132 " NATURE OF THE RADIATIONS [CH. 
radium have been completely absorbed in the gas, and the small 
ionization then observed in the gas is due to the more penetrating 
8 andy rays. The relative amount of the ionization observed at 
a distance from the source will increase with the thickness of the 
layer of active matter, but will reach a maximum for a layer of a 
certain thickness. The greater proportion of the ionization, due 
to unscreened active matter, 1s thus entirely confined to a shell of 
air surrounding it not more than 7 cms. in depth. 
91. The @ rays from different compounds of the same active 
element, although differmg in amount, have about the same average 
penetrating power. Experiments on this point have been made by 
the writer! and by Owens”. For the purpose of comparison of the 
relative power of penetration of the a rays from the different radio- 
elements, it 1s thus only necessary to determine the penetrating 
power for one compound of each of the radio-elements. Rutherford 
100, 

Absorption of Radiation 
by Aluminium Foil 




Intensity of radiation 















; 
Layers of Aluminium Foil (00034 cms. thick) 
Fig. 27. 
and Miss Brooks? have determined the amount of absorption of 
the a rays from the different active substances in their passage 
through successive lavers of aluminium foil ‘(00034 em. thick. The 
1 Phil, Mag. Jan. 1899. 2 Phil. Mag. Oct. 1899.  * Phil. Mag. July, 1900. 
