked effect withcrystalline laminae of the double sulphate ofuranium and potassium, since on developingthe film he saw an image of the laminae andthe shadow of a silver coin which had beenplaced below one of them. It seemed, there-fore, that the phenomenon he had predictedhad really taken place. But Becquerel hap-pened also to obtain the same effect withpoor illumination on a cloudy day, fromwhich he suspected that the effect itself didnot depend on the action of the light. And,in fact, he very soon proved (20) that theuranium salt continually and spontaneouslyemitted rays able to go through opaquebodies and to act on photographic filmswithout the necessity of its being exposedto the light.

Further research showed (21) that the raysfrom the uranium salt share with the X-rays,not only the property of traversing opaquebodies, of acting on photographic films, ofrendering phosphorescent bodies luminous,and, as was later shown, the negative prop-


78/196

$8 RADIO-ACTIVITY

erty of not being susceptible of reflectionand refraction, and hence of polarization, butalso another property which was recognizedshortly after the X-rays were discovered (22);namely, that of ionizing gases throughwhich they pass. A method of studyingthe Becquerel rays, which is more rapidthan the photographic method, is based onthis property; it consists in the measure-ment of the velocity with which an electri-fied body becomes discharged when thesurrounding gas is exposed to the action ofthe rays.

For this purpose we may make use of anyelectrometer connected to a metallic diskplaced at a short distance from a second disk ;and we may experiment in two ways. Eitherthe second disk is put in connection with theearth and then one notes the rate of diminu-tion of an electric charge communicated tothe electrometer when the air between thetwo disks is ionized by the radio-active body ;or -the second disk is charged and the rapiditywith which the electrometer deviates is ob-served. With very active bodies a galvanom-


79/196

RADIO-ACTIVITY 59eter may be employed, but if bodies of weakradio-activity are to be investigated, it isbetter to use a gold-leaf electrometer, or bet-ter still, an electroscope with but a singleleaf consisting simply of a vertical metallicrod, AB (Fig. 10), at the upper end of whicha very light gold or aluminium leaf,CD, is fastened. In order to securegood insulation the rod is supportedby a small piece of sulphur, 6 . Theelectrical capacity of the conduct-ing system ABCD is extremelysmall, and hence it results that thediminution in the divergence of theleaf CD is not too slow. The elec- Btroscope becomes an electrometer,if the position of CD is observed by meansof a microscope and ocular scale. Thepotential corresponding to each division ofthe scale may be determined in advance bythe use of a battery of small accumulators.The writer prefers a form of electrometerslightly different from that which has becomeclassic for the study of feebly radio-activebodies. The sulphur or fused quartz insu-


80/196

60 RADIO-ACTIVITY

lator S (Fig. n) is quite slender and heldwith mastic or gutta-percha to the bottom ofa small bell-shaped piece of metal connectedwith the rod AB. This form of supportobviates or reduces the creeping of thecharge from the rod to the surface of theinsulator. Moreover, the writer in

again adopting an arrangementwhich he devised many years ago,uses an ordinary millimeter scaleplaced at several meters' distancefrom the electrometer, instead ofone annexed to the ocular. A con-verging acromatic lens forms a real

FIG. ii. image of the scale in the plane inwhich the metallic leaf moves, and thus boththe leaf and the scale are seen simultaneouslyin the field of the microscope.

In one of the electrometers lately con-structed by the writer the rod and the goldleaf have scarcely one-fourth the dimensionsshown in Figure n. It is particularly welladapted to the demonstration of radio-activity,since it is so sensitive that, when one of theuranium salts is approached in such a way


81/196


82/196

62 RADIO-ACTIVITY

the peculiarities and the probable originalcause of this interesting phenomenon, or, atleast, all this would have required muchtime and most accurate research.

M. and Mme. Curie chanced to find cer-tain specimens of chalcolite and pitchblende(in particular that which is mined at Joachims-thai), which were somewhat more active thanpure uranium. Recalling that radio-activityis an atomic property, the phenomenon couldnot be attributed to uranium contained inthese minerals, and it was necessary to sup-pose that an unknown substance, more activethan uranium itself, was present. Havingrecourse to physical and chemical methodsof separation, certain compounds of bismuthwere extracted from these minerals having aradio-activity as much as four hundred timesthat of uranium (25). The name poloniumwas given to the unknown substance con-tained in these compounds, the radio-activityof which diminishes slowly as time goes on.Later the Curies and M. Bemont (26) extractedfrom pitchblende a small quantity of a veryactive body, chemically analogous to barium


83/196

RADIO-ACTIVITY 63

and associated with it in almost every re-action, to which they gave the name radium.Another radio-active body associated withthorium and possessing similar chemical prop-erties was discovered by M. Debierne (27),who called it actinium.Various other investigators have suc-ceeded in deriving still other noticeablyradio-active substances from many differentminerals, but especially from pitchblende;the nature of these substances is not yet wellunderstood. In particular Elster and Gei-tel (28) obtained a rather radio-active sulphateof lead which seems to contain simply radif-erous barium. Nor is the radio-active leadobtained by Giesel (29) any better character-ized, while that found by Hofmann andStrauss (30) would, in some respects, seemto resemble polonium. The active lead, orradio-lead of these authors, exhibited oneproperty worthy of mention. Under certainconditions the active sulphate of lead loses alarge part of its radio-activity, which it laterslowly recovers; but if it is exposed to thebombardment of the cathode rays, its radio-


84/196

64 RADIO-ACTIVITYactive properties are fully restored in a fewminutes.

Recently Hofmann and Wblfl (31) havepartially separated the active substance fromthe inactive lead, and have determined thatit differs from the polonium of Mme. Curiein the constancy of its radio-activity.The substance called radio-tellurium byMarckwald (32) also resembles polonium inits properties, the only difference being thatits radio-activity does not seem to diminishwith time. It would seem that radio-tellu-rium is the body to which the activity ofthe bismuth extracted from the Joachimsthalpitchblende is due, and it is considered bythis author to be a new body belonging tothe sulphur and tellurium series, because it isdeposited on sticks of bismuth or antimonyintroduced into an acid solution of activechloride of bismuth. Marckwald obtainedin this manner about six decigrams of quiteactive material from 850 grams of the salt.An active bismuth quite similar to that ofthe other investigators was discovered byGiesel (33), who later separated from it a prod-


85/196

RADIO-ACTIVITY 65uct identical with that obtained by Marck-wald. Finally a supposed new radio-activeelement was separated from thorium byBaskerville, who called it carolinium (34).As we see, a great uncertainty still existsregarding the nature and even the separateexistence of the greater part of the radio-active substances; so much so, in fact, thatthere are some who consider the effects at-tributed to Mme. Curie's polonium as due toan induced radio-activity^ that is, to a transi-tory phenomenon, of which we shall speaklater; there are others who believe radio-active thorium and actinium to be identical.The chemistry of the radio-active bodies isonly in its initial stages, and at presentradium, whose characteristic spectrum is nowknown, is the only substance the existenceof which, as an element distinct from theothers, seems to be sure.

Radium has never been prepared in thefree state, but, nevertheless, several of its saltsare known. For example, the small quan-tity of radio-active substance, which theCuries were first able patiently to obtain from


86/196

66 RADIO-ACTIVITYseveral tons of pitchblende residues, remain-ing after the uranium had been extracted,was the chloride of radium. These residuescontained compounds of almost all the met-als, among which were barium, bismuth, andthe metals of the rare earths. Chemicalprocesses, which it would take too long todescribe here, permit the extraction of thebarium with the radium, the bismuth withthe polonium, and the rare earths with theactinium ; after this there remains the sepa-ration of each of the radio-active bodies fromthe body which accompanied it, in spite ofprevious chemical reactions. This separa-tion, to which Mme. Curie has devoted manyyears of labour, has not yet been completelysuccessful, except in the case of radium,where the following process was employed.The radiferous chloride of barium, ofwhich about eight kilograms is extractedfrom a ton of pitchblende residues, is dis-solved in hot water, so as to form a saturatedsolution. On cooling, crystals, which wewill call A> are thrown down, while on evap-orating the remaining solution, another


87/196

RADIO-ACTIVITY 67chloride, which we will call B, is obtained.Since the chloride of radium is slightly lesssoluble than the barium chloride, it resultsthat the chloride A is richer, and the chlorideB is poorer, in radium than the chloride usedin forming the solution. This may be ascer-tained by determining the relative radio-activ-ity. A second operation exactly similar tothe above is carried out with each of theproducts A and B ; now since the least activepart obtained from A and the most activepart obtained from B are about equallyradio-active, they are reunited, and thus thefour portions which result from the secondoperation reduce to three of different rich-ness in radium. The process is continuedin the same manner ; however, to prevent aninordinate increase in the number of por-tions, no use is made of those whose radio-activity is very small, and the operation isdiscontinued on those portions whose radio-activity is sufficiently great. It is also foundadvantageous to make use of the motherliquor obtained from one operation to dis-solve the crystals obtained in the succeeding


88/196

68 RADIO-ACTIVITYoperation. When the greater part of theinactive substances is eliminated by this pro-cess, it is carried on still further, but now theleast active parts are more freely discarded.In this way a small amount of radium is lost,but the purification proceeds more rapidly,to accomplish which it is advisable to acidu-late the solvent more and more with purehydrochloric acid. Finally, practically purechloride of radium is separated to the amountof two or three decigrams per ton of theresidues employed. At present bromide ofradium is also prepared in the pure state.

Besides the measurement of the degree ofincreasing radio-activity a spectroscopic ex-amination of the product may be employedto estimate its purity, since radium possessesa spectrum which perfectly characterizes itand which was studied by Dema^ay. If animpure product is examined, the spectrumof barium appears with the spectrum ofradium; but as the process of purificationgoes on, the barium lines become weakerand finally almost completely disappear.

After the principal effects due to radio-


89/196

RADIO-ACTIVITY 69

active bodies were discovered, the attentionof physicists was turned to the direct studyof the rays emitted.

All known radiations are, or at least weconceive that they are, of two sorts ; thosedue to waves propagated in the ether, andthose due to the motion of electrified mate-rial particles. Not only the luminous raysproperly so called, and the invisible heat andultraviolet rays, but also, as is thought, the

Rontgen rays belong to the first sort. Tothe second sort belong the cathode rays,which are, in fact, considered to be due tothe motion of negative electrons.

It is not difficult to decide on the natureof new radiations which belong to one or theother of these two categories ; in fact, whilean electric or magnetic field cannot in theleast degree modify the form of luminous orof Rontgen rays, etc., they should cause amarked curvature of the path described byelectrified particles, unless the velocity isexceedingly great. However, there exists, asit appears, certain new radiations to whichtheir discoverer, Blondlot, has given the name


90/196

70 RADIO-ACTIVITYof

n-rays (35), which seem to possess someof the properties of heat rays, but whichappear to possess other very strange proper-ties. We shall not attempt to describe themhere, since their nature is still enigmatic.

In order to acquire a clear conception ofthe nature of the rays emitted by radio-activebodies, it was necessary to subject them tomagnetic or electric forces, and therefore toplace the radio-active bodies either betweenthe poles of a powerful magnet or betweentwo metal plates oppositely electrified. More-over, in order that the possible deformationof the rays might be recognized, they had tobe passed through a small opening or dia-phragm, on the far side of which either aphosphorescent body or a photographic plateprotected from the light by a black papercovering was placed. With the phosphores-cent body the displacement of the luminousspot is at once seen when an electric or amagnetic field deforms the rays, and thesame result is detected when the plate isdeveloped. It is generally preferable to usethe photographic plate, because the length


91/196

RADIO-ACTIVITY 71of exposure, which may be as great as isdesired, eventually compensates for the lowintensity of the radiations which are studied.

Applying these methods, it was recognizedfrom the very first that in general a radio-active body emits rays which are deviatedby the magnetic and the electric field, andat the same time other rays which are notdeviated. It' was further determined thatthe former behave in all respects like cathoderays of high velocity, that is, as though theyconsisted of negative electrons projected instraight lines with enormous velocity. Weshall see further on that this velocity may bemeasured, and that the ratio between electriccharge and mass has practically the samevalue as that found in the case of the cathoderays.

Subsequent researches have proved thatradium and other strongly radio-active bodiesalso produce rays deviated somewhat lessthan the cathode rays by the electric andmagnetic forces, but in the opposite direc-tion. Hence it may be said that radio-activebodies emit three varieties of rays. It is


92/196

72 RADIO-ACTIVITY

probable that this is true of all bodies, since,even if some of these varieties of rays havenot yet been found in the complex emissionof certain radio-active bodies, this is in allprobability due to their being less intenseand hence less easy to detect. An exampleof this is found in Mme. Curie's polonium,which practically only emits rays whosedeviation is opposite to that of the cathoderays.The rays which are subject to deviationcan only be considered as consisting in theemission of electrified particles. The direc-tion of the deviation shows the sign of thecharges of the particles, while the amount ofthe deviation permits an evaluation of thevelocities with which the particles move, andin addition, of the ratio which exists betweenthe charge and the mass of each; conse-quently the mass itself may be determinedif we assume that its electric charge has thatconstant value which pertains to the electro-lytic ion of hydrogen. We will now sum upthe results of the research relating to radio-activity, adopting Rutherford's designation,


93/196

RADIO-ACTIVITY 73a, /3, y, for the three kinds of rays emittedby radium, and perhaps in general by everyradio-active body.The a-rays behave in a manner whichconfirms the hypothesis made by Strutt(36), which is that they consist of positiveions projected in all directions from theradio-active body. In fact, Rutherford (37)proved that they transport positive charges,while Becquerel (38) determined that theirdeviation in a magnetic field is in the oppo-site direction to that of the cathode rays.

If, for example, the rays emanate from asmall quantity of one of the salts of radiumcontained in a little lead vessel, P (Fig. 12),they are propagated

in the direction of thestraight line PC\ but if a magnetic field isset up perpendicular to the plane of the fig-ure, the a-rays separate themselves from theothers and curve in the direction of the arcPA.

It was found, on measuring the velocityand the ratio between the charge and themass of the particles constituting the a-rays,that the velocity may attain a value about a


94/196


95/196

RADIO-ACTIVITY . 75ordinary air ten centimeters thick, or a thinplate of aluminium less than a tenth of amillimeter thick, is sufficient to absorb thegreater part of the rays.The /3-rays behave in all respects like verypenetrating cathode rays. That is, they arenegative electrons projected in all directions ;their velocity is enormous, since it may attaina value but slightly inferior to the velocityof light. This result is deduced from theeffect produced on the rays by the magneticfield, which was determined almost at thesame time by Becquerel (39), Giesel (40),Meyer and Von Schweidler (41), and Dorn(42). Instead of curving like the a-rays,they bend considerably more in the oppositedirection, as is shown in Figure 1 2. The cur-vature of some of the /3-rays may be so greatthat they end by striking a photographicplate, LL, on which the lead vessel P restsin the

region B^ B^The production of rays curved to a greateror less degree by the magnetic field is dueto the fact that the velocities of the variousnegative electrons, of which the /8-rays are


96/196

76 RADIO-ACTIVITY

only the trajectory, have different values.The magnetic field causes those which moveless rapidly to describe semicircles of smallradius and those with the high velocity todescribe the arcs of large radius. It is thusthat the elongated image obtained on thephotographic plate is explained. By meansof this image it is easy to prove that the lessdeflected rays, consisting of electrons withgreat velocity, are also the more penetrating.In fact, a photographic plate placed in thepath of the /3-rays absorbs most stronglythose rays which are the most deviated (B^Fig. 12). Moreover, the variety of /3-rays isquite great; thus, while some are stoppedby aluminium foil one-hundredth of a milli-meter thick, others are able to traverse sev-eral millimeters of lead. It was proveddirectly by the Curies (43) that the /J-raysreally transport negative charges. In theirexperiments the a-rays were arrested by aplate of aluminium so that only the effect ofthe /?-rays was shown by the electrometer.

While the a and ft rays are deviated in anelectric or a magnetic field, the y-rays are


97/196

RADIO-ACTIVITY 77not affected at all. Thus those emitted bythe radio-active body contained in P (Fig.12) preserve their rectilinear motion, PC,even when the magnetic field is set up.The y-rays, like the /3-rays, are not homo-geneous, there being a more penetrating anda less penetrating variety. This propertyrenders them very similar to the Rontgenrays, and as such they are now generallyconsidered. It is true it has been observedthat the conductivity excited by the y-raysin various gases is not proportional to thatproduced by the X-rays, and this seemed toestablish a difference in the nature of thetwo sorts of rays. But recent experimenthas shown that this dissimilarity in behav-iour depends solely on the fact that the y-raysin the aggregate are only comparable withthe most penetrating X-rays. In fact, theratio between the conductivity produced invarious gases by y and X rays tends towardunity, when a comparison is made betweenthe former and Rontgen rays which arefurnished by hard tubes and are made topass through a lead plate before arriving atthe gas to be ionized.


98/196

78 RADIO-ACTIVITYWhen the three sorts of rays strike ortraverse various bodies, they produce effects

which differ according to the nature of thebodies themselves. This is particularly evi-dent when one of the salts of radium is em-ployed

and certain of these effects haveonlythus far been observed with this substance.

It is not possible to completely isolate therays of one species from the others and tostudy separately the phenomena produced bythem ; but it is possible, by means of the in-terposition of absorbing plates to keep backthe least penetrating rays, such as the a, orthe a together with a part of the /3 rays, orfinally, to allow only the y-rays to passthrough ; and this is sufficient in many casesto allow the effects produced by the one orthe other to be recognized.The effects produced by radio-active bodies,and in particular by radium, may be classifiedas luminous, chemical, electrical, mechanical,thermal, and physiological. Besides what hasbeen previously stated, the following may besaid with reference to these effects.

Phosphorescence and fluorescence seem to


99/196

RADIO-ACTIVITY 79result especially from the action of the a andthe ft rays ; but certain bodies become morebrilliantly luminescent when they are struckby the a-rays, others when they are struck bythe j6-rays. For example, hexagonal blendeis particularly luminescent under the actionof the a-rays from radium.

Crookes has constructed a small instru-ment called the spinthariscope (44), by meansof which the effect produced by radium on ascreen coated with phosphorescent sulphideof zinc is observed. A small particle of theradium salt is placed at a distance of abouta half millimeter from the screen, which isviewed through a lens or microscope. Brill-iant points appear here and there and imme-diately become extinguished and thus givean effect of scintillation. According toCrookes each luminous point is caused bythe impact of a p

modern theory of physical phenomena, radio-activity, ions, electrons_a_righi

Documents