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Transactions of the Canadian Institute. [vol. ix 



the object of a further elucidation of the results described in Section III 

 it was thought well first to make an extended study of the phenomenon of 

 the decrease in the intensity of the secondary radiation as made evident 

 by the foregoing experiments. 



V. Experiments on the "Fatigue" of Secondary Rays. 

 The experiments described in Section IV shew that there was a 

 decrease in the secondary radiation sent ofif by carbon bombarded by a 

 rays as the air was pumped out of the apparatus in which the carbon 

 was placed. This decrease may be called a fatigue of the secondary 

 rays for the effect is similar to that observed in the case of the photo- 

 electric fatigue. The phenomenon of the photo-electric fatigue has been 

 studied extensively, and various reasons have been suggested for it. 

 One of the reasons which have been advanced is that the substance which 

 emits the photo-electric radiation becomes impoverished of available neg- 

 ative corpuscles. If the fatigue in the present experiments were due to a 

 decrease in the number of available negative corpuscles, this decrease must 

 have been all at the surface since the exciting « rays have only a small pen- 

 etrability. If the decrease were due to this cause, it is probable that if 

 air were admitted into the apparatus again, the electrode would regain 

 its normal condition. To test this the following set of experiments was 

 performed. 



A fresh piece of carbon was placed in the apparatus, a positive charge 

 of 80 volts was put on the polonium, the Gaede pump was started, and 

 at a definite interval of time after the starting of the pump, readings 

 were taken of the rates of charging the carbon electrode, and of the cor- 

 responding pressures of air in the apparatus. These readings were con- 

 tinued until there was practically no further decrease in the rate of 

 charging of the electrode. 



The readings are given in the following table : 



Table V. 



Air in apparatus. 



