SOME PHYSICAL PROBLEMS 



been for some years the source of uranium and its com- 

 pounds, which, on account of their brilliant colors, 

 have been used in dye-stuffs and some kinds of stained 

 glass. It is a complex mineral, containing at least 

 eight or ten elements, which can be separated from it 

 only with great difficulty and by complicated chemical 

 processes. 



Becquerel's discovery was brought about by a lucky 

 accident, although, like so many other apparently ac- 

 cidental scientific discoveries, it was the outcome of a 

 long series of scientific experiments all trending in 

 the same direction. He had found that uranium, 

 when exposed to the sun's rays, appeared to possess the 

 property of absorbing them and of then acting upon 

 a photographic plate. Since pitch-blende contained 

 uranium, or uranium salts, he surmised that a some- 

 what similar result might be obtained with the ore 

 itself. He therefore prepared a photographic plate 

 wrapped in black paper, intending to attempt making 

 an impression on the plate of some metal body inter- 

 posed between it and the pitch-blende. For this pur- 

 pose he had selected a key ; but as the day proved to be 

 cloudy he put the plate, with the key and pitch-blende 

 resting upon it, in a dark drawer in his desk, and did 

 not return to the experiment for several days. Upon 

 doing so, however, he developed the plate without 

 further exposure, when to his astonishment he found 

 that the developed negative showed a distinct im- 

 pression of the key. Clearly this was the manifestation 

 of a property heretofore unknown in any natural sub- 

 stance, and was strikingly similar to the action of the 

 Roentgen rays. Further investigations by Lord Kel- 



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