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 α rays from the different active bodies are shown in Fig. 38. [Illustration: 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 16·5 from Thorium and Radium 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. =101. Connection between absorption and density.= Since in all cases the radiations first diminish approximately according to an exponential law with the distance traversed, the intensity _I_ after passing through a thickness _x_ is given by $$ I = I₀ e^{–λ x} $$ where λ is the absorption constant and _I₀_ the initial intensity. The following table shows the value of λ with different radiations for air and aluminium. Radiation λ for λ for air aluminium Excited radiation 830 ·42 Thorium 1250 ·69 Radium 1600 ·90 Uranium 2750 1·6 Taking the density of air at 20° C. and 760 mms. as 0·00120 compared with water as unity, the following table shows the value of λ divided by density for the different radiations. Radiation Aluminium Air Excited radiation 320 350 Thorium 480 550 Radium 620 740 Uranium 1060 1300 Comparing aluminium and air, the absorption is thus roughly proportional to the density for all the radiations. The divergence, however, between the absorption-density numbers is large when two metals like tin and aluminium are compared. The value of λ for tin is not much greater than for aluminium, although the density is nearly three times as great.