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IV. The Diffusion of Ions into Gases. 



By JOHN S. TOWNSEND, M.A., Dublin, Clerk-Maxwell Student, Cavendish 



f.'tboratory, Cambridge. 



Communicated by Professor J. J. THOMSON, F.Jt.S. 

 Received April 25, Read May 18, 1899. 



INTRODUCTION. 



THERE are several interesting results in connection with molecular physics which can 

 be obtained from the coefficients of diffusion of ions into gases. From determinations 

 of these coefficients we are enabled to find the number of molecules in a given volume 

 of a gas, and to compare the charge on an ion in a conducting gas with the charge 

 on a hydrogen ion in u liquid electrolyte. In the present paper the principles which 

 are involved in the theory of the interdifrusion of gases are applied to the diffusion 

 of ions produced in a gas by the action of Rontgen rays. 



It will simplify matters if we first consider the general theory of the conductivity 

 of gases. Professor THOMSON has shown that all the phenomena which are met with 

 can l)e explained by supposing that the rays produce ions in the gas, the motion of 

 which, when acted on by an electric force, gives rise to the observed conductivity. 

 When the gas has been removed from the influence of the rays the conductivity 

 gradually diminishes, and the disappearance of the ions may be due to three causes, 

 any of which may predominate. 



1. If an electric force is acting, the ions travel through the gas along the lines 



of force and are discharged when they reach the boundary. 



2. Recombination destroys the conductivity ; the positive and negative ions as 



they move about in the gas come into contact and thus neutralise one 

 another. 



3. The ions diffuse and come into contact with the sides of the vessel which 



contains the gas. This effect we shall denote as the effect of the sides. 

 Like the recombination, it takes place when no electric forces are acting, 

 and it is due to the motion of agitation of the ions. 



In order to illustrate in a simple way the principles which are involved in the 

 experiments which are descril><'<] in the present paper, we will suppose that a gas is 

 contained in a metal sphere ami that it has been made a conductor by the action of 



VOL. rxrm. A. 8 20.9.99 



