﻿xcitecl by the Alpha Rays from Polonium, 



825 



been studied extensively, and various reasons have been 

 suggested, for it. One of the reasons which have been ad- 

 vanced is that the substance which emits the photoelectric 

 radiation becomes impoverished of available negative cor- 

 puscles. 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 u rays have only a small penetrability. 

 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 

 alter the starting of the pump readings were taken of the 

 rates of charging the carbon electrode, and of the corre- 

 sponding pressures of air in the apparatus. These readings 

 were continued 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. 



Fresh carbon electrode. Charge on polonium «= 80 volts. 



Pressure of air in 



vessel. 



Time from initial 



reading taken 7 minutes 



alter starting pump. 



Current to 

 electrode. 



•007 mm. 



•002 „ 



•001 „ 



<-001 „ 



<-001 „ 



<-ooi „ 



<-001 „ 

 < 001 „ 



minutes 

 5 



15 

 25 

 41 

 65 

 87 

 123 



124-7 

 78-2 

 50-7 

 412 

 352 

 31-2 

 28-2 

 26-2 



The carbon was left in the evacuated apparatus, bombarded 

 by ol rays from the polonium, for one week in order that it 

 should get thoroughly fatigued in the production of secondary 

 rays. Then the apparatus was filled with air at atmospheric 

 pressure and again left standing for one week. At the end 

 of this time the experiment described above was repeated, 

 and the readings are given below. 



