﻿836 Secondary Rays excited by Alpha Rays from Polonium. 



polonium was raised above 40 volts, but this decrease was 

 not nearly as great as before. 



The reason for the final slight difference between the 

 behaviour of a carbon and of a brass electrode appears, then, 

 to be due to a difference between the a. ray reflecting power 

 of carbon and brass at different voltages, the brass reflecting 

 more a rays than the carbon as the voltage was increased. 

 If the work done by Geiger and Marsden * on the reflexion 

 of a rays is taken to be applicable to the present experiments, 

 it would seem that a particles cannot be reflected in sufficient 

 numbers to account for this difference. However, the ex- 

 periments described above point definite]y to the reflexion of 

 a. rays as the cause of the slight difference in the behaviour 

 of the carbon and the brass plates under the bombardment 

 by a rays. Moreover, it can easily be shown that with the 

 fields used variations in the speed of the a rays amounting to 

 1*7 per cent, must have ensued. It is just possible that this 

 variation might be sufficient to cause such a change in the 

 amount of a radiation reflected from the carbon and brass 

 electrodes as to contribute, in part at least, to the above 

 effect. It would appear, therefore, that additional experiments 

 should be made on the reflexion of a rays of different velocities 

 at surfaces subjected to low gas pressures before the expla- 

 nation offered above of the effect observed is set aside. 



7. Summary of Results. 



1 . It has been shown that there is a secondary radiation 

 produced when alpha rays fall on a brass or a carbon plate. 



2. This secondary radiation has been proved to be in part 

 due to the presence of gas occluded in or at the surface of 

 the brass or carbon. 



3. When this gas is being removed from the brass or 

 carbon it is found that the secondary radiation decreases, and 

 gives rise to an effect similar to a " fatigue " of the secondary 



4. This fatigue effect is found to be greater for carbon 

 than for brass. This last result was to be expected when the 

 fatigue effect was traced to the presence of occluded gases, 

 since carbon is known to possess a greater capacity for 

 occluding gases than a metal such as brass. 



5. From the experiments which have been described, it 

 will be seen that the secondary radiation emitted by a 



* Geiger and Marsden, Proc. Rov. Soc. Series A, toL lxxxii. July 31, 

 1909. 



