CHAPTER II. 

 THE DISCOVERY OF RADIO-ACTIVITY. 



But Becquerel, abreast of the same wave of investiga- 

 tion, thought as Niewenglowski. He says, "For my part, 

 from the day on which I first had knowledge of the discovery 

 of Professor Rontgen, there came to me, too, the idea of 

 seeing whether the property of emitting very penetrating 

 rays was not intimately bound up with phosphorescence." 

 His thought was soon represented concretely; for, taking 

 fragments of various phosphorescent substances, he placed 

 them one after another on a photographic plate enveloped 

 in black paper, and thus gave them an opportunity of telling 

 their secrets by penetrating the paper and affecting the plate 

 beneath. In this, his work resembled that of Niewenglowski ; 

 but the importance of it is and the luck of it was, that he 

 experimented with different substances. Out of all the dif- 

 ferent substances tried, there was one, uranium, that had 

 waited long for this one precious day. For one day of 

 twenty-four hours this substance lay upon a photographic 

 plate enveloped in black paper, and thus, after ages upon 

 ages of waiting, found utterance. This plate was affected. 

 A glance at Fig. 21 will make it evident; and a close exami- 

 nation will reveal the shadow of the copper cross through 

 which the rays had to pass. The plate is obscure, as would 

 be the picture of the approach of dawn; and it is equally 

 significant. It reveals nothing but the presence of pene- 

 trating rays. " Here I am/' said Nature, "now, tell me, am 

 I Niewenglowski's rays?" "I thought, then," says Bee- 

 (86) 



