192 



Mr, J. S. Townsend. 



" The Diffusion of Ions into Gases." By John S. Townsend, M.A. 

 (Dublin), Clerk Maxwell Student, Cavendish Laboratory, 

 Cambridge. Communicated by Professor J. J. Thomson, 

 F.B.S. Beceived April 25,— Bead May 18, 1899. 



(Abstract.) 



In the paper upon this subject, the principles upon which the theory 

 of interdiffusion of gases depends, are applied to the diffusion of ions 

 produced by Bontgen rays. When a gas has been left to itself, 

 the conductivity gradually disappears. When no electromotive forces 

 are acting, the loss of conductivity is due partly to positive and 

 negative ions coming into contact with each other, and partly to the 

 effect of the surface of the vessel, which discharges those ions which 

 come into contact with it. 



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

 involved, we take the case of a gas contained in a metal sphere, and 

 consider what happens to the ions after the gas has been removed from 

 the influence of the rays. For present purposes we may neglect the 

 effect of recombination. 



The ions may be considered as constituting a separate gas, the 

 molecules of which may be either bigger or smaller than the molecules 

 of the gas in which they are immersed. When an ion comes into contact 

 with the surface of the sphere it loses its charge, so that the metal 

 may be regarded as a body which completely absorbs the ions. The 

 reduction in the conductivity by the diffusion of the ions to the sides, 

 is exactly analogous to the removal of moisture from a gas by 

 bubbling it through sulphuric acid. The more rapidly the water 

 vapour diffuses through the gas, the greater will be the number of 

 water molecules which come into contact with the acid round the 

 bubble. If the quantity of moisture which is removed be found ex- 

 perimentally, the coefficient of diffusion of water vapour into the gas ^ 

 can be deduced.* It would be impracticable to use this method to 

 find the coefficient of diffusion of ions into a gas contained in a large 

 vessel, as the loss of conductivity due to recombination would be large 

 compared with the loss due to the sides. 



The method which was employed was to pass a uniform stream of 

 gas through fine metal tubing, and to allow the rays to fall on the gas 

 immediately before entering the tubing. The bore of the tubing can 

 be so adjusted that the number of ions which come into contact with 

 the sides will be large compared wifh the number which recombine. 

 It is convenient to use tubing of such a length, that the conductivity 

 will be reduced to about one half its initial value. 



* John S. Townsencl, ' Ph.il. Mag.,' June, 1898. 



