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.