212 CONDUCTIVITY AND VISCOSITY IN MIXED SOLVENTS. 



mixture, yet there is a much more pronounced tendency towards a minimum 

 in the 25 per cent mixture than there is in the 75 per cent mixture. We see 

 also that although the mixtures of ethyl alcohol and water show a minimum 

 in the 50 per cent mixture, yet there is a more marked tendency towards a 

 minimum in the 75 per cent mixture than there is in the 25 per cent mixture. 

 The case of mixtures of acetone and water is not quite so clean cut, but here 

 again the minimum in fluidity is in the 50 per cent mixture; and although 

 the actual value of the fluidity in the 75 per cent mixture is greater than 

 it is in the 25 per cent mixture, yet, when the very great difference 

 between the fluidity of acetone and the fluidity of water is considered, it is 

 readily seen that in this case also the tendency towards a minimum in 

 fluidity is more marked in the 75 per cent mixture than it is in the 25 per 

 cent mixture. 



These facts show clearly that in about the 25 and 50 per cent mixtures 

 of the other solvents with water we have the most favorable conditions for 

 the formation of molecular aggregations, such as solvates, between the ions 

 of the dissolved salt and the molecules of the solvents. In other words, 

 we should have the greatest number of simple solvent molecules present in the 

 mixtures above mentioned ; and, as we have already pointed out in another 

 connection, these are the conditions under which we can most reasonably 

 expect the greatest combination between the solvent and the dissolved salt. 



The fact that the molecular conductivities of potassium sulphocyanate in 

 acetone are smaller than they are in water for the more concentrated solu- 

 tions, but are much greater in acetone than they are in water for the more 

 dilute solutions, might be due to several causes. 



First, a much greater degree of dissociation, and a more rapid increase in 

 dissociation with increase in dilution, in acetone than in water. This view, 

 however, is untenable in view of the- fact that acetone is very much less asso- 

 ciated than water, and has a much smaller dielectric constant. Indeed, these 

 facts lead us to the conclusion that potassium sulphocyanate is much less 

 dissociated in acetone than it is in water. 



Second, a much greater velocity of the ions in acetone than in water. That 

 this is the probable explanation is shown by the following considerations: 

 The fluidity of acetone is very much greater than the fluidity of water, and 

 this, to be sure, is one of the factors that governs an increase in the ionic 

 velocity; but a far more important factor becomes manifest from a study of 

 the temperature coefficients of conductivity. It is a very significant fact 

 that the temperature coefficients of conductivity in water are nearly ten 

 times as great as the corresponding coefficients in acetone. This shows quite 

 clearly that the solvates formed in the acetone solutions are very much less 

 complex than those formed in the aqueous solutions. Since the ions in 

 acetone have, as we have just seen, a much smaller atmosphere of solvent to 



