
14.4 NATURE OF THE RADIATIONS [CH. 
cathode than to Rontgen rays. It is well known that Réntgen 
rays produce much greater ionization in gases such as sulphuretted 
hydrogen and hydrochloric acid gas than in air, although the 
differences in density are not large. For example, under the 
influence of X rays sulphuretted hydrogen has six times the con- 
ductivity of air, but under the cathode rays the conductivity is 
only slightly greater than that of air. In an experiment made 
by the writer, in which the testing vessel was filled with sulphu- 
retted hydrogen, it was found that the current for the y rays 
from radium was only slightly greater than it was when the vessel 
contained air. 
Strutt? has recently made a detailed investigation of the rela- ° 
tive conductivity of gases exposed to the y rays of radium. The 
results have already been given in the table in section 45. He found 
that the relative conductivities of different gases compared with 
air were about the same as for the 8 rays of radium, but were very 
different from the conductivities for Rontgen rays. 
The variation of absorption of these rays with density is also 
very similar to that of the cathode rays. On the other hand, 
Benoist? has shown that the relative absorption of Réntgen rays 
by matter depends to a large extent on the kind of rays em- 
ployed. “Hard” rays give ratios quite different from “soft” rays. 
For penetrating Réntgen rays, the absorption of the rays by a given 
weight of material is a continuous and increasing function of the 
atomic weight. 
The y rays thus show properties with regard to absorption 
and ionization unlike those of X rays, but 1t must not be forgotten 
that the y rays are of a far more penetrating character. It has 
not yet been shown that the properties of very penetrating X rays 
with regard to relative absorption and ionization are the same as 
those of the ordinary rays of moderate penetrating power which 
have so far been examined. 
It will be shown later (section 194) that the y rays, hke the 
8 rays, appear only in the last stage of the succession of chemical 
changes occurring in active bodies. Active products which give 
a rays and no 8 rays do not give rise to y rays. The @ and y rays 
1 Proc. Roy. Soc. 72, p. 208, 1903. 
2 C, R. 132, p. 545, 1901. 
