﻿Ionization by Collision. 401 



(3) An ion, however formed, immediately after its forma- 

 tion has a velocity so small th;it it may be neglected. 



(4) A collision, whether or not it results in ionization, 

 does not change the nature of the ion in such a way that it 

 is incapable of ionization at subsequent collisions after a free 

 fall through the potential difference V'. 



3. Assumption (1) is the basis of the theory. (2) is 

 almost certainly not accurately true ; an ion after collision 

 retains some portion of its previous velocity ; but the as- 

 sumption is made for simplicity of calculation. The error 

 probably does not lead to an error in the form of the formulae 

 deduced, but only to values of N or V' which are somewhat 

 too small. (3) is considered at length in the following paper * 

 on " Ionization by a, rays." 



(4) is deserving of further consideration. When the 

 theory of gaseous ionization was first advanced by Thomson 

 and Rutherford, it was believed that the negative ion con- 

 sisted of a group of several molecules collected round an 

 electron. Ionization was thought to consist in the liberation 

 of an electron from a neutral molecule, the collection of 

 other neutral molecules round it being effected at subsequent 

 collisions. If this view, already considerably discredited, 

 were correct, a negative ion immediately after its formation 

 would be a system different from a negative ion which had 

 already made several collisions. After these collisions it 

 would resemble the positive ion more than the original 

 electron. Accordingly, if the great difference in V for the 

 negative and positive ions is to be attributed to a difference 

 in their structure, it would be expected that this difference 

 should disappear after a few collision*, and that no ionization 

 by collision of negative ions which had made such previous 

 collisions would occur. 



This view is not adopted by Townsend ; he assumes that 

 the properties of the negative ion remain unchanged how- 

 ever many collisions it makes. If his assumption is true, the 

 contusion would seem to follow that the negative ion must 

 remain an electron throughout its history, and not collect 

 round it neutral molecules. The theory will be developed 

 first on this assumption ; later it will be considered how the 

 theory is modified if the negative ion changes its properties 



P It IT" 



alter a tew collisions. 



4. The gas is supposed lo be contained between parallel 



electrode?, of which the distance apart, parallel to (he. axis 



of ./•, is /. The difference of potential between the electrodes 



is V, and the uniform electric intensity between them 



* To be published in the April number. 



Phil Mag. S. 6. Vol. 23. No. 135. March L912. 2 E 



