136 NATURE OF THE RADIATIONS [CH. 
The absorption is least in hydrogen and greatest im 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 im the case 
of hydrogen. Results for the relative absorption by air of the a rays 
from the different active bodies are shown in Fig. 30. 
100 


Absorption of Radiation by 











Intensity of radiation 





(0) 2 + 6 8 10 12 14 16 18 20 22 24 26 
Distance in mms. 
Fig. 30. 
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 See bit ms! aes sae or 4:3 
Radium aes Se an eats 586 nae 75 
Thorium Ane eae nae rap vas ae 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. 
