318 Mr Kleeman, On the Dependence of the Relative Ionisation 
the ionisation due to the absorption of the secondary y rays by 
the gas in the chamber, and the other the ionisation produced — 
by the @ rays produced by the secondary y rays in the walls of 
the chamber. . 
When the softness of the primary y radiation is increased, 
that of the secondary radiation from the walls of the chamber 
must be increased also. Thus the behaviour of the relative 
ionisations of the gases in the chamber with a variation of the 
‘ 
ry 
+4 
softness of the primary y rays, is an index of a general nature of — 
the variation of the chance of an atom becoming the source of a 
8 ray under the influence of y pulses with the softness of the 
latter. Some experiments* that I have carried out previously 
have a bearing on this point. It was found that the ionisation of 
a gas relative to that of air increased with the softness of the 
y rays when the molecules contained atoms heavier than those in 
air. But it decreased with the softness of the rays in the case of 
hydrogen. ‘This fits in with the results obtained by Crowthert 
with X rays of different hardness. The results are thus evidence 
of the identical nature of y and X rays. 
It will be of interest here to consider more closely the nature 
of the process of the ionisation by y or X rays. According to the 
experiments of C. T. R. Wilson mentioned the ionisation is 
produced by the 8 rays ejected by the rays. In these experi- 
ments photographs of the ions were obtained by making them 
serve as nucle1 to a fog in a cloud-producing apparatus of 
appropriate construction. Previously, however, Crowther} had 
shewn that the ionisation by X rays is largely direct ionisation. 
The method he used involved the principle that the ionisation 
produced, varies directly as the pressure, while that produced 
by the secondary @ rays varies as the square of the pressure. 
Beatty§ later found that the amount of direct ionisation 
greatly depended on the nature of the gas and hardness of 
the rays. ‘These experiments do not fall into line with those of 
C. T. R. Wilson. But the disagreement is, I think, more ap- 
parent than real. An inspection of the photographs obtained by 
C. T. R. Wilson will shew that a blotch occurs at the beginning of 
each streak which shews the ions produced along the course of a 
B or cathode ray. It may be mentioned that a blotch also occurs 
at the end of each streak, being due to the zig-zag path and 
intense ionisation by the cathode particle before it ceases to 
ionise. In the former case it seems to be most likely due to 
the cathode particle ionising one or more atoms of its parent 
* loco. cit. 
+ Proc. Roy. Society, A, vol. 82, p. 115 (1909). 
+ Ibid., p. 103 (1909). 
§ Ibid., vol. 85, p. 230 (1911). 
