Page:Radio-activity.djvu/188

 The absorption is least in hydrogen and greatest in carbonic acid, and follows the same order as the densities of the gases. In the case of air and carbonic acid, the absorption is proportional to the density, but this rule is widely departed from in the case of hydrogen. Results for the relative absorption by air of the [Greek: alpha] rays from the different active bodies are shown in Fig. 38.

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 the order of absorption by the metals and solid substances examined.