FLUIDITY AND CONDUCTIVITY. 161 



This idea of ionic hydration, or ionic spheres, was further extended by 

 Kohlrausch, 1 who proposed the hypothesis that 



About every ion there moves an atmosphere of the solvent, the dimensions of which 

 are determined by the individual characteristics of the ion. . . . The electrolytic 

 resistance is a fractional one that increases with the dimensions of the atmosphere. The 

 direct action between the ion and the outer portions of the solvent diminishes as the at- 

 mosphere becomes of greater dimensions. For a slow-moving ion there would be only 

 the friction of water against water, and the electrolytic resistance will have the same 

 temperature coefficient as the viscosity of water, providing the atmosphere does not 

 change its dimensions with temperature. However, if the atmosphere becomes smaller 

 with rise in temperature, the temperature gradient of the conductivity might be greater 

 than that of the fluidity. This seems to be true for the slowest-moving, univalent 

 ion, Li. 



It will be recognized that this is essentially the theory of hydrates proposed 

 by Jones. 



In the case of the alcohols and water, this minimum of conductivity is 

 entirely accounted for by some investigators on the basis of the formation of 

 hydrates. This view was suggested by Zelinsky and Krapiwin. 2 The mini- 

 mum in fluidity is attributed to the formation of molecular aggregations, 

 which are formed by mixing the solvents. In the case of methyl alcohol and 

 acetone, or ethyl alcohol and acetone, since the fluidity curve is a straight 

 line, we conclude that the molecular aggregations of these solvents are not 

 changed in size when the solvents are mixed. This is what we should expect 

 if the fluidities are additive, a fact which has been shown recently by Bing- 

 ham 3 to be true. From the above, we see that the conductivity minimum is 

 generally accompanied by a fluidity minimum, and that both minima are more 

 marked at the lower temperatures. Also, that an increase in temperature 

 tends to shift the minimum towards the mixture containing a greater per cent 

 of alcohol or acetone, as the case may be. Thus, we believe that a diminution 

 in the fluidity of the solvent, which would bring about a corresponding decrease 

 in ionic mobility, is an important factor in causing the minimum of conductivity. 

 In the case when the conductivity minimum is in the 75 per cent mixture, 

 while the minimum of fluidity is in the 50 per cent mixture, as it is with ethyl 

 alcohol and water, we believe that the explanation is to be found in the fact 

 that the ethyl alcohol and water mixtures have a much greater viscosity than 

 those of methyl alcohol and water. When the minimum shifts with an in- 

 crease in dilution, there may be an increase in dissociation. 



However, we do not believe that the above explanation accounts entirely 

 for the conductivity minimum. Since conductivity is dependent upon the 

 number and velocity of the ions, there is no doubt that an increase in 



J Proc. Roy. Soc., 71, 338 (1903). 3 Amer. Chem. Journ., 35, 195 (1906). 



Ztschr. phys. Chem., 21, 35 (1896). 



