PRBSIDKNT: S ADDRESS SBCTION A. 



35 



Mr. Welliscli further shows that if d denotes the coefficient of 

 interdift'usion of a molecule through the gas, 



Both in the case of the mobility and in that of the coefficient of 

 diffusion the agreement between the calculated and the observed 

 values is, on the whole, quite satisfactory, the conclusion being that 

 the beha\'iour of the ion can be explained on the supposition that it 

 consists of a single molecule associated with a charge equal to that 

 carried by the monovalent ion in electrolysis. 



Mr. Wellisch read an account of this investigation of the 

 mobility and diffusion of the ions before the Cambridge Philosophical 

 Society at its meeting held on the 9th November, 1908, and com- 

 municates a paper on the same subject to this Section. 



Mr. Sutherland, to our regret, is unable to be present at this 

 meeting of the Association, but he allows me to communicate to the 

 Section a letter of his on the theory of the small ion written to me 

 on the 6th February, 1908, and permits me to mention the results of 

 his investigation at this stage of our proceedings. 



Amplifying the discussion developed in his Viscosity paper by 

 the addition, in the energy expression, of a term representing the 

 electrical potential energy of ion and molecule when in contact, Mr. 

 Sutherland, in his letter, proceeds to investigate the relation between 

 tlie mobility and temperature, and deduces for the mobility of the 

 ion the simple expression — 



k = 



A6"^ 



1 + 



0-6' 



where A is a constant, 6 the absolute temperature, 6' the absolute 

 boiling point, under the experimental pressure, of the substance of 

 the gas in which the ions are formed, and C a constant similar to 

 that represented by C in his now well-known viscosity formula. 



* See Jeans, Dynamical Theory of Gases, p. 253. 

 + Townsend, Phil. Trans. A 193, p. 129, 1900. 



