September 3, 1908] 



NA TURE 



415 



rotation of the plane of polarisation of light by the 

 influence of the earth's magnetism. During the years 

 from 1879 to 18S3 he was associated with his father 

 in a series of joint memoirs on the temperature at 

 the surface of the earth, and beneath the surface to a 

 depth of 36 metres, using thermoelectric methods for 

 the subterranean observations. In 1879 he investi- 

 gated the temporary magnetic properties of cobalt 

 and nickel, and further examined the magneto-optic 

 rotatory power of gases. He also determined the 

 specific magnetic properties of ozone. Then he 

 turned to the subject of phosphorescence, which his 

 father had studied for so many years. One of the 

 phenomena of phosphorescence — discovered originally 

 by no other than Goethe — was the hastening of the 

 fading out of the light of a phosphorescent body when 

 exposed to the red rays at the hot end of the spectrum. 

 Becquerel saw in this fact a means of studying the 

 distribution of the intensity of the invisible infra- 

 red rays of the spectrum. These cannot be photo- 

 graphed by ordinary photographic means. The 

 method of exploring the infra-red spectrum by the 

 thermometer or the thermopile is too coarse to give 

 satisfactory results. The bolometer of Langley had 

 not yet been invented. Becquerel exposed a brightly- 

 phosphorescing strip of prepared material — one of 

 the sulphides of the alkaline earths, so much studied 

 by his father — to the action of the invisible infra-red 

 spectrum, and found it to become striated with dark 

 and light lines and bands, according as the radiation 

 had hastened the decay of the luminosity. These 

 phosphorographic studies he extended to include an 

 investigation of emission spectra, in the same region, 

 of incandescent metallic vapours. From 1886 to 1890 

 he was conducting experiments on the absorption of 

 light in crystals, and on the anomalies in this absorp- 

 tion in different directions. 



In 1892, on the death of Edmond Becquerel, Henri 

 became professor in the Musee d'Histoire naturelle. 

 In 1894 he was named Ingenieur en chef des Fonts et 

 Chaussees, and in 1895 he was given a chair at the 

 Ecole polytechnique. Beyond giving an account of 

 the laws of emission of light by phosphorescent 

 bodies, he published little in these years. But in 1896 

 came the chief of his scientific successes. At the close 

 of 1895 Rontgen had described the rays of peculiar 

 penetrating power which he had observed to be 

 emitted from highly exhausted Crookes's tubes, rays 

 which he discovered and investigated by their singu- 

 larly effective action in stimulating the luminescence 

 of phosphorescible bodies. Associated as these rays 

 were, both in the tubes whence they were emitted and 

 on the platinocyanide screens where they were re- 

 ceived, with the plienomena of phosphorescence, the 

 association seemed to suggest a further inquiry. Was 

 it not possible that in the phenomena of ordinary 

 phosphorescence and fluorescence there might also be 

 an emission of penetrating rays? Such a query sug- 

 gested itself independently to several physicists in 

 more than one country. Henri Becquerel was the 

 first to publish any certain facts. In the Cotnptes 

 rendus of February 24, 1896, there is a note by 

 him, " Sur les Radiations ^mises par Phosphor- 

 escence." His experiment was as follows : — .\ photo- 

 graphic drv-plate was enclosed in opaque black paper. 

 Over it was laid a thin plate formed of encrusted 

 crystals of the double sulphate of uranium and potas- 

 sium, and the whole was exposed to the sun for 

 several hours. On developing the photographic 

 plate it was found that the uranium salt (which has a 

 brief phosphorescence) had emitted radiations capable 

 of traversing the opaque paper and of reducing the 

 silver salts. Metallic objects such as coins, inter- 

 posed, left their silhouettes printed on the photo- 



N"0. 2027, VOL. 78] 



graphic plate. Such was the first announcement. 

 On March 2 came a second note, " Sur les Radi- 

 ations invisibles <Jmises par les Corps phosphorescents." 

 He has now found that the crystals of uranium salt 

 produce the same effect when shielded from exposure 

 to the sun's rays, and even when kept in darkness, 

 and concludes that the invisible radiations emitted by 

 phosphorescence continue to act long after the tem- 

 porary phosphorescence has ceased. He recognises 

 that here is a new order of phenomena. One week 

 later he sends a third contribution. He has dis- 

 covered that, like Rontgen "s rays, the radiations 

 emitted from the phosphorescent salts can discharge 

 an electroscope, and he begins to employ this electric 

 test quantitatively. He also announces that these 

 new rays can be reflected, and possibly refracted. He 

 tries different substances as to the amount and 

 duration of their activity, finding the uranium salts 

 to surpass by far the alkaline sul|jhides and the zinc- 

 blende preparations. By March 23 he communicates 

 another notice, in the title of which it is significant to 

 observe that he has dropped all reference to phos- 

 phorescence. It is called " .Sur les Radiations invisibles 

 emises par les Sels d'Uranium "; for he finds that a 

 non-phosphorescent solution of uranium is also active. 

 He has also been studying the absorption of these 

 rays, and has, he thinks, confirmed their refraction. 

 On March 30 he reads to the academy another note on 

 the differences between the radiations of uranium and 

 the Rontgen rays. He insists that the former can not 

 only be reflected and refracted, but that they can 

 show double refraction and polarisation if transmitted 

 through tourmaline. He has also obtained them 

 from non-phosphorescing compounds of uranium. 

 That he was mistaken in respect of reflection, refrac- 

 tion, and polarisation does not detract from the merits 

 of the great discovery. Before six months from the 

 date of his first note he was able further to announce 

 that metallic uranium, furnished by his friend 

 Moissan, far surpasses its salts in activity, the first 

 example, he declares, of a metal presenting a 

 phenomenon of the order of an invisible phosphor- 

 escence. 



The subsequent development of the new branch of 

 physics — radio-activity — thus opened out by the dis- 

 covery of the Becquerel rays is known to all 

 students of science. In 1898 Schmidt and Mme. 

 Curie independently observed that thorium was also 

 radio-active. M. and Mme. Curie set out on a syste- 

 matic examination of other minerals, and Mme. 

 Curie, after finding that certain uraniferous minerals 

 were more active than uranium itself, embarked on 

 the laborious search which yielded her the successive 

 discoveries of polonium and radium. Rutherford, in 

 the Cavendish Laboratory, repeated and extended 

 Becquerel 's measurements on the electrical properties 

 of the uranium radiations, and pushed the investi- 

 gation into new regions by demonstrating the various 

 stages of phenomena explicable only on the hypothesis 

 of the degradation of the uranium atom and the suc- 

 cessive evolution of new elements of transitional 

 tvpes. Becquerel continued to investigate the 

 radiations, and their divisibility into three kinds which 

 differ in penetrating properties and in the deviations 

 which they suffer when subjected to magnetic and 

 electric forces. In 1903 he united in a large quarto 

 memoir of 360 pages, under the title of " Recherches 

 sur une Propri^te nouvelle de la Matiere," his hitherto 

 scattered contributions. This memoir, written with 

 admirable lucidity of phrase and illustrated with many 

 plates, remains a witness to his powers of investiga- 

 tion and scientific acumen. He had since 1889 been 

 a Membre de I'.Xcademie des Sciences; he was also 

 Officier de la Legion d'Honneur; and with the repu- 



