from Faraday to J. J. Thomson. 403 



conductivity was also found to be destroyed when an electric 

 current was passed through the gas a phenomenon for which 

 a parallel may be found in electrolysis. For if the ions were 

 removed from an electrolytic solution by the passage of a 

 current, the solution would cease to conduct as soon as 

 sufficient electricity had passed to remove them all ; and it 

 may be supposed that the conducting agents which are produced 

 in a gas by exposure to X-rays are likewise abstracted from it 

 when they are employed to transport charges. 



The same idea may be applied to explain another property 

 of gases exposed to X-rays. The strength of the current 

 through the gas depends both on the intensity of the radiation 

 and also on the electromotive force ; but if the former factor be 

 constant, and the electromotive force be increased, the current 

 does not increase indefinitely, but tends to attain a certain 

 " saturation " value. The existence of this saturation value is 

 evidently due to the inability of the electromotive force to do 

 more than to remove the ions as fast as they are produced by 

 the rays. 



Meanwhile other evidence was accumulating to show that 

 the conductivity produced in gases by X-rays is of the same 

 nature as the conductivity of the gases from flames and from 

 the path of a discharge, to which the theory of Giese and 

 Schuster had already been applied. One proof of this identity 

 was supplied by observations of the condensation of water- 

 vapour into clouds. It had been noticed long before by 

 John Aitken* that gases rising from flames cause precipita- 

 tion of the aqueous vapour from a saturated gas; and 

 E. von Helmholtzf had found that gases through which an 

 electric discharge has been passed possess the same property. 

 It was now shown by C. T. E. Wilson, % working in the 

 Cavendish Laboratory at Cambridge, that the same is true of 

 gases which have been exposed to X-rays. The explanation 



* Trans. R. S. Edinb. xxx (1880), p. 337. 



t Ann. d. Phys. xxxii (1887), p. 1. 



% Proc. Roy. Soc., March 19, 1896 ; Phil. Trans., 1897, p. 265. 



2 T> 2 



