for the relative absorption by air of the α rays from the different active bodies are shown in Fig. 38.
The initial observation was made about 2 mms. from the active surface, and the initial current is in each case taken as 100. The current, as in the case of uranium, falls off at first approximately in geometrical progression with the distance. The thickness of air, through which the radiation passes before the intensity is reduced to half value, is given below.
| Distance in mms. | |
|---|---|
| Uranium | 4·3 |
| Radium | 7·5 |
| Thorium | 10 |
| Excited radiation from Thorium and Radium | 16·5 |
The order of absorption by air of the radiations from the active substances is the same as