THE DISCOVERY OF RADIO-ACTIVITY. 87 



querel, "that it was necessary previously to expose the sub- 

 stance to light in order to provoke this very penetrating 

 emission, but a short time after / recognised that the emission 

 of the rays was produced spontaneously, even when the sub- 

 stance had been kept completely sheltered from any previous 

 exposure to light." This settles the question. Niewenglow- 

 ski's rays were directly due to the action of the sun upon the 

 substance which emitted them; Becquerel rays arise from a 



Fig. 21. Becquerel's first experiment. 



substance whose natural property it is, not only to emit 

 them, but, apparently, to manufacture them. It may be 

 stated here that since this discovery, the rays emitted by 

 this particular fragment of uranium compound have shown 

 no signs whatever of diminution. They are, apparently, a 

 permanent property of this form of matter. Furthermore, 

 it was soon seen to be a matter of indifference what uranium 



