THE ADVANCE OF SCIENCE 73 



were given off by the uranium : the black paper was 

 penetrated, and the form of the copper cross was printed 

 on a dark ground (fig. 9). The copper was also penetrated 

 to some extent by the rays from the uranium, so that its 

 image was not left actually white. Only one step more 

 remained before Becquerel made his great discovery. It 

 was known, as I stated just now; that sulphide of calcium 

 and similar substances become phosphorescent when ex- 

 posed to sunlight, and lose this phosphorescence after a 

 few hours. Becquerel thought at first that perhaps the 

 uranium salt acquired its power similarly by exposure to 

 light ; but very soon, by experimenting with uranium 



FIG. 9. HENRI BECQUERBL'S DISCOVERY OF RADIO-ACTIVITY. 



Photographic print or skiagraph of a copper Maltese Cross produced 

 by uranium salt placed as a heap of powder on the surface of black paper 

 wrapped round a sensitive plate. Between the paper and the uranium 

 powder the flat copper cross was interposed. The rays from the uranium 

 salt have penetrated the black paper, but have been intercepted to a large 

 extent by the copper cross so that the sensitive silver plate is darkened 

 all about the cross over an area corresponding to that of the heap of 

 uranium salt, but is left pale where the copper figure blocked the path of 

 the active rays given off by the uranium, partially but not wholly. It was 

 thus proved that the rays from the uranium salt can pass through 

 blackened paper and also though to a less extent through a plate of 

 copper. 



salt long kept in the dark, he found that the emission of 

 penetrating rays, giving photographic effects, was pro- 

 duced spontaneously. The emission of rays by this 



