178 



NA TURE 



[December 22, 1904 



I 



metallic copper on a crack in a test-tube containing 

 a solution of cupric sulphate, and immersed in another 

 solution of sodic sulphide. The investigation of this 

 phenomenon was full of interest, and not the least was 

 the suggestion that the deposition of metals in veins 

 in rocks is due to the same cause as that which he 

 observed in the broken test-tube. 



A member of the French Academy from 1829, eight 

 years before being called to the chair which he filled 

 to the end of his life, he was also a corresponding 

 member of the Royal Society, and received from it its 

 greatest honour, the Copley medal, and from the 

 Emperor Napoleon III. the Cross of Commander of the 

 Legion of Honour. Thus with him there closed a 

 chapter, a long, an interesting, and an eventful chapter, 

 in the history of the Museums d'Histoire naturelle. 



Edmond Becquerel, although a pupil of his father 

 and for a considerable time his assistant at the 

 museums, did not teach there, and, indeed, as Sir 

 William Crookes has said in his obituary notice of 

 him in the Proceedings of the Royal Society, of which 

 he was a foreign member, it may " be remarked 

 that though he had early distinguished himself by 

 scientific works of high value, and as the son of an 

 eminent and much respected Academician he was not 

 without influence, yet none of the great scientific 

 establishments of his country offered him an appoint- 

 ment." He finally, however, secured a permanent 

 position at the Conservatoire des Arts et Metiers, and 

 there the abilities so long latent had full play, and 

 manifested themselves by the success of his subsequent 

 career. 



At the death of his father, in 1878, he succeeded to 

 the chair of physics at the museum, and this im- 

 portant position he continued to hold until his death in 

 1892. Brought up as he was in a scientific atmo- 

 sphere, he evidently inherited from his father his 

 "acute power of observation," and that "infinite 

 capacity of taking pains which seems to be the 

 essential characteristic of the Newtons, the Faradays, 

 and the Darwins, and, in short, of all the great leaders 

 of science." 



Since 1892 Henri Becquerel has been professor at 

 the Museum d'Histoire naturelle, and has continued 

 those studies which his ancestors in days gone by 

 pursued with ardour and with success, not the less 

 marked, although perhaps, on the whole, notwith- 

 standing their brilliant achievements, less fruitful in 

 revealing that knowledge which was to come; for by 

 his memoirs on the radio-activity of matter Henri 

 Becquerel has given to the world o"f science the results 

 of a very remarkable scries of researches. 



There are four methods of studying the infra-red 

 parts of the spectrum : the thermopile, as employed 

 by Tyndall and others, the radiomicrometer of Boys, 

 the bolometer as used by Langley, and the phos- 

 phorescent screen of Becquerel. After exposure to the 

 violet rays, and if the screen is subjected to the action 

 of the infra-red, the phosphorescence becomes so intense 

 that the energy accumulated is radiated so rapidly 

 that the parts thus acted upon become quite dark 

 relatively to the other parts of the screen. Thus a 

 map of the infra-red can be produced and studied at 

 leisure so long ns the phosphorescence of the screen 

 lasts, or, indeed, photographs of the screen thus 

 affected may be taken. The effect is due most prob- 

 ably to heat, and is therefore a case of thermolumin- 

 escence. Under the influence of heat the collisions 

 between molecules become more frequent and more 

 violent, and the energy absorbed from the more re- 

 frangible rays, and stored up in the substance, by 

 some means at present not very clearly understood, is 

 once more yielded up to the aether and radiated awav. 

 The energy is stored up in unstable molecular aggre- 

 NO. 1834. VOL. 71] 



gates which gradually disintegrate, as radio-active 

 molecules have been found to do,' the change of 

 absorption which accompanies fluorescence being due 

 to the formation of these molecular groups. 



The absorption spectrum of crystals exhibits many 

 anomalies, from which Becquerel has extracted a most 

 important principle. If a crystal is composed of twa 

 isomorphous substances the molecular elasticity of 

 which varies in different directions, so that the absorp- 

 tion varies too, the absorption spectrum will likewise 

 vary in different directions, so that it is thus possible 

 to detect the presence of different substances, since 

 in two isomorphous substances the directions of 

 molecular elasticity do not correspond, and therefore 

 the directions of absorption would likewise differ. 

 Each chemical substance, therefore, affects the direc- 

 tion of propagation and of absorption. 



If the directions of absorption do not coincide with 

 the optic axes, it is due to the presence of different 

 isomorphous substances in the crystal. The absorp- 

 tion spectrum of each substance remains different and 

 in its own particular direction, whilst that of refrac- 

 tion is the resultant effect. By this contrivance the 

 composition of crystals has been examined and after- 

 wards confirmed by chemical means, whilst in many 

 instances the presence of substances in quantities too 

 minute for the chemist to notice has been detected by 

 this elegant method of analysis. 



But the most striking' work that has issued from 

 Becquerel 's laboratory relates to the radio-activity of 

 matter. Of this great discovery, separating as it does 

 the ideas of this century from those of the last, so much 

 has been written, upon their far-reaching importance, 

 so many ideas have been discussed in these columns, 

 that to discourse upon them here would be but vain 

 repetition of all that has been said before; yet, para- 

 doxical though it may seem, it is unquestionably the 

 work of all works that most definitely separates, and 

 at the same time most closely unites, the two sciences 

 of physics and chemistry, whilst it brings into promin- 

 ence what may appropriately be called a new science — 

 that of radio-activity — a science which neither physics 

 nor chemistry can claim within its old province, and 

 yet neither can disclaim, nor would it very readily do 

 so if it could. 



What is the influence which these laboratories have 

 exerted and exert? We may ask, what is the in- 

 fluence of the Royal Institution? Is it not to be 

 measured by the work which has been done there and 

 bv the ideas which have been scattered from those 

 great fountains of thought — if they can be measured? 

 How many youthful imaginations, how many 

 enthusiastic aspirations have been aroused within 

 those venerable halls, of the Becquerels as of the 

 Davys, the Faradays, and the Tyndalls? Parisian 

 lecturers are savants, philosophers, and orators. For 

 although the Teuton regards the gift of eloquence 

 (we hope it is his own) as the gift to be designated as 

 " gab," the southerner or the Celt thinks it indis- 

 pensable in the expression of a clear mind and of a 

 great soul, at once saturated with thoughts and the 

 grandeur of its subject; and in France this counts for 

 more than it usually does among us. 



These lectures arc a source of inspiration to the 

 multitude as well as to the gr;ive, and their importance 

 cannot be overrated. 



Having said thus much of the lalxiratories of the 

 Museum d'Histoire naturelle, wc may perhaps be per- 

 mitted to add a word as to the central figure in this 

 centre of scientific thought, of M. Henri Becquerel; 

 from none need we expect greater freedom, greater 

 ease, or kindlier consideration. The brief summary 



* British Associali 

 P/iil. Trans., 1897' 



and Phil. .Ua:; 



