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OisCance. from dssumeot Umb. 



Eadio-activity of Uranium." By Sir William Ceookes, F.E.S. 

 Eeceived May 3 — Eead May 10, 1900. 



[Plate 5.] 



1. The researches of M. Henri Becquerel have shown that com- 

 pounds of uranium possess the property now called " radio-activity " ; 

 that lis, rays emitted by them affect a sensitive photographic plate 

 through bodies usually considered opaque to light; they discharge 

 an electrometer when brought near it ; and they are deflected by a 

 ■magnet. These rays are now called " Becquerel rays," or " uranic 

 rays." 



2. On the discovery by M. and Mdme. Curie of polonium and 



