produced by the Action of Thorium Compounds. 171 



the purpose of comparison. It will be observed that radio- 

 activity is produced on the plates some distance away from 

 the thorium oxide, and is roughly proportional to the emana- 

 tion-current at the plate. We may conclude from this experi- 

 ment that the radioactivity is, in some way, due to the 

 " emanation," or to something that accompanies it, but is not 

 caused by the direct action of a radiation from thorium 

 oxide. 



Absorption of the Radiation by Substances. 



All radio-active substances, as well as bodies made radio- 

 active in the manner described, ionize the gas in their neigh- 

 bourhood and act upon a photographic plate in the dark. A 

 simple method of testing whether two types of radiation are 

 the same, is to determine the absorption of the radiation by 

 layers of thin metal foil. If the absorption is different for 

 the two type 3 of radiation, we may consider them distinct 

 kinds of radiation. 



The current between two parallel plates, one surface of 

 which was radio-active, was determined when successive layers 

 of a substance of equal thickness were placed over the radio- 

 active plate. The following table is an example of the way the 

 current (which is proportional to the intensity of the radiation) 

 diminishes with successive layers of aluminium-foil over a 

 plate of zinc, which had been made radio-active : — 



Zinc plate = 12 x 18 cms. 

 Thickness of foil='0004 cm. 

 95 volts between plates. 



$o. of layers 



Current for radia- 



Current for thin 



alum. foil. 



tion from zinc. 



layer of thorium oxide, 







1 



1 



1 



•71 



•57 



2 



•55 



•36 



3 



•43 



•23 



4 



•32 



•13 



5 





•084 



6 



V 155 



•056 



The third column of the table gives the variation of the 

 current with thickness of foil for a thin layer of thorium oxide, 

 and serves as a basis of comparison with the excited radiation. 

 The current for the bare radio-active surface is in each case 

 taken as unity for the purpose of comparison. 



Fig, 5, curves A, B, show these results graphically when 

 the ordinates denote current and the abscissae thicknesses of 

 aluminium. 



