778 ANNUAL EEPORT SMITHSONIAN INSTITUTION^ 1908. 



term ought to be used for each band, so that if there are two bands in 



the spectrum the formuhi becomes 'i = io ( r-, ) +( } ) • He 



verified this formula in several cases, so that it seems proved that 

 the phosphorescent phenomena follow a law probably adaptable to 

 some mechanical explanation but much more complicated than acous- 

 tical resonance or other analogue to which we are at j^resent ac- 

 customed. 



From that moment, for Becquerel, phosphorescence became a source 

 offering mj^sterious properties, the unraveling of the secret of which 

 would embrace a multitude of new discoveries. 



RADIO-ACTIVITY. 



When the discovery of Rontgen was announced, Becquerel, like 

 many others, at once tried to see whether phosphorescent bodies 

 emitted photographic or phosphorogenic radiations which would 

 traverse opaque bodies. And here we may still better appreciate 

 the subtlety of Becquerel's mind. In the midst of a maze of seem- 

 ingly contradictory facts, he knew, by his marvelous intuition, how 

 to avoid the paths to error and to take that which would lead him 

 by infinitely small manifestations to the fundamental phenomena of 

 radio-activity, that immortal discovery which has already revolution- 

 ized modern physics and promises to lead the physics of the future 

 into fields as yet unrealized. 



The biography now becomes difficult. It could be made nothing 

 more than the renumeration of Becquerel's astonishing discoveries 

 without explaining the extraordinary conditions under which they 

 were produced. Those physicists who may read this will recall 

 that fever of excitement among men of science which followed in 

 1896 the anouncement of Eontgen's discovery. They will recall, too, 

 the first experiments of Becquerel, which raised the doubts of the 

 older school and the curiosity of the younger. Then Becquerel, 

 aroused by the daily disclosure of new truths and by the increased 

 publication of his works, accumulated in three years a mass of re- 

 sults that confounds us. And we should also note at this time the 

 devoted collaboration of his assistant, M. Matout, in whom he inspired 

 admiration as a man of science and an unlimited personal attachment. 



Ordinary phosphorescent substances give off no emanation capable 

 of traversing black paper. But it is not so with the compounds of 

 uranium, whose peculiar properties Becquerel had already recorded. 

 By first covering a photographic plate'with black paper and placing 

 over the latter a salt of uranium excited by direct sunlight, he suc- 

 ceeded in obtaining an impression upon the plate. But one day the 

 sunlight disappeared a moment after the exposure had been started 



