- 1 -

Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW) Citation: Becquerel, H. & Becquerel, J. & H. Kamerlingh Onnes, On phosphorescence at very low temperature,in:KNAW, Proceedings, 12, 1909-1910, Amsterdam, 1910, pp. 76-88 This PDF was made on 24 September 2010, from the 'Digital Library' of the Dutch History of Science Web Center (www.dwc.knaw.nl)

> 'Digital Library > Proceedings of the Royal Netherlands Academy of Arts and Sciences (KNAW), http://www.digitallibrary.nl'

- 2 -

( 76. )

same spol, WhCl'~ I fonnd a, feu stratum of 8 metres thickness, on 1\hLl'ch 20-21 Hl91 at Iltc time of Ihe I.Jzl'\RilIAN-expccliLion.

At tbe same time I tellCler m.r hearty tlk'tnks to my friend, General G. M. BUCKi\IANN, for his kind assistance, by which my' request was transmitted to Mr .. L G. I.JARIVl<\ throllgh the valnable intervention to the Resident of SlllI1atl'a's Easl coast.

Leiden, May 29th 1909.

Physics. - "On Plwsplw1'escence at veJ'y 1010 tempemtu1'es." By HENRI and JEAN BECQUEREL, allel H. KAlIiERUNGH ONNES. Oom­

munication N°. 110a from tbe Physical Laboratory, Leiden.

(Communicated in the meeting of Apul 23, 1909).

§ 1. lntrocluction. It is known tüa,t in the case of tüe salts of the rare em·ths both in their cl'ystalline f0I'111 and in solution, absorptioll spectra with particularly clear and fine tines can be obtained by the applicatlOn of very low temperatures, and that the investigation of the optical alld magneto-optical propertles of these substances at the tempel'atures which can be reached by means of hquid air alld liquid hyc1l'ogen opens up a new method for the study of seleetive absol'ption 1).

A pl'oblem, not less important than this, is that of the emission of light at temperatUl'es below th at of beginning incandescence. For certain substances, such as the umny1sa1ts ~), there exists such an inti­mate conl1ection between the bands in the emission spectrtt of their phosphorescence anel the bands of theit· abSOl'ptlOn spectm, that their distl'ibution in each spectrum follows the same ]a,w, so th at the bands in the one spectrum form the continuation of those in the other, while they have even two bandb in common. Any peculial'ity in the occurrence of one of the bands in any of the compounds pel'iodically l'ecUl'S in aU the other bands, as weU in the emission as ia the absol ption spectl'l}m.

Thub, when the absorption spectrum of tl,ny of the urany 1 compounds

1) JEAN BECQUEREL Compt. Rend. 1906, 1907, 1908, 1909. Ie Radium t. lV. p. 49, p. 107, p. 328, p. 38:3. 1. V p. 5; Physik. Zeitschr. l. V UI p. 632, p. 929.

JEAU BECQfTEREL ::md H. KAMERLINGH ONNES, These Proc. l·'ebr. 1908. p.592. Comm. ft·. the phYS1C. Labor. at. Leiden n". 103. Compt. Rend. t. CXL\l1 p. 625. Ie Radium t. V p. 227.

~) Em,WND BECQUEREL Mém. de I' Ac. d. Sciences t. XL. Ann de Ch. et de Phys. fle ser t. X p. à. HNNRI BECQUNREL Compt. RemJ. l. UI 1885 p. 1252.

- 3 -

( 77 )

changes when brollght under the influence of a very low temperature, the same change is also to be expected in the emission spectrum of its phosphol'escence. This expectation is fully Suppol'ted by our experimenls. The more or less bl'o~"td anel diffuse banels which follow each other in regulal' sequence in the spectra of the uranyl salts at orelinary tempel'ature, are, at the temperature of liquiel air, resolveel into mnltiple banels, which, in some cases, are pal'ticularly fine and intense; in that respect, all the banels in both emission anel absorption spectra nnelergo ielentical -changes. The appearance of these spectra at liqlliel air tempel'ature has all'eaely been elescribed 1). At the Leielen cl'yogenic laboratory we have extendeel the investigation of these phenomena to the temperature of solid hyelrogen (140 K.), and the most important of the results tlms obtaineel concerning emission spectra at very low temperatures are here published.

§ 2. ExpB1'imental met/wd. The apparatus here Ilsed fol' the in­vestigation of the spectra of phosphorescence was the same as that which had already served for the investigation of absorption spectra, and fol' a description of which we refer to former papers 2).

Here we have only to mention that the source of the spectrum was a ROWLAND plane gl'ating with a lens of 1.30 m. foral length arranged in anto-collimatlOn, and that Ihe phosphorescent substances, encloseel in thin-walled glass tubes, were placeel in a non-silvered vaCUllm vessel containing liqnid hydrogen; in the latter case the glass containillg liquid hydt'ogen was pItteed in a second vacuum vessel with liquid air so as to rednee the vaporisation of the hydrogen. The light of an arc-lamp, flltereel through an ammoniacal solution of copper sulphate, served to illuminate the salts.

§ 3. BeTtavioul' of tTte spectm wlten t!te tempemture is lowel'ed. TTte displacement anc! t!te lil1ûting position of tTte pTtosp/wl'escence bands of u1:anyl salts.

When uranyl salts are cooleel to lt temperalU1'e of 14° K. (soliel hydrogen), the lustre which they show at orelinary tempel'ature is not diminished anel vanishes with tlle illumination. Rence, lowe1'ing of tTte te1T/,jJemtu1'e of 'llmnyl salts in no way jJ1'events n07' p1'olongs 01' shol'tens 3) thei1' emission of light.

The bands which, between ol'dinal'y tempel'atul'e anel that of liquid air a]ready undergo subdivision, become groups of mnch finer lines at the tempel'atUl'e of liquid hydrogen.

I) HENRI BECQUEREL Compt. Rend. t. CXLIV J 907 p. 459 and p. 67 f. 2) Proc. 29 I~ebr. 1908; Comm. Phys. Lab. Leiden N0. 103. Le Radium t. V,

p. 227. ~) Added in thc lranslation

--------~--=~-

- 4 -

, I

( 78 )

Fig. 1 which gives a part of the spectrum of the double sulphate of potassium and uranyl at temperatures of 2880 K (ordinary tempe­rat ure) , 800 K. (liquid nih'ogen) and 200 K-: (liquid hydroge~1), shows this very clearly. Tbe group in tlle bluish green is very weak, bnt three groups of bands in the green are very f:Strongly developed. The mmmer in which the broad, diffuse patches at ordinary temperature are resolved at the lower temperatures into groups of bands is here strikingly shown. Figs. 4 and 5 (obtained with the second spectrum of the grating) give the details of some groups of bands of the same salt. Some bands which still remained complex at 800 K. have been resolved between 800 and 20°K. The spectra a and b of fig. 1 were obtained alongside each othel' upon the same plate by projecting the image of the phosphorescent salt upon two different parts of the slit in succession . From them we can see that by lowel'ing t/te temperature the emission-maxima a1'e disjJlaced fowa1'Cls the smaller wave-lengths. This has al ready been obsen'ed 1), but it was not then shown that the displacement of the maxima was the l'esllIt of a change in the period of vibration; it could a,lso have heen the result of an unusllal strengthening of bands which existed already at ol'dinary temperature bnt had then a very small intenE>ity. Tlle observation of thl3 spectrum at temperatures at which the ballds are clistinctly separated from each other shows that we have here to deal with a displacement resulting fi'om the lowering of the temperature. The following measnrements, obtained by eomparison with the iron spectrum, show a distinct elisplacement of the bands in the case of the double sulphate of nranyl anel potassium bet ween 80 0 K and 20° K.

80° K. 200 K.

p.p.

511.48 511.35

p.p.

534.24 534.10

lIP 1'1' 559.09 586.31 558.89 586.05.

Between 80° K. and 20° K. the ratio of the displacement to the temperature diffel'ence expl'essed in degl'ees is much smaller than between ordinary tempera.ture (288° K.) and 80° K. (It is then ti'om 2 to 31'1').

The displacements obtained by fnrther lowering of the temperatme fi'om 200 K. to 14° K. are scarcely noticeable (fig. 5 a anel h), as was to be expected by extrapolation paying attention to the small diffel'ence in degl'ees bet.ween these tempel'atures. I The order of magnitude is fi'om 0,01 to 0,02 !-L~t. Hence, it is ve1'J! possible tltat as the tempemtul'e is lowel-ed, tlte bands app1'oach asymptoticall;lj a b:müi-n.1J ZJos7:tion. Theil' change, thel'efol'e, is of the same nature as the change of vohllTIe and, pel'haps, as (he change of dielectl'ic constant.

1) HENRI BECQUEREL1 Campt. Rend. t. CXLIV 1007 p. 671.

- 5 -

( 79 )

~ 4. COJnpa1'ison oJ the d~tJèl'ent gl'OZtpS oJ bands oJ tlw same salt.

The photographs of Pl. I anel II show bettel' than any descl'iption the spectra, of a number of uranyl compounds. The data fol' the separate spectra are given witb the explanat,ion of the figlll'es.

The speètrum of a definite salt consists of different groups (we observed from 7 to 8), which though not quite the same, still show a very small difference uetween sucêessive groups. OnIy the m05t l'efl'angible group, some b~nds of which, as we shaH see later, belong both to the emission and absol'ption spectrum, is somewhat different from the othel::;. We shall call t110se bands homo1ogous which occupy the same relath e position in the different groups. Snch, for instance, are the bands whose wave-Iengths are marked on fig. 7. (Double acetaLe of uranyl and sodium).

The d~lf'etence in the alJpearance oJ tlte dijje7'ent gl'oups is the 1'etntlt oJ successiüe ascending changes in t/te l'elative intensities oJ t/te bands in these g1'OUpS. This can be seen in all the figures, especially in fig. 7. The banel 566/J I' .34 (fig. 7) is the most intense in the least l'efl'acted group visible in the figul'e; in tbe gl'OUpS succeeding this olie towards tbe side of tbe smaller wave-lengths, th€' homologous bands 540/J

/J45. 516,:J I'-78, 495.1JI'02 become steadily weaker, anel

in the b1ue group the band which should be homologous with the foregoing seems to have clisappeared.

On the other hand, the bands which O(,Clll' on eithel' side of the above-mentioned homo10gous series, pl'epondertl,te in the more refi'actecl portion of the spectrum, but beeome less distinct as the greatel' wave­lengths are approachecl.

The spectra of uranyl sulphate has the same general appeal'anee quite independently of whetber it is combinecl with othel' slllphates Ol' not. (See figs. 1, 8, 9, 10). It was noticed 101lg ago 1) th at the stl'ueture of tbe spectrum depends ehiefly upon tIte acid with which the salt is formed, anel is little influenceel by tlle othel' base in the double salt. But this is shown much more elearly by observations a.t temperatures at which the bands no longer overlap.

§ 5. Law of succession oJ tlw bands. One of the most important pl'oblems is 10 ascertain the la.w eonnecting the frequencies of Sl1C­cessive hamis. Obsel'Yaüons at orelinal'y temperature had already ShOWll that the broad bands (sueh as those of fig. ia) were at such c1istanees apart, that the c1ifferencc between tbe fl'eql1eneies ofsnccessive

1) Em.rOND BECQUEREL, Mém. de l' Acad. d. Sc. t XL p. 8 aud 15. \

~ -,,--- ~ -::..=-- -=="""""

- 6 -

( 80 )

TAB L E.

Double sulplwte of uranyl and lJotas~ium. SerIes U1 winch tbe stl'ongest ballels OCCUl', at 20° K.

PP PP PP PP PP \ I. 4901.34 - 511.35 - 534,.10 - 558.89 - 586.05

ILl ~ XI0 7 837 833 830 829 À.

The same serie& at 80° K. PP PP PP PP PP ,J. 1 4904,2 - 511.48 - !J34.24, - 559.09 - 586.31

ILl - XI0 7 836 833 831 830 I.

Double sulphate of umnyl ancl sodiwn. Series of strong bands 80° K.

~P PP P' PP

I À. 489.15 - 510.12 - 533.00 - 557.85

1 ' Ll-XI07 tlJO 8·U 835

)

Anothel' serIes of strong bands at 80° K. PI' ,"P PI'

512.54 - 535.47 - 560.46

837 833

Dottble sulphate of lt1'anyl and ammonium. SerIes of strong bands at 80° K.

PI' PI' PI' PI' Ijl' p-p \ I. 1 491.26 - 512.45 - !'i35.54 - 560.74 - 588.11- 618.40

ILl - XI0 7 842 841 839 829 833 À.

Another series of strong band& at 80° K. p-p 1'1' PI' PI' PI' I'P-I I. 493.4 7 - 514.78 - 538.01 - 563.33 - 590.80 - 621.57

ILl ~ X 1 07 839 839 835 825 838 À.

UranylsuqJlwte. Series of strong bands at 80° K:

~ " H " ~ ~ \ 1 1 491.83 - 513.39 - !'i36.94 - 562.68 - 591.01- 621.95

ILl - X 1 07 854 853 852 tl52 844 I.

- 7 -

( 81 )

Anothel selles of strong bands at 80° K.

1'1' /'1' 1'/' 1'1'-516.02 - 539.58 - 565.40 - 593.88

843 850 848

Umnylnit1·ate.

SerIes of the sil'ongesL b,tnds at 80° K. I' I'- I' I' 1'-1' 1'-1'

506.96 - 530.15 - 555.46 - 583.29

862 859 858

Double acetate of umnyl ancl sodium.

Six series- at 80° K.

1'-1' 1'-1'- 1'-1'- p.p. 1'-1'-

\). 1 473.25 -=- 493.25 - 514.86 - 538.45 - 564.27

/6 -XI07 855 852 851 850 À

1'-1' 1'-1'- 1'-1' 1'-1'-

495.02 - 516.78 - 540.45 - 566.34

851 848 846

1'1' 1'-1' 1'-1'- 1'1' 1'-1'-

\). 1 47623 - 496.49 - 518.49 - b42.40 - 568.58

/6 --;;- X 107 857 855 849 849 .iI.

" ~ ~ ~ ~ \). 1 476.92 - 497 2J - 519.22 - 543.16 - 56g.46

/6-X107 856 853 848 850 À

1'-1'- 1'-1'-

\ À 1 478.58 -- 499.02

/6 -X107 856 À

1'-1'- 1'-1'- 1'-1'- 1'-1'-

\ l 1 479.46 - 499.92 - 522.14 - 546.35

/6 -XI07 854 851 849 ).

6 s ' Royal Acad. Amsterdam. Vo 1. XI 1.

- ---- --~ -==-. ===

- 8 -

( 82 )

bands was almost constant 1). But as the bands at th is tempcrature are broad, diffuse, and complex, it wa.s not possible to determine the accuracy of the relation. In the spectra obtained at low temperatures (he wave~lengths could be measured; ·this was done by compal'ison with the iron-spectrum wbich was photogl'aphed upon the same plate as the phosporescence iSpectrum (see, for instance, tigs. 6, 7, 8, 10).

A high degree of accuracy is possible in these measurements, partif!ularly at the temperature of liquid hydl'og'en: in the case of the double' sulphate of uranyl and potassium (fig. 6) and of tl1e double acetate of sodinm and uranyl (fig. 7) the error in the position of the principal bands is not more than ± 01'1' ,03. 'The table given on pp. 80 and 81 contains the results of the measurements for the bands of certain salts, homologous bands being placed upon tile same line.

TlJe values of wave-Iengths given above were measured in air, but the differel1ces between the reciprocal wave~lengths are l'edtlCed to vacnum, so that the values givel1 for these differences are pro~ pOl'tioual to the frequencies.

From the table it appears that t/te d((fel'ence between t/te freq'l.6en­cies of two successive lwnwlogous bands is pmctically constant, not only f01' the sanw, but also for all se1'ies of lwmologous bands of t/te same salt. PUl'thé1', the values of tMs constant f01' the va1'ious salts dijJer but slightly f1'om eac1L otller.

As regards the position of the bands, the spectrum of a uranyl salt is thl1s represented on the scale of freql1encies by successive ' idel1tical gl'OllpS at a,n equal distance from each other. The deviations ' fl'om this law are, as a general rule, so smalI, that they eau be ascl'ibed to errors in the meaSUl'ements of the wave-lengths; never­theless thel'e is a tendency towal'ds decrease in the fl'equency-diffe­ren ces with incl'easing wave-Iength, and it is possible, and even probable, that t11ere is a deviation from the Iaw in this direction, although in that case, the deviation is extremely smalL

In the fOl'egoing we have recalled the influence of the acid upon the strnclUl'e of the spectrnm; now we must eaU attention to the fad th at, although these spectra are also spectm of nwlecules, which diffcl' in the val'ious salts, )1et their charader is due io the uranium, which subjects the spectra of all the salts to the law just mentioned.

The' law according to which the homologous bands correspond with equal differences between the frequencies is the simplest that could connect successive lines in the spectrum. --------~ \

1) EDMOND BECQUEREL loc. cito § 4 etc. HENRI BECQUEREL Compt. Rend. t. Cl 1R85 p. 1259.

- 9 -

( 83 )

In this connection we may recall thai lhi::; Jaw, regarded as holding with close approxima,tion,. had long ago dl'awn aUention to the nrany 1 salts in particular as being, from the point of view of optical phenomena, of remarkable molecl1lar stl'uctnre. This formed the starting point of the first expel'imcnts which ware undertaken with a view to di13covering rays analogous to those w hich had been observed in vacuum tubes. 1) They led to the discovery of radioac­tivity.

§ 6. Charactel' ancl nature of tl~e plwsplwrescence spectm of the 'uranyl sa lts.

We have seen what t.he law is that govems the succession of the homo10gous bands in the various groups. Is, nOVir, lhe disposition of the bands in the same group governed by a pal'ticular law? In the most intense pOl'tion of the green of the spectrum the bands' are numerous, especially at the temp61'atul'e of liquid hydrogen, forming a system in themselves. But l'egal'ding the bands at tile ends of the spectrum, we find, especially in the last gl'Ol1p in the red, that they become simpIer, and in many cases thera remains O'nly a succession of bands lying close togethel' and showillg marked similal'ity with the gl'onps of bands in tbe channeled spectra of gases (llitl'ogen, carbon) when these are observed witll a weak dispersion. In the least refrangible gl'OUpS in the spectrum of llrany I sulphate (fig. 10) this is particularly well mal'ked. This gl'OUp consists of a stl'ongly developed head towards the side of the small wave-lengths folIo wed by seven Ol' eight bands at l'egulal' distances from each other, and decl'easing regulal'ly in intensity as thoy go further way fl'om the head. Each of these bands is a little asymmetrieal, fol' tlle side towards the smaller wave-lengths is more sba1'ply defil1ed than the other. The blue group in the spectrum of the same salt (fig. 11) is a.lso a chal1ueled spectrum, but it is not so extended and seems to be formed out of VttriOllS series of overlapping bands.

The bands are not sufficiently fine and especially they are not sufficielltly numerous for us to examine if the sllccessi"e bands in the same group and the initial bands in the val'iol1s gl'onps follow DESLANDRES' ln,w. Nevertheless this law l'eceives as mllch support as might be éxpected especially ti'om the groups in the Ol'ange and red in the case of the simple sulphate.

It seems to us, thel'efore, thai these spectra aee channeled speclra of the same character as t11e band spectra of gases.

1) HeNRI BECQUEREL, Compt. R~l1d. 'CXXII, p. 420. (1896).

- 10 -

( 84 )

We have investigated if a magueLic field has any influence .upon the spectra of the uranyl salts. In a ,field that co~dd exceed 20 Kilo­gauss in value, no influence even upon the na1TOW bands at 14° K. (solzd Itydrogen) was to be obsel'vec!. This negative result forms a new analogy between these spectra and the band spectra of gases, which likewise, in the ma:jority of cases undergo no change in a magnetic field, although the strength of ihis is raised to ihe highest value at present possible 1).

§ 7. Relation between the emission by plwsplwrescence and the abso1'ption of the uranyl sa lts. Reve7'sible anc! i?'I'eversible bands.

In the foregoing we have recalled the existence of g'roups which contain at the same time emission and ausorption bands. Ol'ystals of autunite (donbie phosphate of uranyl and calcium), a mineml fr om which very transparent plates may be obtained by cleavage, are especially suitable for the study of absorption.

Fig. 12 shows the emission and absorptiön spectra in the blue and green region of the spectrum of a plate of autunite at [t temperature of 14° K. (solid hydrogen). 1t can be seen th at two bands lying very close beside each other are coml11on to both spectra; a is weil developed both as an emission and absorpiion band, ~ is strong as an emission and weak as tUl absorption band.

Fig. 13 gives the spectrum of autllnite at 80~ K. In can be shown experimenlally in a stl'iking mannel' that some

bands can arbitl'arily become emission Ol' absorpLion baneIs. For this purpose an intense beam of light is transmitted through the crystal plate and the position of the ab&orption band fixed; the intensity of the transmitted light is then diminished anel at the same time the plate is iIluminated by violet light whose intensity al ways incl'eases. The cla?,k band is seen to be tmnsformed into a light band occupyin,CJ exactly the same position. This experiment, which becOlues possible at the lowest tempel'a(ures, resembles the classical expcwiment of the reversal of the lines in the case of the sodium-flame.

The bands in the less refmngible groups cannot oe reversed; they all belong to the emission spectrum.

The markecl difference between the gl'OUpS which are common io both spectra, and those which belong only to the emissiOl1 spectrum, ean be attributed to the revel'sible bands whose relu.tive intensity,

1) This, ho wever, is not generally the case. As is known, A. DUFOUR has found numerous bands in the spectra of the chlorides anel tluorides of the alkaline earths which show a ZEEMAN effect, and one of us has also observed this effect in the

yttrium·bands.

- 11 -

( 85 )

compal'ed with that of the othel' emission bands, is much modified l

by this property. The experiment concerning the reversal of certain bands of the'

uranyl sa lts is the first pro of that cel'tain emission bands occupy exactly the same position as the absorption banels. Further, the irre­versible homologous bands of tbe other gl'OUpS cOl'l'espond with vibration-fl'equencies of the absorbed light separated froIll these bands by constant differences.

There seems, therefore: to exist an immediate and simple relation between absorption and emission in the uranyl salts.

We may recall that the manosalts, wlticli do not plwspho1'esce, have absol'ption spectra in which the bands with mal'ked regularity follow the law of succession of the emission bands of the phosphor­escent uranyl eompounds -').

§ 8. Plwsphol'escence of the sulpllUl' compounds at low temperatzwes. We have made but very few observations of the phosphorescenee

at low temperatures of phosphorescent substa,nces other than the uranyl salts. As regards sulphul' compounds of the alcaline earths we conld adel nothing to the masterly l'esearch of LENARD anel KUTT 2).

Zinc sulphide is very luminons when exeited by violet light at 80 K. allel remains lllminous for a long time aftel'Vv~rds; 4 or 5 seconds aftel' excitation eeases, the stronger raeliation comes, to an end, a,nd then for a long time a,fterwards the zinc sulphide shines with a soft glow. From the moment that warming begins as it returns to ol'c1inal'y tempel'atul'e, zinc slliphiele sbows a pal'LÎcularly s11'0ng lustre.

At hydrogen tempel'alures lhe light emitteel c111ring excitation, and by ,varD1Îng aftel' excitation, becomes le&s.

The coloUl' of the light eOllling from zinc sulphide seems to be the same at various temperatl1l'e8, anel between 80') IC and ordinal'y temperalql'e no change can be seen in the spectL'lllTI, whieh was not unexpecteel seeing that Lhe spectrum was continuous.

§ 9. Conchtsion. The uranyl salts must be regal'ded as a class by thernselves shttl'ply ddfel'entiaLed from othel' classes of phosphorescent substances.

From the researches of Ll!;cocQ. DE BOISBAUDRAN, LENARD and KLATT,

alld UltBAIN on the rare eartlU:l, the sulphul' eomponnds ofthe alkaline

1) HENRI BECQUER:mL Uompt. Rend. t. Cl. 1885, p. 1252. 2) P. LENARD and V. KLATT, Ann. d. Phys. iV. 15. 1904, p. 225-282 and

425-672.

- 12 -

r-'~'-------~---- ---

( 86 )

earths, and the oxides of the earth metals, it is lmown that these substances in the pure state do not phosphoresce; and :iJ appears that the influence of impurity, as was - first 110ticed by EDlIWND

BECQUEREL, is essential to the fOl'mation in those substances of complex molecules, LENARD'S "centra", which, as dlstinguished from the molecules of the substancea themselves, have the power of emitting light.

It is completely differenl with the salts of uranyl. These compQunds always phosphoresce, even wh en quite pure, and each of them is characterised by a specifically distinct emission spectrum.

And in other respects the uranyl salts diffel' from those just mentioned as phosphol'escent only by the influence of admixtUl'e. Even at a tempel'atme so low aS that of liguid hydrogen, in intensity the emitted light does not alter and vanishes with the illumination 1).

The uranyl salts do not exhibit thel'moluminescence. Finally, the spectra of these salts are of remarlmble structure, while an immediate relation exibts between the emisslOn and absorption spectra.

These optical phenomena at low tempel'atures, by w hich conside­rabIe light is thl'own on 111e characteristic properties of the phosphor­escence of uranyl salts, thus lead 10 a distll1ction between two kinds of phobphorescence, which seem to be of different origins. On the one hand, in tile case of the rare eal'ths, the sulphur compounds of the alkaline eal'ths, and the oxides of the eal'th metals, according to the beautiful tbeo1'Y of LENAHD 2), tlle electrons, which, unde1' the photo-electric influenre of the absorbed light, are ~jecteà by the a10ms of the metal and by their return to these cause phosphol'escence are supplied by that metal of which a small qualJtity is added to the experimental su bstance so as to cause 11 to phosphoresce by the formation of "centra", of which the structure of the molecular rom­pi exes play an important part in the phosphorescence. On the other hand, following Îlr the track of the same theo1'Y, the explanation of the phenomena exhibited by uranyl salts must be sOllght in the motion of the electrons contained in tile uranium atom itself and in the structure of the molecules of the substance itself.

In this case especially considel'ing the small obstruction cold offers to the return of the electrons 3) the question obtrudes itself if, perhaps, the whole process of phosphorescence takes place within the uranium atom, or at least within t11e uranyl gl'OUp, and electl'ons, undel' the photo-electric influence of the absorbed light, al'e projected from a part of thiR atom to other parts, remain there temporarily, and, by

1) Added in the translation. 2) P. LENARD, Verh. d. Naturhist. Med. Ver Heidelberg 5 Febr. Hl09. 3) 'l'he emission vanishing with the illumination. (Added in the transjation).

- 13 -

HENRI and J eAN BECQUEREL and H. KAMERLINGH ONHES : "On ph".·

pho.e lCen<e at very low I<mp ... lu, ... "

• • , ,

• • ,

• • , • , I ,

• • ,

• • ,

• •

~~ Ir' ~ -- - --I I , , .... "

I .. J IJl I [ , , ,

":'"

, .... t<

Fig. 3.

Fi~. 4. .,,~. ,

- I " • --- - , -

...... ,

, "'~I

. , . • ,. '1 ., -,-----, --,

.... tI

Fig. 6 .

, -,-

I'rooeedin". nortl M.ad. Am>ltn.lo.m. \'uI. XII.

PI. J.

2SI! ' K.

~

- 14 -

HENRI an d JEAN BECQUEREl ond H, KAMERLING" ONNES, "On pho • •

• •

phoru« n« ot " ~ ry lo l" t~ mp~rat ur • • . "

" •

" • ,

• •

• • ,

, .... 1>

, .-" .. " ,

, , ...... "',0>

Fof. 1 .

Fig. 6.

fi g, 9.

, _.

• • 'I I ,

Fog, 12. ., _ .. • •

FiK. 13.

, -

-,_._. -- .-._---

•

PI . 11 .

80" K .

14' K.

- 15 -

( 87 )

retnrning to their original position of equilibrium, cause the emission of phosphorescent light. The motion of the electrons causing ph os­phorescence, and therefore, th~ position of the bands in the spectrum will, of course, llndergo different modifications for the various salts by the inflnence (of the electric field) of the other atoms, which with the atom or uranium form a complete molecule.

EXPLANATION OF THE FIGURES.

Fig. 1. Potassium zt1"anyl sulpha,te a) at 288" K, b) at 800 K, c) at 20° K. First spectrum, ROWLAND grating. The spectra a) and b) photographed beside each other on the same plate allowadireet comparison of the positions of the bands at ordinary temperature and at thaL of .liquid nitrogen.

Fig. 2. a) Autunite (Calcium uranyl phosphate). b) Pota8sium ztranyl chloride. c) Uranyl nitrate. d) Pota8BÏum uranyl sulphate. Temperature 80° K. First spectrum, ROWLAND. grating.

Fig. 3. a) and c) Potas8ium uranyl chloride. b) and f) Uranyl nitrate. g) Potassium uranyl sulphate. d) Autunite (calcium uranyl phosphate). Tempent1ure 140 K. Second spectrum, ROWLAND grating. These photos

, give the details of tbe groups at the temperature of solid hydrogen. Fig. 4, and 5. Potas8izwn ztranyl szûphate.

a) at 140 K., b) at 20?K., c) at 800, K.

Second spectrum, ROWLAND grating. Details of the groups at these tbree temperatures.

Fig. 6. Adjacent spectra of a) potassium uran1'l sulphate at 20° K. and b)

Fig. 7.

Fig. 8.

Fig. 9. Fig. 10.

. the iron arc. Second spectrum, ROWLAND grating. Example of the measurement of wave-Iength; short exposure so as to make an accurate measurement possible. Sodium uranyl acetate at 800 K. First spectrum, ROWLAND grating. Mcasurement of wave-Iengths, and details of the groups. a) 11'on (trc, b) Sodium uranyl szûphate, c) Ammonium uranyl sulphate. Temperature 80° K. First spectrum, ROWLAND grating. Measurement of wave lengths, and details of the groups. Uranyl sztlphate at 800 K. a) Iron arc, b) Uranyl sztlphate. Temperature 80° K. First spectru,m, ROWLAND grating. Channeled spectrum in the yellow and orange.

- 16 -

( 88 )

Fig. 11. Uranyl sulphate. Tempel'ature 80° K. Second spectrum, ROWLAND grating. Channeled spectrum in the green·blue.

Fig. 12. Absorption and emission of autunite (calcium uranyl phosphate) at 14° K. Second spectrum, ROWLAND grating. a) emission spectrum, b) absorption spectl'um, Cf- and~, bands common to both spectra (reversibie bands).

Fig. 13. Absorption and emission of autunite at 800 K. First spectrum, ROWLAND grating. a) and b) two of the chief absorption spectra of the crystal plate, c) phosphorescence !'pectrum, ct) reversible band.

ER RAT U M.

In the Proceedings of the Meetjng ot Oct. 31, 1908.

p. 335 line 5 from the bottom: for Pt' d read Ptd. the head of Table I read: Compal'ison beiween Pt'l and Pt"d'

p. 341 line 1 from the boHom: fol' 1807 read 1907.

(June 25, 1909).