PRESIDENT S ADDRESS SECTION A. 



would otheiTvise be avoided, and consequently the ions act as mole- 

 cules of greater than the normal size, the apparent diameter decreas- 

 ing as the temperature rises. 



For the movement of an ion through a gas, M. Langevin* has 

 given for the mobility, k, and the coefficient of diffusion, D, the 

 equations — • 



h= eL/MV; D = LV/3 



where e denotes the ionic charge, L the mean free path of the ion, 

 M its mass, and V its mean velocity of thermal agitation. Mr. 

 Wellisch, in his investigation, calculates the mean free path of the 

 ion, taking into account the effect of the ionic charge in increasing 

 the collision frequency, and substituting in the above equations 

 reaches general expressions for the two quantities under considera- 

 tion. If the mass and dimensions of ths ion are taken as the same as 

 those of the molecule, the expression for the mobilitv becomes at 

 O"^ C. 



Ic 



9^P (■ 



(K, - 1) TT A y 



-1 



and that for the coefficient of diffusion at the same temperature 



D 



p L 



(K, - 1) TT Ahf 



where A (= 1"30 x lOi" electro-static units), is the product of the 

 number of molecules per cubic centimetre and the ionic charge, »/ the 

 coefficient of viscosity of the gas, K its specific inductive capacity,. 

 f) the density and p the pressure in dynes per cm-., the symbols 

 with subscripts referring to values under the standard conditions as 

 to temperature and pressure. 



To test the theory Mr. Wellisch gives the following table of 

 comparison between the observed and the calculated values, the 

 observed mobilities, except in the case of air, hydrogen, nitrogen, 

 and oxygen, being the results of a series of determinations recently 

 made bv him. 



Langevin. Ann. de Chimie et de Physique, V, 28, p. 289, 1903. 



