of the Ions formed in Gaseous Media. 



p = nm 



We deduce 



When the mass and dimensions of the ion are the same as 

 those of a molecule we have M = m, s' = s, and the expression for 

 the mobility becomes 



p^p I 4i'7rnmv^s'^ 

 This expression can be transformed into 



,.^|i+(^^ii-^r ...w. 



PiP { ^PiPi ) 



In a similar manner we obtain for the case when M=m,s' = s, 



^^,| (z,-ii,^y> w. 



P [ ^PiP^ ) 



Consider the expressions (a) and (/3) which have been found 

 for the mobility and coefficient of diffusion through a gas of an 

 ion regarded as a molecule carrying a charge equal to that 

 associated with the monovalent ion in electrolysis. 



For a given medium K^, pi and p^ are constant; whence 

 k varies inversely as p provided rj is constant. Now by Maxwell's 

 law* the coefficient of viscosity of a gas is independent of its 

 density ; consequently over the range of pressures for which this 

 law holds good we should expect the ionic mobility to vary 

 inversely as the pressure, a conclusion which has been verified 

 by experiment. Similarly we should expect the coefficient of 

 diffusion to vary inversely as the density over the same range. 



The expressions for k and D involve only known physical 

 constants of the gas and are therefore directly comparable with 

 the results of experimental observation. The results obtained 

 by substituting the observed experimental values of the quantities 

 involved are given in Table I. The values of the viscosity 

 coefficients and of the dielectric constants have been taken from 

 Landolt and Bornstein's Tables (3rd edition); the constant p^ 

 was taken as 1,013,610 (dynes per sq. cm.). 



The seventh column in the table affords an indication of the 

 effect on the mobility of the electric polarisation of the molecules 

 by the ionic charge; it will be seen that the effect is quite 



* Vide Jeans, Dijnamical Theory of Gases, p. 252. 



.JW%. 



