130 MR. J. S. TOWNSEND ON THE DIFFUSION OF IONS INTO GASES. 



Rontgen rays. Let us consider what takes place when the gas has been removed 

 from the action of the rays, disregarding for the present 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 experi- 

 mentally, 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 with the number which 

 recombine. 



It is convenient to use tubing ot such a length that the conductivity will be 

 reduced to about one-half its initial value. 



In order to obtain the coefficient of diffusion, when the reduction in the conduc- 

 tivity is known, the following problem presents itself. 



If a small quantity of a gas, A, is mixed with another gas, B, and the mixture 

 passed along a tube, the sides of which completely absorb A, to find what quantity 

 of A emerges from the tube with B. 



It will be immediately seen that if the gases diffuse rapidly into each other, a 

 large proportion of the molecules of the gas A will come into contact with the 

 surface of the tube, and will there be absorbed. If on the other hand the rate 

 of interdiffusion is very small, the molecules of A will travel down the tube in 

 straight lines parallel to the axis of the tube, and practically none of them will come 

 into contact with the surface. 



The complete solution of the above problem, taking into account the variation 

 of the velocity at points along a radius of the tube, is given in Section I. The 

 results of the experiments and the conclusions to which they lead are contained in 

 Section II. 



* JOHN S. TOWN-SEND, 'Phil. Mag.,' June, 1898. 



