A N T O I N E H . B E C Q U E R E L On ra dioactivity, a ne w pro perty of matter

Nobel Lect ure, Dece mber 11, 1 9 0 3

T h e s u bj e ct w hi c h I pr o p os e t o s p e a k t o y o u a b o ut h as b e c o m e, i n o nl y a f e w y e ars, s o v ast t h at i n or d er t o d e al wit h it i n a si n gl e le ct ur e I s h o ul d have to confine myself to listing the main facts follo wing the chronological or der of their discovery. But as M. an d M me. Curie are to describe to you t h eir fi n e w or k o n r a di u m, I will si m pl y s u m m ari z e t h e s u bj e ct a n d gi v e so me acco u nt of my o w n researc h. At the beginning of 1896, on the very day that ne ws reache d Paris of the ex peri ments of Röntgen an d of the extraor dinary pro perties of the rays e mit- te d by the phos phorescent walls of Crookes’ t ubes, I tho ught of carrying o ut researc h to see w het her all p hos p horesce nt material e mitte d si milar rays. T he res ults of t he ex peri me nt di d not j ustify t his i dea, b ut i n t his researc h I e n- countered an unexpected pheno menon. Of all t he p hos p horesce nt materials, ura ni u m salts see me d partic ularl y s uitable for t he i nvestigatio ns, beca use of t he exce ptio nal str uct ure i n dicate d by the har monic series of the bands making up their absorption and phos- p horesce nce s pectra. T h us I place d s heets of do uble s ul p hate of ura ni u m a n d potassiu m on photographic plates envelope d in black paper or protecte d by a s h e et of al u mi ni u m a n d e x p os e d t h e m t o li g ht f or s e v er al h o urs. O n d e- velo pi ng t he plates, I fo u n d t hat t he ura ni u m salt ha d e mitte d rays w hic h re- pro d uce d t he sil ho uettes of t he cr ystalli ne s heets t hro u g h t he blac k pa per a n d vario us scree ns of metal or t hi n glass lai d o n t he plates. U n der t hese co n ditio ns t he p he no me no n co ul d be ascribe d to a tra ns- for mation of solar energy, like phos phorescence, b ut I soon recognize d that the e mission was in depen dent of any fa miliar source of excitation, such as li g ht, el e ctri cit y or h e at. We were t h us face d wit h a s po nta neo us p he no me no n of a ne w or der. Fig ure 1 sho ws the first print, which reveale d the s pontaneity of the ra diation e mitte d by the uraniu m salt. The rays passe d through both the black paper w hi c h e n v el o p e d t h e pl at e, a n d a t hi n s h e et of c o p p er i n t h e s h a p e of a cr oss. Fi g ur e 2 s h o ws t h e r a di o gr a p h of a n al u mi ni u m m e d al; t h e n o n- u nif or m absor ption of the ra diation by the different thicknesses of metal reveals the Fi g. 1. effi g y. As t he ura ni u m salts use d ha d bee n pre pare d a ver y lo n g ti me before- hand, it was to be supposed that the intensity of the pheno menon was in- de pen dent of ti me, an d hence that the e mission was constant. All the later experi ments have sho wn that the activity of uraniu m does not di minish ap- pr e ci a bl y wit h ti m e. I n obtai ni ng t hese first res ults I notice d t hat t he ra diatio n of ura ni u m dis- charge d electrically-charge d materials locate d so me distance a way, an d this pheno menon provided a second method of studying the ne w rays. The pho-

Fi g. 2. 5 4 1 9 0 3 A . H . B E C Q U E R E L togra p hic met ho d was pri marily a q ualitative o ne, t he electrical met ho d gave nu merical data, and the early measure ments revealed the constancy of the r a di ati o n wit h ti m e. T h e t w o m et h o ds s h o w e d t h at all ur a ni u m s alts, w h at e v er t h eir ori gi n, e mitte d ra diation of the sa me type, that this property was an ato mic prop- erty co n necte d wit h t he ele me nt ura ni u m, a n d t hat metallic ura ni u m was a b o ut t hr e e a n d a h alf ti m es as a cti v e as t h e s alt us e d i n t h e first e x p eri m e nts. A sphere of charged uraniu m, which discharges spontaneously in the air u n d er t h e i nfl u e n c e of its o w n r a di ati o n, r et ai ns its c h ar g e i n a n a bs ol ut e vac u u m. T he exc ha nges of electrical c harges t hat take place bet wee n c harge d bo dies un der the infl uence of the ne w rays, are the res ult of a s pecial con- d uctivity i m parte d to the s urro un ding gases, a con d uctivity that persists for several mo ments after the ra diation has cease d to act. These funda mental properties of the radiation e mitted by uraniu m were verifie d later b y n u mer o us i n vesti gat ors; of t hese I will o nl y me nti o n R ut her- for d, w ho esta blis he d t hat t he co n d ucti n g pro perties of t he gases t hro u g h which the ra diation of urani u m passes, are co m pletely eq ual to the ioniza- tio n ca use d by ot her factors. D uring the co urse of these first ex peri ments, the observation of a n u mber of pheno mena that were hitherto unexplaine d le d me asi de fro m the path to w hic h my later ex peri me nts were to bri ng me back. Vario us sa m ples of p h os p h or es c e nt c al ci u m s ul p hi d e r esti n g o n s m all pl at es of gl ass a n d c o v er e d wit h a s mall bell-jar of glass ha d bee n lai d o n a p hotogra p hic plate protecte d b y a s h e et of al u mi ni u m 2 m m t hi c k, as s h o w n i n Fi g. 3. T h e pri nt d e v el o p e d after 48 h o urs ( Fi g. 4) reveale d sil ho uettes of t he plates of glass, re pro d uce d wit h t he details w hic h wo ul d ha ve bee n pro d uce d b y refractio n a n d total

Fi g. 3. R A D I O A C T I V I T Y . N E W P R O P E R T Y O F M A T T E R 5 5

Fi g. 4. reflection of the light rays. Altho ugh at the sa me ti me Troost an d Nie wen- glo wski observe d the e mission of penetrating rays by phosphorescent mate- rials, the above experi ment coul d not be repro duce d; the cause of the mo- mentaneo us a p pearance an d the disa p pearance of the activity of these pro d- ucts is u nk no w n. B ut si nce, i n t he ex peri me nts wit h ura ni u m, t he sil- ho uettes of all t he scree ns p ut i n bet wee n are e dge d wit h w hite li nes o utli n- ing projecte d sha do ws, an d si milar to those of the print which we have j ust me ntio ne d, I was le d to attrib ute t he pro perties of lig ht to t he ra diatio n fro m urani u m, while all later ex peri ments have sho wn that this ra diation cannot be reflecte d or refracte d li ke li g ht ra ys. I n 1898 Sc h mi dt a n d M me. C urie observe d al most si m ulta neo usly t hat t hori u m has pro perties si milar to t hose of ura ni u m; t hese pro perties were st u die d by O we ns a n d by R ut herfor d. M me. C urie, after meas uri ng, by m e a ns of t h e i o ni z ati o n of air, t h e r a di o a cti vit y of a l ar g e n u m b er of mi n er als containing uraniu m or thoriu m, noted the re markable fact that so me mine- rals were more active than metallic urani u m. After having verifie d the fact that the activity acco mpanie d the uraniu m molecule in its various co mbina- tions, M. an d M me. Curie conclu de d fro m this that there must be present in the minerals mentione d a substance more active than uraniu m, an d they set a b o ut is ol ati n g it. T h e y tr e at e d o n e of t h e m ost a cti v e of t h es e mi n er als, Joachi msthal pitchblen de, an d first separate d fro m it active bis muth which they supposed contained a ne w substance, poloniu m, and then soon after- war ds t he y o btai ne d extre mel y acti ve bari u m, associate d wit h a ne w ele- ment, ra di u m. These pro d ucts were obtaine d by fractional crystallizations in co nj u nctio n wit h electro meter rea di ngs. T he acti vity of t he pro d ucts i n- 5 6 1 9 0 3 A . H . B E C Q U E R E L crease d with their richness in ne w ele ments. The activity of p ure ra di u m is abo ut one million ti mes greater than that of urani u m. Giesel has s uccee de d in obtaining very active pre parations, an d Debierne has st u die d a pro d uct i nti mately associate d wit h t hori u m, w hic h he has calle d a cti ni u m. Of t hese differe nt pre paratio ns, ra di u m alo ne has t he c haracteristics w hic h we associate with si mple substances; it has an e mission spectru m for me d of lines not belonging to any other kno wn ele ment, and the molecular weights of ra di u m salts i ncrease wit h t heir ra di u m co nte nt. The intensive ra diation of ra di u m excites the phos phorescence of different materials a n d rest ores t he a bilit y of cr ystals t o p h os p h oresce u n der heat, w he n t h e y h a v e l ost t his as a r es ult of a pr e vi o us ris e i n te m p er at ur e; m or e o v er, ra di u m salts beco me l u mi no us s po nta neo usly u n der t he i nfl ue nce of t heir o w n r a di ati o n. The first sa mples of poloniu m and radiu m that M. and M me. Curie were goo d eno ugh to len d me, reveale d a striking difference in the nat ure of the ra diatio n e mitte d b y eac h of t hese s ubsta nces. T he y were p ut i nto s mall pa per cylin ders close d at the botto m by very thin flakes of mica or al u mini u m, an d the cylin ders were place d on a photogra phic plate. The print in Fig. 5 sho ws t h at t h e r a di u m r a di ati o n h as e asil y p ass e d t hr o u g h t h e v ari o us e n v el o p es while the poloni u m ra diation has not penetrate d the wall of the pa per cyl- in der; th us the poloni u m rays are only slightly penetrating. To war ds the en d of 1899, first Giesel an d then Meyer an d von Sch wei dler observe d that the ra diation of active pre parations is deflecte d by a magnetic

Fi g. 5. R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 5 7 fiel d. At the sa me ti me, una ware of these experi ments, I note d the sa me in the case of ra di u m ra diation, after first having note d that in a non- unifor m fi el d t h e r a di ati o n is c o n c e ntr at e d o n t h e p ol es, as ar e c at h o d e r a ys i n t h e ex peri me nt of Birkela n d. Fig ures 6 a n d 7 s ho w so me pict ures w hic h were o bt ai n e d i n t h e f oll o wi n g w a y:

Fi g. 6. Fi g. 7.

Se veral grai ns of ra dio-acti ve matter are p ut o n a p hoto gra p hic plate e n velo pe d i n blac k pa per a n d place d horizo ntall y bet wee n t he poles of a magnet; on developing the plate after a fe w mo ments’ expos ure, it will be see n t hat besi de t he mar k i n dicati n g t he p ositi o n of t he acti ve s o urce a str o n g i m pression has been pro d uce d d ue to the ra diation reflecte d by the fiel d an d c oll e ct e d o n t h e pl at e, o n o n e si d e o nl y. Al m ost at o n c e I r e alis e d t h at t h e r a ys e mitt e d b y p ol o ni u m w er e n ot d efl e ct e d u n d er t h e a b o v e e x p eri m e nt al c o n diti o ns a n d t h at t h er e ar e t w o ki n ds of r a ys, o n e hi g hl y d efl e ct e d b y a m a g n eti c fi el d, a n d t h e ot h er a p- p ar e ntl y n ot d efl e ct e d. W h e n st u d yi n g t h e e missi o n fr o m r a di u m, M. a n d M me. C urie o bser ve d t he si m ulta neo us prese nce t here of t he t wo t y pes of r a ys a n d s a w t h at t h e r a ys fr o m p ol o ni u m, i n c o m m o n wit h t h os e of t h e sa me ty pe e mitte d by ra di u m, were i ncreasi ngly absorbable accor di ng as t he y ha d passe d t hr o u g h a greater t hic k ness of a bs or be nt material: t he i n-

Fi g. 8. 5 8 1 9 0 3 A . H . B E C Q U E R E L v ers e o c c urs i n t h e c as e of t h e h et er o g e n e o us b e a ms of t h e ot h er r a ys. T h e p h ot o gr a p h i n Fi g. 8 s h o ws t h e t w o t y p es of r a ys k n o w n n o w a d a ys as α -ra ys for those e mitte d by poloniu m, an d β -r a ys f or t h e m a g n eti c all y d efl e ct a bl e p art of t h e r a di ati o n fr o m r a di u m. I f urt h er f o u n d t h at t h ori u m e mits t h e t w o t y p es of r a ys a n d t h at, e v e n i n a v a c u u m, ur a ni u m e mits o nl y p-r a ys (Fig. 9) not excl u di ng t he existe nce of m uc h less active, no n- deflectable rays.

Fi g. 9.

T here is, i n fact, a t hir d ty pe of rays, y-rays, not deflecte d by a mag netic fiel d, atte ntio n to w hic h was first dra w n by a n ex peri me nt co n d ucte d by Vill ar d a n d w hi c h s e e m a n al o g o us t o X-r a ys. T he actio n of a ma g netic fiel d e nables t he vario us co m po ne nts of t he ra diation fro m ra dioactive s ubstances to be se parate d an d analyse d. b - Rays behave like catho de rays; it may be assu me d, as has been for the latter, t hat t he y are ma de u p of masses m carr yi n g at vel ocit y v ne gati ve c harges e. I n a u nifor m mag netic fiel d of i nte nsity H t he traject ories n or mal t o t h e fi el d m ust b e cir c ul ar p at hs, t h e r a di us R of w hi c h is gi v e n b y t h e relati o n R H = ( m/e)v. F or a n ori gi nal directi o n ma ki n g a n a n gle a wit h t h e li n es of f or c e, t h e trajectories are helices w hic h wi n d o n c yli n ders of ra di us R si n a. I ha ve verifie d the vario us geo metrical conseq uences of this si milarity. T he bea m of p-ra ys is ma de u p of a n i nfi nit y of ra ys wit h traject ories h a vi n g diff er e nt r a dii of c ur v at ur e; t h e m a g n eti c fi el d dis p ers es t h e m as a pris m dis perses the light rays of vario us colo urs. A photogra phic plate with- o ut black pa per wra p ping can be place d in, an d parallel to, a unifor m, hor- izo ntal mag netic fiel d, for i nsta nce, t he n o n t he plate a s mall lea d dis h co n- taining a fe w grains of ra difero us bari u m for ming a so urce of very s mall di- a meter. The ra diation is directe d at the plate an d pro d uces an i mage on j ust o ne si de. If s mall stri ps of differe nt s ubsta nces - pa per, al u mi ni u m or vario us m et als - h a v e b e e n pl a c e d o n t his si d e it will b e f o u n d t h at i n t h e diff us e i mage w hic h re prese nts a ki n d of s pectr u m t here are rays wit h varyi ng pe ne- tration which, un der each screen, give i mages whose li mits are different an d w hic h co nstit ute absor ptio n s pectra. It will be see n i n t he follo wi ng t hat t he i m a g e is m ai nl y c a us e d b y t h e s e c o n d ar y r a di ati o n w hi c h ori gi n at es at t h e face where the inci dent rays e merge. Figure 10 is an exa mple of the photo- R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 5 9

Fi g. 1 0. Fi g. 1 1. gra p hs obtai ne d; t he scree ns are: a stri p of black pa per, a stri p of al u mi ni u m 0. 1 m m t hi c k, a n d a pl ati n u m f oil 0. 0 3 m m t hi c k. T o s e c ur e w h at mi g ht b e ter me d a p ure s pectr u m, i.e.s uch that each point on the plate is str uck by a single bea m, all of whose trajectories have the sa me c urvat ure, the ra diation fro m the point source must be made to pass through a narro w aperture. The result is the sa me as the foregoing (Fig. 11). The photograph sho ws, more- over, an i mage pro d uce d by the secon dary rays e mitte d by the insi de face of a lea d se mi-cylin der which covere d the so urce an d in which was pierce d the s mall a pert ure in q uestion. Finally, when the ra diation is gathere d thro ugh a seco n d s mall a pert ure, t he res ult is a si ngle ray. E a c h si n gl e r a y m a y b e d efi n e d b y t h e v al u e of t h e pr o d u ct R H c orre- s p o n di n g t o it. I n t h e a b o v e tests, t h e R H v al u es of t h e a cti v e r a ys r a n g e d fr o m 600 t o a b o ut 2,500. A more co mplete analysis of the radiation can be made by applying the follo wing general metho d: The active material is place d at the botto m of a dee p, narro w groove in a s m all bl o c k of l e a d t o pr o d u c e a v er y t hi n v erti c al b e a m iss ui n g fr o m a li n e ar s o ur c e a f e w milli m etr es l o n g. T his s yst e m is pl a c e d i n, a n d p ar all el t o, t h e unifor m fiel d of a magnet. A photogra phic plate is then arrange d above the

Fi g. 1 2. 6 0 1 9 0 3 A . H . B E C Q U E R E L R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 6 1 so urce, at right angles to the fiel d an d interce pting the deflecte d bea m. The very obliq ue rays reaching the plate pro d uce thereon an i mage which differs very little fro m the act ual trajectory of a ski m ming ray e mitte d nor mally to the fiel d. On the photogra phic plate are place d a n u mber of screens pierce d b y s m all slits n or m al t o t h e pl at e; t h es e t h e n all o w eit h er p orti o ns of p ur e s pectra or single rays to pass. To eli minate the light e mitte d by the so urce, the latter is covere d with a thin al u mini u m foil. In practice the screens are fixe d by means of an a dhesive to a glass plate which serves to press the m against the photogra phic plate. Fig ures 12, 13, 14 an d 15 sho w the arrange- ment of the screens and so me of the photographs obtained by this method. F or t w o of t h es e p h ot o gr a p hs a n al u mi ni u m f oil 0. 1 m m t hi c k, c o n c e ntri c wit h t he pierce d scree n, was m o u nte d be y o n d t he p ure s pectra. It ca n be see n t hat t he least deflecta ble ra ys p ass t hr o u g h t his f oil as if it w er e n ot t here; ot her more deflectable rays pro d uce, o n e mergi ng, seco n dary rays w hic h are photogra phically more active than the inci dent rays, an d lastly the most deflectable rays do not pass thro ugh the al u mini u m foil an d pro d uce at the entry face secondary rays which give an intense i mage. The general i mage wit hi n t he co nto urs of t he scree ns is also attrib utable to seco n dary ra diatio n. T h e m a g n eti c fi el d h a d a n i nt e nsit y of 8 5 9 c. g.s. u nits. I n t h es e tests t h e α − r a ys w er e bl o c k e d b ut t h e γ -r a ys gi v e str ai g ht li n e i m a g es r e v e ali n g a dis- co nti n uity bet wee n t hese rays a n d t he less deflectable β -ra ys; f or t hese latter t he pro d uct R H is a b o ut 1 0 4 . I n ge neral t he not very deflectable β -ra ys are ver y pe netrati n g a n d t he ra ys w hic h are hi g hl y deflecte d are also ver y rea dil y absorbe d. These tests ill ustrate the a dvantages of this metho d for analysing t he effects pro d uce d for eac h si ngle ray. When a p plie d to secon dary ra diation this metho d has sho wn that the rays were deflecte d by a mag netic fiel d i n t he sa me directio n as t he cat ho de rays. W hil e I w as c o n d u cti n g t h es e e x p eri m e nts, M. a n d M m e. C uri e d e m o n- strate d t hat t he β -rays fro m ra di u m act ually carry negati ve electrical c harges; t he bo dies w hic h receive t he ra diatio n beco me negatively c harge d w hile the so urce itself beco mes positively charge d. For this do uble pheno menon to be o bser ve d, all t he co n d uctors a n d t he so urce itself m ust be co m pletel y s urr o u n de d b y i ns ulati n g materials, e. g. paraffi n, or be place d i n a vac u u m. O n t h e ot h er h a n d I h a v e s h o w n t h at t h e β -r a ys fr o m r a di u m w er e d e- fl e ct e d b y a n el e ctr ost ati c fi el d. W h e n F is t h e i nt e nsit y of t his fi el d, t h e tr aj e ct or y of a si n gl e r a y, as c h ar a ct eri z e d b y t h e q u a ntiti es m, e a n d v defi ne d a bo ve, is a para bola wit h para meter ( m/e) ( v 2 / F), a n d t h e si z e of t his p ar a m et er co mbine d with that of the ra di us of c urvat ure of the sa me ray’s trajectory in 6 2 1 9 0 3 A . H . B E C Q U E R E L a kno wn magnetic fiel d enables m/e a n d v to be deter mined. The experi ment p erf or m e d wit h t h e a p p ar at us i n Fi g. 1 6 pr o vi d e d t h e i m a g e s h o w n i n Fi g. 1 7 w hic h reveals t he electrical deflectio n by t he s ha do w projecte d by a vertical screen nor mal to the fiel d. By co mbining for a single bea m the crosse d elec- trical an d magnetic deflections, Ka uf mann has ma de m uch more precise m e as ur e m e nts t h a n t h e o n es t h at c a n b e d e d u c e d fr o m t h e e arli er e x p eri- m e nts. His m e as ur e m e nts s h o w e d hi m t h at t h e r ati o e/ m was a f u nctio n of t h e v el o cit y v w hi c h, f or t h e l e ast d efl e ct a bl e b -r a ys, t e n ds t o w ar ds t h e s p e e d of light. Inter pretation of this fact in ter ms of Max. Abraha m’s conce pts s ug- gests t hat t he mass of t he electr o ns is at least i n part, if n ot e ntirel y, t he o ut- co me of electro mag netic reactio ns - a r es ult w hi c h pr o m pts fr es h i deas abo ut t he nat ure of t he i nertia of matter. A part fro m t he b -r a ys, i d e nti c al wit h c at h o d e r a ys, α -rays make up an i m portant part of the ra diation fro m active s ubstances. I mentione d earlier h o w I h a d o bs er v e d t h e m f or t h e first ti m e wit h p ol o ni u m, a n d h o w t h eir a p p ar e nt n o n- d efl e ct a bilit y a n d t h e p e c uli ariti es of t h eir l o w p e n etr a bilit y

Fi g. 1 6.

Fi g. 1 7. R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 6 3 ha d ca use d t he m to be classifie d a part. By mea ns of a very delicate electrical ex peri ment, R utherfor d discovere d that the α -rays co ul d be deflecte d very slightly by a very strong magnetic fiel d, an d that the deflection was in the o p posite directio n to t hat of b -rays. They hence behave as if carrying posi- ti ve electrical c harges a n d a p pear i de ntical wit h Gol dstei n’s < < Ka nalstra hle n > >.

Fi g. 1 8.

Usi n g t h e a b o v e arr a n g e m e nt I r e c or d e d p h ot o gr a p hi c all y t h e tr aj e ct or y d es cri b e d i n a m a g n eti c fi el d b y α -r a ys fr o m r a di u m a n d t h e r a ys fr o m p ol o- ni u m w hic h are i de ntical. Fig ure 18 s ho ws a p hotogra p h (e nlarge d) obtai ne d i n a ma g netic fiel d of 10,000 c. g.s. u nits. T he t wo co nc urre nt pat hs eac h correspon d to a direction of the magnetic fiel d which has been reverse d in t he mi d dle of t he ex p os ure. T he y d o n ot prese nt a n y trace of dis persi o n, w hi c h all o ws us t o r e g ar d t h e a cti v e b e a m as h o m o g e n e o us. It is n ot e d, moreover, t hat t he ra di us of c urvat ure calc ulate d for t he differe nt poi nts of t h e tr aj e ct or y c o nti n u es t o i n cr e as e as t h e p at h i n t h e air i n cr e as es. T his pheno menon can be attrib ute d to the fact that the positive charges, with a v el o cit y w hi c h is te n ti m es less t h a n t h at of t h e β -ra ys a n d a r e al or s u p p os e d mass a t ho usa n d ti mes larger, attract t he ne utral molec ules of t he air, or are dis c h ar g e d pr o gr essi v el y i n t h e i o ni z e d air. As t h e α -ra ys are ver y a bsor ba ble, t he y a p pear i n co nse q ue nce to co n- stit ut e t h e m ost a cti v e p art of t h e r a di ati o n w h e n t his is m e as ur e d b y t h e ionization of the air in the neighbo urhoo d of the so urce. These rays are also the most active in exciting the phos phorescence of zinc blen de, an d of dia- mon d, whereas bariu m platinocyani de beco mes equally lu minous un der the i nfl ue nce of t he α - a n d β -rays an d the phosphorescence of the double sul- phate of urani u m an d potassi u m is partic ularly excite d by the b -ra ys. T he c urio us effect of Sir W. Crookes’ s pintharisco pe sho ul d be attrib ute d to the 6 4 1 9 0 3 A . H . B E C Q U E R E L cc-rays, an d this effect see ms d ue to the cleavages acco m panie d by flashes i dentical with those pro d uce d when vario us crystals are fract ure d. T h e t hir d ki n d of r a ys, γ -rays, are c haracterize d by t heir great pe netrabil- ity a n d t heir no n- deflectability i n a mag netic fiel d. I n t he attac he d ill ustra- tion (Fig. 19) ma de by the metho d describe d earlier, the α -ra ys ha ve bee n st o p p e d n e ar t h eir s o ur c e, t h e b -r a ys ar e d efl e ct e d b y t h e m a g n eti c fi el d, w hilst t h e γ -rays a n d t he lig ht e mitte d by t he ra di u m salt for m a rectili near bea m w hic h falls u po n a q uartz pris m. T he l u mi no us ra ys are deflecte d, w hereas t he γ -rays leave a trace which can be follo we d witho ut deflection not only past the pris m b ut even thro ugh the pris m itself. T he great pe netra bilit y of t he γ -r a ys a n d als o of t h e l ess d efl e ct a bl e b -ra ys means that neither the ionization of the air nor the photogra phic plate can gi v e a n e x a ct i d e a of t h eir i nt e nsit y, si n c e t h e y p ass t hr o u g h t h e g as es a n d t h e silver salts wit ho ut bei ng absorbe d. If fairly t hick metal plates are place d i n t heir pat h, t hese rays are tra ns- for me d a n d, eit her o n t he e ntry face or o n t he exit face, give rise to seco n d- ar y ra ys w hic h are m ore a bs or ba ble, s o m uc h s o t hat t he effect o bser ve d i m me diately behin d these screens is more intense than if this screen di d not

Fi g. 1 9. R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 6 5 exist. This transfor mation re min ds one of the effect pro d uce d by placing a fl uoresce nt scree n i n t he pat h of a bea m of i nvisible ra diatio ns. A photogra phic plate receiving the ra diation of ra di u m filtere d by lea d 1 c m t hick is affecte d more u n der a stri p of lea d 1 m m t hick t ha n i n t he regio ns not co vere d by t his scree n. Fig ure 2 0 sho ws the effect pro d uce d by the ra dia- tion leaving thro ugh the walls of a lea d parallele pi pe d after passing thro ugh 5 t o 1 2 m m of m et al.

Fi g. 2 0.

T hese seco n dary p he no me na ca n acco u nt i n part for t he a p peara nce of c ast s h a d o ws o n t h e e d g es of all t h e m or e or less tr a ns p ar e nt scr e e ns pl a c e d o n t he p hotogra p hic plates. Ur a ni u m a n d p ol o ni u m e mit p e n etr ati n g r a ys w hi c h a p p e ar t o b e i d e n- ti c al wit h γ -r a ys. Ra diation of ra dioactive s ubstances pro d uces vario us che mical actions; it acts u po n t he s ubsta nces use d i n p hotogra p hy, a n d t he α - a n d β -r a ys ar e t h e most acti ve i n t his res pect; it colo urs glass violet or bro w n, a n d t he alkali ne salts are c ol o ure d yell o w, vi olet, bl ue or gree n. U n der its acti o n paraffi n, celluloi d an d paper turn yello w; white phosphorus is transfor me d into re d phosphorus. This transfor mation has been note d with β -ra ys, b ut it is pr o b- a bl e t h at α -r a ys ar e e q u all y a cti v e. O z o n e is pr o d u c e d i n t h e air ar o u n d a cti v e b o di es. N ot o nl y g as es b ut als o li q ui d di el e ctri cs ( p etr ol e u m, li q ui d air) a n d ins ulating soli ds like paraffin are ionize d when they have been penetrate d by ra di u m ra diatio n, a n d t hey preser ve t heir co n d ucti ve pro perties for a fe w 6 6 1 9 0 3 A . H . B E C Q U E R E L mo ments after the ra diation has cease d to act. Giesel has observe d that an aqueous solution of radiu m bro mide continuously liberates oxygen and hy- drogen at the rate of a p proxi mately 1 0 c m 3 per gra m per day. Accor ding to R a ms a y a n d S o d d y t h es e g as es w o ul d als o c o nt ai n h eli u m. M. C uri e h as dis c o v er e d m or e o v er t h at r a di u m s alts c o nti n u all y e mit h e at; p ure ra di u m wo ul d e mit a p proxi mately 80 calories per gra m per ho ur. C urie and De war observed that in plunging a tube containing radiu m into liquid hy drogen there was a contin uo us release of hy drogen gas; with 0.7 g of ra- di u m br o mi d e 7 3 c m 3 of hydrogen were obtained per minute. Vario us physiological effects have been observe d with ra di u m rays; they excite phos phorescence in the interior of the eye; when an active pro d uct is bro ught near to the te m ple, a sensation of light is perceive d. They act u pon the e pi der mis an d profo un dly disorganize the skin, as do X-rays. The effect is pr o d u c e d wit h o ut a n y s e ns ati o n b ei n g f elt at first a n d it o nl y d e v el o ps after se veral wee ks; it t he n pr o d uces m ore or less dee p lesi o ns w hic h ca n ta ke several months to heal an d which leave scars. At present an effort is being ma de to utilize t his actio n i n t he treat me nt of l u p us a n d ca ncers. Ra di u m rays have an active effect on the nerve centres an d can then ca use paralysis a n d deat h; t hey see m to act wit h partic ular i nte nsity o n livi ng tis- s ues i n t he process of evol utio n.

U p to no w we ha ve o nly s poke n of t he ra diatio n w hic h is tra ns mitte d t hro ug h glass, mica, o pa q ue bo dies a n d metals. I n t he e missio n of ra dioacti ve bo dies there is another pheno menon of a different nature which appears to b e i nti m at el y c o n n e ct e d wit h r a di o a cti vit y, if it is n ot t h e pri m or di al p h e- no menon. Thoriu m and radiu m e mit energy in a particular for m; the resul- tant activity is propagate d in the for m of an active vapo ur which has been c all e d e ma natio n a n d w hic h is arreste d by a ny coveri ng, ho wever t hi n, w hic h is i m per meable to gases. T his e m a n ati o n s e e ms t o s ettl e u p o n all b o di es i n or d er t o m a k e t h e m ra dioactive, b ut the activity disa p pears when the latter are no longer un der the infl uence of the active so urce, even when they are in a close d cha mber. T hese facts were disc o vere d si m ulta ne o usl y at t he e n d of 1899 b y R ut her- ford for thoriu m, and by M. and M me. Curie for radiu m. Rutherford when st u d yi n g t h e a cti vit y of t h ori u m s a w t h at b esi d es t h e or di n ar y r a di ati o n t h er e was a n effect pro d uce d by a n e ma natio n co mparable with an active vapour. T his l att er is d e p osit e d o n all b o di es, pri n ci p all y o n t h os e w hi c h ar e n e g- atively charged, and it makes the m mo mentarily active. R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 6 7 At the sa me ti me M. and M me. Curie discovered that under the influence of ra di u m the bo dies beco me te m porarily active; this induced acti vit y persists for so me ti me after the bodies have been re moved fro m the radiu m; in the o p e n air it di mi nis h es b y a b o ut o n e h alf i n h alf a n h o ur. The pheno menon is produced with regularity and with great intensity in a close d s pace; t he i n d uce d activity is t he n t he sa me o n all t he bo dies; it is in de pen dent of the nat ure of the gas an d of the press ure within the cha mber, b ut t h e a cti v ati o n is n o l o n g er pr o d u c e d if a v a c u u m is c o nst a ntl y m ai n- t ai n e d b y r e m o vi n g t h e g as es w hi c h ar e li b er at e d as s o o n as t h e y ar e pr o- d uce d. Sol utio ns of ra di u m salts pro d uce acti vatio n wit h greater i nte nsit y t h a n t h e s oli ds. T h e w at er of cr yst alli z ati o n e xtr a ct e d fr o m t h e a cti v e s alts, or t h e w at er s e p ar at e d fr o m a n a cti v e s ol uti o n b y a s e mi- p er m e a bl e c ell ul oi d w all, b e- co me strongly radioactive. The bodies activated by the radiu m produce the s a m e eff e cts as r a di u m; t h e gl ass w alls a cti v at e d b y t h e e m a n ati o n e mit a penetrating ra diation which passes thro ugh the m an d makes the m l u mino us, w hilst t h e a cti v ati n g s ol uti o n c a n o nl y e mit fe e bl e r a di ati o n. T h e a cti v ati n g pr o p ert y is diff us e d gr a d u all y t hr o u g h t h e g as es i nsi d e a close d cha mber, thro ugh ca pillary t ubes or i m perce ptible fiss ures; the bo dies beco me more active as t he vol u me of gas at t heir s urface is greater. A gas which has been kept near to radiu m and has acquired the property of maki ng soli d bo dies ra dioactive is itself ra dioactive, b ut it o nly e mits rays with very little penetrating po wer which will not pass thro ugh a glass wall. When it is re move d fro m the ra di u m it continues to e mit rays an d to cause r a di o a cti vit y. Its a cti vit y fr o m t his d o u bl e p oi nt of vi e w di mi nis h es b y h alf d uring each s uccessive perio d of fo ur days an d finally beco mes extinct. This perio d of fo ur days is a ti me constant characteristic of ra di u m e manation. Air c harge d wit h e ma natio n pro d uces p hos p horesce nce of vario us s ub- sta nces (glass, zi nc ble n de, etc.) a n d t his p he no me no n allo ws vario us striki ng experi ments to be con ducte d with relation to the propagation of e manation. Radiu m e manation behaves like a gas fro m many points of vie w. M. and M m e. C uri e h a v e s h o w n t h at it di vi d es li k e a g as b et w e e n t w o i nt er c o m- m u nicati ng gas c ha mbers a n d t hat it is diff use d i n t he air i n accor da nce wit h t he diff usio n la w for gases; its diff usio n coefficie nt i n air a p pears to be close to t hat of carbo nic aci d. R ut herfor d a n d So d dy have discovere d t hat t he e ma natio n co n de nses at the te m perat ure of liq ui d air. All ex peri ments lea d one to regar d the e mana- tio n as a material gas. Ho wever, t he hy pot hesis of t he existe nce of s uc h a gas 6 8 1 9 0 3 A . H . B E C Q U E R E L is base d solely on the ra dioactive manifestations, an d contrary to what ha p- pe ns wit h or di nary matter, t he e ma natio n disa p pears s po nta neo usly i n a seale d t ube w hic h e ncloses it. Ra msay and Soddy h ave recently fo un d that by enclosing the e manation fr o m r a di u m i n a s e al e d t u b e a n d st u d yi n g t h e e m a n ati o n s p e ctr u m, t h e spectru m of heliu m could be seen progressively appearing, which was not observable at first. T he writers ex plai n t hese facts by a tra nsfor matio n t hat w o ul d b e h alf c o m pl et e i n f o ur d a ys, a n d f ull y c o m pl et e i n 2 8. Wit h o ut resorti ng to t he hy pot hesis of t he tra nsfor matio n of matter, w hile o ne is still waiti n g for a more co m plete de mo nstratio n of s uc h a n i m porta nt fact, it wo ul d be possible to ex plain the facts by agreeing that the heli u m alrea dy exists in the gases of the e manation in the seale d t ube, b ut that the s pecial st at e of t h e e m a n ati o n pr e v e nts t h e li n es of t h e h eli u m s p e ctr u m fr o m a p- peari ng. T he latter wo ul d gra d ually a p pear as t he e ma natio n is tra nsfor me d; an d it was observe d that the activity of the e manation was re duce d by half by the en d of fo ur days. F urt her, it is releva nt to q uote a striki ng met ho d of activatio n w hic h lea ds one to make certain reservations abo ut the concl usions to be dra wn regar d- ing the presence of ne w ele ments in ra dioactive pre parations. Every inactive s u bst a n c e w hi c h is p ut i nt o s ol uti o ns of ur a ni u m, r a di u m or t h ori u m, a n d t he n preci pitate d o ut, beco mes ra dioactive, b ut slo wly loses its ra dioactivity. T his f a ct w as first n ot e d b y C uri e a n d Gi es el; t h e l att er a cti v at e d bis m ut h i n t his w a y. Wit h ura ni u m, a trace of bari u m preci pitate d as t he s ul p hate beco mes n oti c e a bl y m or e a cti v e t h a n ur a ni u m; w h e n a cti v at e d i n t his w a y b ari u m, li k e ur a ni u m, gi v es off o nl y β -r a ys. Aft er pr e ci pit ati o n, t h e ur a ni u m s alt, w hi c h is c oll e ct e d as a s oli d, is l ess a cti v e t h a n b ef or e. T his r e d u cti o n i n acti vit y ca n be carrie d f urt her b y re peati n g t he pr ocess, b ut t he pr o d ucts gra d ually regain s pontaneo usly their for mer activity. The te m porary re d uction in activity after dissol ution is a general pheno m- e no n. Ra di u m salts, w he n t hey are take n o ut of sol utio n agai n are less active t ha n bef ore t he y were diss ol ve d; t heir acti vit y t he n i ncreases o ver se veral m o nt hs b ef or e r e a c hi n g a m a xi m u m. T h e y gi v e off h e at f oll o wi n g t his i n- crease i n acti vit y. Accor di ng to Debier ne, bari u m acti vate d by acti ni u m ca n be se parate d fro m i nactive bari u m; it fractio nates as ra difero us bari u m c hlori de, t he least active portion being least sol uble in water an d hy drochloric aci d. Debierne obtained by this method a product which was one thousand ti mes more ac- R A D I O A C T I V I T Y , N E W P R O P E R T Y O F M A T T E R 6 9 tive than urani u m. Activate d bari u m behaves like a false ra di u m, b ut differs fr o m tr u e r a di u m i n l a c ki n g t h e c h ar a ct eristi c s p e ctr u m, a n d i n l osi n g its a cti vit y wit h ti m e. T h e s p o nt a n e o us r e c o v er y of r a di o a cti vit y b y s u bst a n c es i n w hi c h it h as been re duce d may be explaine d in ter ms of the substances the mselves trap- pi n g t h eir e m a n ati o n, eit h er o n t h e a cti v e m ol e c ul es, or o n t h e i n ert m ol e- c ules associate d with the m. Follo wi ng o n t his researc h, vario us workers have fo u n d more exa m ples of ra dioactivity, so meti mes as trace ele ments in metals, so meti mes in nat ural pheno mena. Elster an d Geitel discovere d that at mos pheric air sho ws to a slight extent t he sa me pro perties as activate d gases. By stretc hi ng lo ng negatively-c harge d wires in the air they were able to recover traces of activate d s ubstances on t he m. Gases tra p pe d i n e nclose d s paces, a n d air extracte d fro m t he gro u n d or near waterfalls, s h o w t hese pr o perties, as d o gases extracte d fr o m certai n ty pes of water.

To s u m up, the ra dioactive s ubstances whose nat ure is no w well establishe d ar e: ur a ni u m, t h ori u m, r a di u m, a n d p ol o ni u m; a cti ni u m c a n b e a d d e d, al- t h o u g h ver y little i nf or mati o n has bee n p u blis he d a b o ut t his last pr o d uct. Reservatio ns m ust be ma de abo ut vario us ot her pro d ucts obtai ne d by Giesel, an d abo ut a pre paration of active bis m uth or active tell uri u m obtaine d elec- tr ol yti c all y b y M ar k w al d. Ur a ni u m gi v es off β - a n d γ -rays; it does not give off a n e ma natio n i n air, b ut t he acti vati o n w hic h it pr o d uces i n s ol uti o n ca n be ex plai ne d as t he effect of a n e m a n ati o n. Thori u m an d ra di u m give off α -, β - a n d γ -rays, an d an activating e mana- ti o n i n g as es. P ol o ni u m d o es n ot gi v e off β -r a ys. It gi v es off α - a n d γ -ra ys, b ut l oses its a cti vit y wit h ti m e. Actini u m is sai d to possess a re markable activation po wer. Besides uraniu m and thoriu m, only radiu m has characteristics which en- able it to be co nsi dere d as a n ele me nt wit h pro perties relate d to, b ut disti nct fr o m, t h os e of b ari u m. H o w e v er, it is w ort h n oti n g t h at t his s u bst a n c e is never fo un d, even as a trace ele ment, in or dinary bari u m minerals, an d that it is only met with in urani u m minerals, where it is fo un d with bari u m. This f a ct m a y w ell h a v e a si g nifi c a n c e w hi c h will b e c o m e cl e ar t o us l at er. Ra dioacti ve s ubsta nces, es pecially ra di u m, gi ve off e nergy i n all t he k no w n 7 0 1903 A. H. B E C Q U E R E L f or ms: h e at, li g ht, c h e mi c al r e a cti o ns, el e ctri c al c h ar g es, γ -ra diatio n. T he y s e e m t o m ai nt ai n t h e s a m e st at e i n d efi nit el y, a n d t h e s o ur c e fr o m w hi c h t h e y derive t he e nergy t hey give off esca pes us. A m o n g t h e h y p ot h es es w hi c h s u g g est t h e ms el v es t o fill t h e g a ps left b y c urrent ex peri ments, one of the most likely lies in s u p posing that the e mis- sio n of e nergy is t he res ult of a slo w mo dificatio n of t he ato ms of t he ra dio- acti ve s ubsta nces. S uc h a mo dificatio n, w hic h t he met ho ds at o ur dis posal are u n a bl e t o bri n g a b o ut, c o ul d c ert ai nl y r el e as e e n er g y i n s uffi ci e ntl y l ar g e q ua ntities to pro d uce t he o bser ve d effects, wit ho ut t he c ha n ges i n matter being large enough to be detectable by our metho ds of investigation. I n t his s c h e m e, t h er e w o ul d still b e s c o p e t o w o n d er w h et h er t h e tr a ns- for mation of the ato m co mprises a slo w, spontaneous evolution, or whether it is t he res ult of t he absor ptio n of exter nal ra diatio n beyo n d t he ra nge of o ur s e ns es. If s u c h a r a di ati o n w er e t o e xist, o n e c o ul d still pi ct ur e t h e r a di o a cti v e s u bst a n c es tr a nsf or mi n g it wit h o ut t h e ms el v es b ei n g alt er e d. S o f ar n o e x- peri ment has confir me d or invali date d these hy potheses.