166 DISCUSSION OF EVIDENCE. 



would be to increase the viscosity of the mixture over that of either 

 pure solvent. 



Maxima in the conductivity of electrolytes in the mixed solvents 

 were shown to correspond to maxima in the fluidity of the mixed 

 solvents. Maxima in fluidity are probably due to an increase in the 

 size of the molecules of the solvent, due to a combination of one 

 solvent with the other. This would diminish the viscosity and conse- 

 quently increase the velocity of the ions, which would increase the 

 conductivity. This factor must also be taken into account in explain- 

 ing conductivity maxima. 



The temperature coefficients of conductivity in the above-named 

 mixtures of liquids with water are a maximum in the 25 and 50 per 

 cent mixtures. These are just about the mixtures in which the sol- 

 vents have the least association. The molecules of the solvents being 

 in the simplest condition, would be most favorable for chemical action. 

 In such mixtures the solvents probably combine to the greatest extent 

 with the dissolved substance the solvation is at a maximum. The 

 effect of rise in temperature breaking down these solvates would there- 

 fore be a maximum where solvation is a maximum. Solutions of 

 potassium sulphocyanate have greater conductivity in acetone than 

 in water. This was shown to be due to the greater fluidity of the 

 acetone. 



This same salt when dissolved in water lowers the viscosity of the 

 water. An examination of the literature showed that certain salts of 

 potassium and salts of rubidium and caesium are practically the only 

 ones known to lower the viscosity of water. In the case of certain 

 salts of potassium the positive effect of the anion on the viscosity of 

 water may more than offset the negative effect of the potassium ion. 



The following explanation of the above-named phenomenon was 

 suggested. If the atomic volume of the ions dissolved in the solvent 

 was larger than the molecular volume of the solvent, the larger ions 

 would diminish the size of the frictional surfaces coming in contact 

 and would lower the viscosity. 



It is well known that potassium, rubidium, and caesium occupy the 

 maxima on the atomic-volume curve, and have much larger atomic 

 volumes than any other known elements. Potassium has a smaller 

 atomic volume than rubidium, and rubidium than caesium. Potassium 

 chloride lowers the viscosity of water less than rubidium chloride, and 

 rubidium chloride less than caesium chloride. 



If we study the salts which raise the viscosity of water, we will find, 

 in general, that the amount of increase in the viscosity bears a relation 

 to the atomic or ionic volumes of the dissolved substances. Smaller 

 ions tend to increase the viscosity of water more than larger ones. It 

 would therefore seem that the above explanation contains a large 

 element of truth. 



