172 Prof. E. Rutherford on Radioactivity 



It will be observed that the radiations from zinc and thorium 

 oxide are quite different in character, the radiation from the 

 former being far more penetrating as regards aluminium. 

 Both types of radiation are approximately homogeneous. The 

 current, which is proportional to the intensity of the radiation, 

 diminishes approximately in a geometrical progression as the 

 thickness of the metal increases in arithmetical progression. 



Fis\ 5. 



The same general difference is shown for the two types of 

 radiation by testing their comparative absorption by thin 

 layers of paper, gold-leaf , silver-foil, and Dutch metal. 



The following table is an example of the absorption of the 

 radiation from a zinc plate and a thin layer of thorium oxide 

 for thin tissue-paper : — 



Thickness of layer of paper = '0030 cm. 

 Potential-difference between plates = 50 volts. 



Current. 





t — 





•, 



No. of lavers 



Radiation from 



Radiation from 



of paper. 





zinc. 



thorium oxide. 









1 



1 



1 





•57 



■37 



2 





•35 



•16 



3 





•20 



•080 



4 





•12 



•055 



