4 Mr Wellisch, The Laius of Mobility and Diffusion 



_ K-l e^ 



Sttii ' r^ ' 

 when the molecule is polarised by the field due to the ionic charge. 

 This expression for R assumes that the polarising field is 

 uniform throughout the volume of the molecule. Langevin* has 

 obtained the general expression for R in the case of a spherical 

 molecule and finds it to be given by a series of "which the above 

 is the most important term. 



Expressions for the mobility and coefficient of diffusion of the 



ion. 



Let 7] denote the coefficient of viscosity of the gas, p its 

 density, p the pressure in dynes per sq. cm. and I the molecular 

 mean free path. Let n^, pi, pi, K^ denote the values of n, p, p, K 

 respectively corresponding to a temperature of 0° C. and a pressure 

 of 760 mm. of mercury. 



The charge e carried by the ion is taken as equal to the 

 charge {E) on the monovalent ion in the electrolysis of solutions. 

 This equality was established from measurements of the mobility 

 and rate of diflFusion of gaseous ionsf. The exact value of the 

 ionic charge is not required in the present treatment inasmuch 

 as e only enters in the expression n-^e = n^E, which has been shown 

 from experiments in electrolysis to have the value 1'30 x 10^", 

 E being measured in electrostatic units. The product n^E is 

 denoted by A. 



The gas is supposed throughout to be at a temperature of 0° C, 



We have the following equations : 



7? = A nmvl 

 l~^ = TT \/2 ns^ 



/-, ^ of-, ^RA T, 1/1 



L-^ = 'jr7i\/ l-\ 0-2 J 1 -I k where o" = As + ^s 



y m [ mir ] 



K-l e- 



Jtla = 



Stt?? o""' 



MV^ = mv^ (equipartition of energy) 

 e = E 



* loc. cit. p. 317. 



t For the evidence in support of this equality the reader is referred to J. J. 

 Thomson, Conduction of Electricity through Gasen, 2nd edit. Art. 39. 



