RECENT DISCOVERIES IN RADIATION. 483 



was to expose uranium to strong sunlight for a long time, and then to 

 notice whether a photographic plate, which was wrapped up carefully 

 in perfectly opaque paper and placed beneath the uranium, received any 

 impression from it. He found that it did; but he further found that 

 the exposure of the uranium to sunlight was altogether unnecessary; 

 that the uranium itself in a perfectly dark room would affect, in the 

 course of ten or twenty days, a photographic plate from which it was 

 separated both by opaque black paper and by a thin sheet of metal. In 

 fact he obtained in this way a radiograph of a metallic object similar 

 in all respects to the pictures which Rontgen had obtained with X-rays. 

 This showed, in the first place, that the fluorescent light had nothing 

 whatever to do with the production of the photograph, but it showed also 

 something much more important than this, namely, that the mineral 

 uranium is all the time spontaneously emitting rays of some sort, 

 which are capable of penetrating opaque objects in just the way the 

 X-rays do. 



This discovery, which has been one of the most fruitful in the 

 history of science, is immediately due to the accident of a few cloudy 

 days in Paris, during which Becquerel, since he could not expose his 

 uranium to sunlight, set away his plate with the uranium on the top 

 of it, to wait for fair weather. When the fair weather returned and 

 he was ready to continue his experiments, it fortunately occurred to 

 him that it might be worth while to develop the plate upon which the 

 uranium had rested to see if anything had happened to it. The dis- 

 covery of radio-activity was the result. Those who recall the story 

 of the discovery of photography will remember that it was made quite 

 as accidentally and under quite similar circumstances. 



Becquerel further found that the rays emitted by uranium are 

 also emitted by all uranium compounds. He therefore named them 

 uranium rays. Another property which he found that the rays pos- 

 sessed, in addition to that of affecting a photographic plate, was the 

 important property of rendering a gas through which they pass a 

 conductor of electricity, or, to state the same thing in another way, 

 the property of discharging any electrified body which is brought into 

 their neighborhood. 



The Discovery of Radium. 



It was but a few months after this that Madame Curie, one of the 

 few women who has attained eminence in the pursuit of science, and 

 who together with her husband, with whom most of her work has been 

 done, deserves a large share of the credit for our present knowledge of 

 radium, set about investigating all the then known elements to see if 

 any of the rest of them possessed this remarkable property which 

 Becquerel discovered in uranium. She found that one, and but one, 

 of the remainder of the elements, namely, thorium, the element which is 

 one of the chief constituents of Welsbach mantles, was capable of pro- 



