I9I2J 



Secondary Rays Excited by the Alpha Rays. 



163 



The carbon was left in the evacuated apparatus, bombarded by a 

 rays from the polonium for one week in order that it should get thoroughly 

 fatigued to 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 re- 

 peated and the readings are given below. 



Table VI. 



Fatigued carbon electrode. 



Air in Apparatus 



Charge on Pol. = 80 volts. 



After this experiment was completed the apparatus was immediately 

 filled with air at atmospheric pressure and left standing for 22 hours. 

 Then the above experiment was again repeated with the following results. 



Table VII. 



The results given in the last two columns of Tables V-VII are rep- 

 resented by curves shewn in Fig 4. The upper curve is plotted from the 

 results given in Table V. the middle curve from Table VI and the lower 

 curve from Table VII. 



It will be seen on looking at the curves that each has a gradual drop. 

 This drop represents both the decrease as time went on in the rate at 

 which the carbon electrode sent out secondary rays and the* decrease in 

 the ionisation current across the air gap between the polonium and the 

 electrode beginning at a certain definite interval of time after the first 



