Page:Radio-activity.djvu/84

 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 M^cClung and Eve, 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).