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conditions precluding mechanical motion or convection. Or diffusion may 

 be defined as the mixing taking place in a system that is isolated, 

 thermally and mechanically. 



■ 2. Diffusion may be defined as mixing due to random motion of molec- 

 ular particles, in distinction from mixing due to directed motion of masses 

 of the substance, e.g., mechanical mixing or convection. 



3. Diffusion may be defined as mixing due to the decrease of the free 

 energy of the system (increase of the entropy) in accordance with the 

 second law of thermodjmamics. 



It should be noted that just as diffusion may be defined from the view- 

 point of the kinetic theorj^, so by a reciprocal process we may say that the 

 facts of diffusion furnish the most striking, and to many minds the most 

 convincing, proof of the kinetic theory. 



In general it would follow as a consequence of the Idnetic theory that 

 the rate of diffusion should be a function of the temperature, of the size 

 and nature of the molecule, and the medium through which diffusion takes 

 place. (The mathematical treatment of diffusion when the temperature, 

 kind of molecule, and medium, are constant, has been taken up in detail 

 for our particular case on a previous page.) Nernst has developed a very 

 complete theory of diffusion for electrolytes which has already been men- 

 tioned in this article. In so far as an electrolyte is dissociated the separate 

 ions will have independent motions ivithin a certain narrow range — this 

 range is very sharply limited by the large electrostatic forces that must 

 come into play as soon as an accumulation of one kind of charge is per- 

 ceptible in any part of the solution (34). 



On the above theory as a basis, Nernst deduces the expression already 

 given for the velocity of diffusion of an electrolyte, viz. : 



K = 1^ g RT 10-^ 



u -\- V 



where u and v represent the ionic mobilities of the cation and anion re- 

 spectively, and g is a constant depending on the unit in which K is given. 

 It is assumed that conditions during diffusion are the same as Avhen u 

 and V are measured. This is not the case for us, and there is no way in 

 which we can do more than guess the ionic mobilities under conditions of 

 varying concentrations of KI. 



Theory of the "Reststrom." If a very low potential difference be main- 

 tained between two electrodes immersed in a conducting solution it is 

 found that a weak current flows, even when this potential difference is far 



