﻿Ionization hy Alplia Rays. 481 



The last conclusion confirms the view that the alpha rays 

 ionize every atom which they pass through, whatever its 

 nature, so long as they have any ionizing power at all — a 

 conclusion which may be deduced from the number o£ ions 

 made by an alpha particle in travelling through a gas. 



From (A) we may conclude that no important part of the 

 energy of ionization appears as energy of the ejected electron. 

 This conclusion appears to me of the utmost importance, for 

 it seems to suggest that the energy expended when gases are 

 ionized is not expended directly in producing ionization, but 

 that it is expended on some other process of which ionization 

 is a bye-product. This view has been already put forward 

 by Prof. Bragg to explain the fact that, whereas the energy 

 expended in ionizing a gas by alpha rays is determined, 

 solely by the nature of the atoms in the gas, the amount of 

 ionization produced is not so determined, but depends upon 

 the nature of the molecules in which the atoms are com- 

 bined ; this fact also suggests that the energy does not go 

 directly to producing ionization. The most plausible view 

 seems to be that the energy is expended by the rays in 

 penetrating the atoms through which they pass, and that the 

 ionization is produced by the electrons drifting out of the 

 atoms owing to the internal disturbance of the atom caused 

 by the presence of the alpha particle within it. 



(B) suggests very strongly the conclusion that the initial 

 speed of the delta rays is not only small compared with 20 volts, 

 but is actually zero, or at least a very small fraction of a 

 volt. For if this initial velocity is independent of the 

 exciting rays and of the material on which they impinge, on 

 what can it be dependent? Possibly on the temperature of 

 the material, as in the case of free electrons in metals, but 

 then the velocity would only be a small fraction of a volt. 

 The only other physical hypothesis which would lead to a 

 value for the initial velocity universally constant is to the 

 -effect that in ionization an " explosive " action of some 

 system common to all atoms is concerned. An hypothesis 

 of this nature has been put forward by J. J. Thomson, but 

 it was based on the view that the actual process of ionization 

 required the expenditure of energy. But we have seen that 

 the experiments are quite consistent with the view that 

 ionization itself requires no energy, that the energy which 

 is spent actually in producing ionization is spent in producing 

 changes in the atom, of which ionization is only a bye- 

 product ; it is conceivable that ionization might be produced 

 without any expenditure of energy, even it' it is not actually 

 possible so to produce it. (In the case of the five electrons 



Phil. Mag. S. 6, Vol. 23. No. 136. April 1912. 2 K 



