of different gases when acted on by the rays.** For a given intensity of radiation, the rate of production of ions in a gas varies for different gases and increases with the density of the gas. Strutt33 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 α rays. When the active substance is covered with a layer of aluminium ·01 cm. in thickness, the ionization is mainly due to the β or cathodic rays, and when covered with 1 cm. of lead, the ionization is solely due to the γ or very penetrating rays. Experiments on the γ 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.
| Gas | Relative Density | α rays | β rays | γ rays | Röntgen rays |
|---|---|---|---|---|---|
| Hydrogen | 0·0693 | 0·226 | 0·157 | 0·169 | 0·114 |
| Air | 1·00 | 1·00 | 1·00 | 1·00 | 1·00 |
| Oxygen | 1·11 | 1·16 | 1·21 | 1·17 | 1·39 |
| Carbon dioxide | 1·53 | 1·54 | 1·57 | 1·53 | 1·60 |
| Cyanogen | 1·86 | 1·94 | 1·86 | 1·71 | 1·05 |
| Sulphur dioxide | 2·19 | 2·04 | 2·31 | 2·13 | 7·97 |
| Chloroform | 4·32 | 4·44 | 4·89 | 4·88 | 31·9 |
| Methyl iodide | 5·05 | 3·51 | 5·18 | 4·80 | 72·0 |
| Carbon tetrachloride | 5·31 | 5·34 | 5·83 | 5·67 | 45·3 |
With the exception of hydrogen, it will be seen that the ionization of gases is approximately proportional to their density for the α, β, γ rays of radium. The results obtained by Strutt for Röntgen rays are quite different; for example, the relative conductivity produced by them in methyl iodide was more than 14 times as great as that due to the rays of radium. The relative conductivities of gases exposed to X rays has been recently re-examined by McClung34 and Eve35, who have found that the conductivity depends upon the penetrating power of the X rays employed. The results obtained by them will be discussed later (section 107).
This difference of conductivity in gases is due to unequal absorptions of the radiations. The writer has shown36 that the total number of ions produced