﻿Ionization by Collision. 411 



TI13 resulting equations are so complicated that no attempt 

 has been made to apply them to experiments. But it mav 

 he noted that, in place of the simple exponential iorm (17) 

 when ;/ = 0, we obtain 



i/e = P^*+-QZ + B (17') 



Accordingly, if this theory were correct, the curves obtained 

 by Townsend should not have been accurately exponential. 

 Sufficient measurements are not given to test this point, but 

 the fact that consistent values of the exponential coefficient 

 were obtained from different pairs of points, on the assump- 

 tion that the curve was exponential, is some evidence against 

 the second theory. But the distinction between the two 

 theories can be best made by examining the calculated and 

 observed relations between n x and n' and those constants in 

 the equations which involve them. Townsend gives no 

 values for %, and hence this test cannot be applied. The 

 experiments described in the next paper appear to provide a 

 conclusive proof in favour of the first theory and against the 

 second. 



Summary. 



1-3. The physical assumptions made in Townsend's theory 

 of ionization by collision are analysed and discussed. It is 

 pointed out that one of them is to the erfect that the negative 

 ion does not undergo any permanent change in its properties 

 as a consequence of repeated collisions with neutral mole- 

 cules. This assumption is contradictory to the older view of 

 ionization by collision, according to which the negative ion, 

 when first formed, is an electron, but subsequently becomes a 

 "complex ion" by gathering round it neutral molecules. 



4, 5. Townsend's mathematical argument, by which he 

 applies these assumptions to certain cases of the discharge 

 through gases, is sketched briefly. 



G-8. It is pointed out that this argument contains an error. 

 No attempt is made to arrive at completely, accurate formulae, 

 but a nearer approximation is made. 



8, 9. The new formulas are applied to Townsend's measure- 

 ments, and shown to agree with them better than those which 

 he used. 



10-12. Similar formula? are briefly sketched based on the 

 older view that a negative ion after collisions with neutral 

 molecules becomes a complex ion. Townsend's experiments 

 are not so well suited for distinguishing between the two 



