1] IONIZATION THEORY OF GASES 61 
For example, if the active matter is uranium, pd,= 1'6 for the 
a rays at atmospheric pressure. If d,=3, and d, =1, the saturation 
current reaches a maximum when the pressure is reduced to about 
1/3 of an atmosphere. This result has been verified experimentally. 
45. Conductivity of different gases when acted on by 
the rays. For a given intensity of radiation, the rate of pro- 
duction of ions in a gas varies for different gases and increases 
with the density of the gas. Strutt’ has made a very complete 
examination of the relative conductivity of gases exposed to the 
different types of rays emitted by active substances. To avoid 
correction for any difference of absorption of the radiation in the 
various gases, the pressure of the gas was always reduced until 
the ionization was directly proportional to the pressure, when, as 
we have seen above, the ionization must everywhere be uniform 
throughout the gas. For each type of rays, the ionization of 
air is taken as unity. The currents through the gases were 
determined at different pressures, and were reduced to a common 
pressure by assuming that the ionization was proportional to the 
pressure. 
With unscreened active material, the ionization is almost 
entirely due to a rays. When the active substance is covered with 
a layer of aluminium ‘01 cms. in thickness, the ionization is mainly 
due to the @ or cathodic rays, and when covered with 1 cm. of lead 
the ionization is solely due to the y or very penetrating rays. 
Experiments on the y rays of radium were made by observing the 
rate of discharge of a special gold-leaf electroscope filled with the 
gas under examination and exposed to the action of the rays. 
The following table gives the relative conductivities of gases 
exposed to various kinds of ionizing radiations. 
With the exception of hydrogen, it will be seen that the ioniza- 
tion of gases is approximately proportional to their density for the 
a, 8, y rays of radium. The results for Rontgen rays are quite 
different; for example, the conductivity produced by them in 
methyl iodide was more than 14 times as great as that due to 
the rays of radium. The y rays of radium appear to be more allied 
to the 8 rays of radium than to Rontgen rays. 
1 Phil. Trans. A, p. 507, 1901 and Proc. Roy. Soc. p. 208, 1903. 
