Density 

 of 



Relative 

 Ionization 



Gas. 



of Gas *. 



1 



1 



•07 



•33 



1-18 



6 



1-53 



1-4 



2-19 



6-4 



696 Mr. C. Gr. Barkla on Secondai 



Rel. Intensity 

 Gas. of 



Sec. Radiation. 



Air 1 



Hydrogen '17 



Sulphuretted Hydrogen . 1*075 



Carbon Dioxide .... 1*45 



Sulphur Dioxide . . . 2* 11 



From these results there is obviously a proportionality be- 

 tween the intensity of secondary radiation and the density of 

 the gas, while there is no obvious connexion with the rate 

 of ionization. 



It should be noted that the discrepancy in the case of 

 hydrogen can be accounted for by the presence of a small 

 quantity of air or other impurity in the gas chamber. As 

 the gas was introduced by simply displacing the gas previously 

 occupying the box, this impurity was undoubtedly present. 

 The same consideration would bring the other gases into even 

 closer agreement. 



During the course of these experiments, the changes in the 

 density of the gases experimented upon due to variations in 

 atmospheric pressure and temperature between the observa- 

 tions were insignificant. 



To examine the absorption of the radiation from different 

 gases, plates of aluminium of thickness '0105 cm. were placed 

 before the side window of the gas chamber. The intensity 

 of ionization produced by the radiation which passed through 

 the aluminium was compared with that produced when no 

 plates intercepted the secondary radiation. .Experiments 

 were made with the radiation from hydrogen and from 

 sulphuretted hydrogen, and the differences due to absorption 

 of the secondary radiation in the two cases amounted to 

 36 per cent, and 37*5 per cent, of the normal ionization. 

 This was much smaller than the possible error, and in later 

 experiments with air and carbon dioxide no difference in the 

 penetrability of the rays was found. 



The experimental results may be stated as follows: — 



(1) All gases subject to X-rays are a source of secondary 



radiation. 



(2) The absorbability of the secondary radiation is (within 



the limits of possible error — about 10 per cent.) the 

 same as that of the primary radiation producing it. 



* J. J. Thomson, Cambridge Phil. Soc. Proa x. pp. 10-14 (1898). 



