Rate of Recombination of Ions in Gases. 211 
doubt whether the saturation current gives a measure of (. 
I think it gives a measure of the excess of the rate of pro- 
duction of ions over the rate of recombination ; and I see no 
reason for assuming that the latter is negligible. It is equal 
to aN,’?, where N, is the number of ions per unit volume when 
the steady state, with the saturation current passing, is reached. 
It certainly cannot be zero, and may be comparable with Q. 
In conclusion I should like to express my great obligation 
to my colleague Mr. James Gray, for his kindness in revising 
the tedious arithmetical calculations. 
Physical Laboratory, 
he University, Glasgow. 
26th March, 1904. 
XXII. Reply to Mr. G. W. Walker's Paper on the “ Rate of 
Recombination of Ions in Gases.” By Kh. K. McCune, 
M.A., Trinity College, Cambridge*. 
HROUGH the courtesy of Mr. Walker I am enabled to 
make a reply to the criticisms of my experiments on 
the rate of recombination of ions contained in his paper, which 
appears in the present number of this Magazine. These 
criticisms are evidently due to a want of knowledge of the 
exact conditions under which the experiments, described in 
my paper, were made, and the relation of the different expe- 
ments to one another. Iam very sorry that Mr. Walker has 
wasted so much valuable time in making what must have 
been such very tedious arithmetical calculations, for the 
numbers obtained by him are quite valueless, and consequently 
the criticisms based upon these numbers are quite beside the 
mark. 
Although at the beginning of his paper he mentions three 
of my papers which have appeared in the Philosophical 
Magazine, yet he confines his criticisms to the one on the 
effect of pressure on the rate of recombination of ions. I 
shall therefore also confine my remarks to the experiments 
described in that paper. 
In the first place, Mr. Walker makes the following state- 
ment in the first partof his paper:—“ In testing the formula (1) 
it appears to me that he assumes the relation between N,, N, 
and ¢ to be of the same form as (1). This, as I shall show, is 
not the case in his experiments. It would be true if the beam of 
rays were cylindrical.” In deriving formule (1) and (2) no 
assumption whatever is made as to the shape of the beam of 
* Communicated by the Author. 
P2 
