Metals under the Influence of Alpha Hays. 



317 



results obtained by Geiger.* It is possible that an explanation 

 of this effect may be found in the phenomena to be discussed 

 in the following section. Beyond five thick foils, the number 

 of a-particles diminishes rapidly; this is doubtless due to their 

 absorption in the aluminium foils, the more oblique rays being 

 the first to be stopped. 



By dividing the abscissae of lb by those of II, a curve could 

 be obtained which would represent the ionization or S-ray 



Fig. 2. 



effect produced by a fixed number of a-particles, which is, 

 strictly speaking, what should be given by an ionization curve. 

 Nevertheless, it does not seem advisable to use results thus 

 obtained in the present investigation ; the a-ray currents are 

 small and the errors introduced by using them as factors might 

 be considerable. Moreover our purpose is primarily to com- 

 pare the effects with different metals. The ionization curves 

 of gases and vapors have also usually been obtained without 

 allowance for the decrease in the number of a-particles, so that 

 a more direct comparison with them is possible by using the 

 curve lb. An estimate of the number of S-electrons due to 

 one a-particle may be obtained, however, by dividing the 

 abscissae of lb by those of II. Allowing for the fact that the 

 charge on an a-particle is twice the electronic charge, we find 

 that the number of S-electrons per a-particle emitted by an 



* Proe. Eoy. Soc, lxxxiii, 492, 1910. 



