WORK OF HENRI BECQUEREL BROCA. 777 



covery of radio-activity. Uranium forms two distinct series of salts. 

 Edmond Becquerel had shown the phosphorescence of one set, the 

 second does not phosphoresce. Henri Bacquerel soon noted that the 

 latter salts have characteristic bands of absorption in the visible and 

 the infra-red spectrum. Studying further the compounds of the first 

 class, he noted that most of them phosphoresce as had already been 

 shown and that they have in general a discontinuous spectrum of 

 seven or eight bands or groups of bands between the C and the F 

 lines; these bands vary according to the nature of the compound. 

 The compounds have selective absorption bands which correspond to 

 all the radiations which will excite the phosphorescence. If the body 

 is excited by the light of the wave length of any one of these latter 

 bands it will give out its total emission spectrum of all the wave 

 lengths proper to its phosphorescence. Becquerel formulated this 

 law : The difference in the oscillation frequencies in passing from one 

 band to another is a constant, the bands of absorption continuing the 

 series formed by those of emission. The latter seem to be the sub- 

 harmonics of the former. Often one or two of the less refrangible 

 absorption bands coincide with the more refrangible ones of emission. 

 It seems probable that the absorption forms some kind of synchronism 

 with the periods of the emission, but it is not expected that they will 

 be found to be subharmonics. They are essentially distinguished from 

 incandescent vapors in absorption. The ordinary theories of reso- 

 nance are incapable of explaining them, but the simplicity of the law 

 which binds them gives hope some day of the possibility of a mechan- 

 ical explanation. 



It is remarkable that the second series of uranium salts which do 

 not phosphoresce but seem to degrade into heat the selectively ab- 

 sorbed radiation, should have bands which follow with a notable 

 regularity the same law which holds for the emission bands of 

 the other series. The bands have not, however, the same relative 

 intensities. 



We have just seen the theoretical difficulties offered by these 

 phenomena. Edmond Becquerel had already studied and formulated 

 the variation of the intensity of the phosphorescent emission with 

 the time. His formula, however, held only for very short periods of 

 time. Nor was the one derived by Wiedemann sufficient. The 

 formula, i = %e~"^ was deduced theoretically from the hypothesis that 

 the molecular degradation of energy is proportional to the velocity of 

 what w^e will now call electrons. Becquerel thought it better to make 

 this degradation proportional to the square of the velocity and so 



obtained the formula i = iJ j- i . As the photometric measures of the 



total phosphorescence intensity did not agree with this, Becquerel, 

 remembering the changes of color, thought it possible that a similar 



