Electrons by Metals under Influence of Alpha Buys. 407 



Here there does appear to be a slight alteration in the form 

 of the ionization curve, and in the direction expected. But 

 the differences are so slight that no great confidence can be 

 placed in the result. Further experience with the method 

 did not give any reason for hoping that the question as to 

 the effect o£ adsorbed gases could be definitely settled in 

 this way. 



The results of this experiment are susceptible of two 

 quite different interpretations. We may say that, since the 

 heating of the platinum strip did not alter the shape of its 

 ionization curve, we may conclude that this shape is due to 

 the properties of the metal itself and not to adsorbed or 

 occluded gases. On the other hand, we may put the emphasis 

 on the fact that, under certain conditions, at least 30 per 

 cent, of the S-ray effect is due to such gases, and that when 

 these are removed there is no change in the shape of the 

 curve. If the residual effect is really due to the metal, we 

 must suppose that platinum and the adsorbed gases have 

 ionization curves of very nearly the same shape, which is not 

 altogether probable considering the variations which are met 

 with in the curves of gaseous ionization. From tins point 

 of view, therefore, it seems more probable that the 8- ray 

 emission (slow electrons) is mainly due to such a layer of 

 gas, which may be reduced but not entirely removed by the 

 methods which we have employed *. 



§ 2. Determination of the Ionization Curves of Various 

 Metals. 



In the determination of the ionization curve of platinum in 

 the preceding section and of those of gold and aluminium 

 in the previous paper, the metals were so thin that the a-rays 

 passed through them, except when they were near the end of 

 their range. This has the advantage that, at least until the 

 top of the curve is reached, no correction for the charge of 

 the a-rays is necessary, and one need not apply a magnetic 

 field in order to determine this charge. On the other hand, 

 a difficulty results if one wishes to make a careful comparison 

 of different metals. It is impossible to obtain foils of 

 different metals which have the same retarding effect on the 

 a-rays ; and hence the emergence S-radiation from the different 

 metals corresponds to different speeds of the a-rays even when 

 the same number of aluminium foils are interposed. This is 



* Since the present investigation was completed a paper has appeared 

 (Pound, Phil. Mag. xxiii. p. 813, May 1912) in which the effect of 

 occluded, gases upon the magnitude of the 8-radiation is clearly brought 

 out. 



