﻿482 On Ionization by Alpha Rays. 



in a metal, however, it is possible that we have ionization 

 without the expenditure o£ energy.) 



This view that ionization itself requires the expenditure of 

 no energy, and that the initial velocity of the delta ray is 

 zero, or determined merely by the temperature, requires, of 

 course, additional confirmation. But if it can be accepted 

 it may lead to a considerable change in our views in other 

 directions. For instance, most of the difficulties connected 

 with the nature of radiation, which have been the subject of 

 so much discussion lately, appeared originally because, on 

 the old theory of radiation, it was difficult to see whence 

 came the energy which was supposed to be necessary to 

 ionize. If no energy is required to ionize, the difficulties 

 disappear. 



But at present there is one piece of experimental evidence, 

 to which reference was made in the second paper on delta rays, 

 which is difficult to reconcile with the hypothesis that the 

 delta rays have no initial velocity. This is the fact that a 

 body emitting delta rays undoubtedly raises itself to a finite 

 positive potential. I am inclined to believe that this action 

 has nothing to do with the emission of delta rays from it ; 

 experiments are being undertaken to investigate the matter 

 further, and to discover whether it is as serious an objection 

 to the view that the delta rays have no velocity as appears 

 at present. 



Summary. 



2-5. Information as to the quantity and quality of delta 

 rays may be obtained by experiments in a gas which they 

 ionize by collision, as well as by experiments in a high 

 vacuum. Some qualitative experiments of this nature are 

 described which lead to the following conclusions : — 



(a) There is no difference in the "emergent" and 

 " incident"" radiations from the same metal. 



(b) The delta rays excited by a given stream of alpha 

 rays in two different metals are the same in quantity and in 

 quality. The conclusion as to quantity is, however, much 

 more certain than the conclusion as to quality. 



6. Experiments were next undertaken to investigate the 

 matter more thoroughly by extensive and accurate measure- 

 ments based on the theory of ionization by collision given 

 in the previous paper. 



7-10. The apparatus and the methods of measurement 

 are described, and the numbers obtained given in detail in 

 Tables Y.-XIX. 



11-15. The method by which the measurements are inter- 



