Viscosities of C cesium Salts. 9 



of the 75 per cent mixture. These facts are in perfect accord with the 

 results obtained in earlier work with salts of potassium and rubidium. 

 In the 50 per cent mixtures the two salts manifest somewhat different 

 behavior. The chloride in 50 per cent methyl alcohol shows an 

 increase in viscosity at all temperatures for the most dilute solutions, 

 and a tendency of the more concentrated solutions to pass over into 

 negative viscosity. The nitrate in the same solvent showed a decided 

 negative viscosity effect. 



In 50 per cent ethyl alcohol a decrease in the viscosity of the solu- 

 tions as compared with that of the solvent was noted at the lower 

 temperature, a transition to positive viscosity taking place at higher 

 temperatures. Again, the nitrate decreases the viscosity of the solvent 

 at all temperatures. 



With regard to the 50 per cent acetone-water mixture, an increase 

 in viscosity at all temperatures is to be noted in the case of the chloride, 

 while the nitrate in 50 per cent shows a tendency to decrease the 

 viscosity of the solvent, although it increases it at the higher tempera- 

 tures. The difference in the effect produced by these salts, in com- 

 parison with that produced by the salts of rubidium, lies in the shifting 

 of the transition-point from negative to positive viscosity towards the 

 mixtures containing the larger percentage of acetone. This follows 

 from the different molecular volumes of the cations of rubidium and 

 caesium. When acetone and water are mixed the principal changes 

 take place in the association of the acetone. Consequently, until 

 considerable water has been introduced the solvent particles are quite 

 large, so that even caesium nitrate having the largest negative viscosity 

 coefficient in water increases the viscosity of the 75 per cent mixture and 

 even of the 50 per cent mixture, except at lower temperatures. For 

 rubidium salts, no examples of negative viscosity are to be noted until 

 the 37.5 per cent mixture is reached. No data are available for com- 

 parison with rubidium salts in alcohol-water mixtures, but it has been 

 shown that potassium iodide increases the viscosity of these mixtures to 

 that concentration containing 40 per cent alcohol. But in this instance, 

 caesium nitrate decreases the viscosity of the 50 per cent mixture, posi- 

 tive viscosity manifesting itself only in the 75 per cent mixture. 



This shifting of the transition-point from negative to positive vis- 

 cosity towards the more concentrated solvents (regarding water as the 

 diluent), with increase in the molecular volume of the salt, brings out 

 clearly the gradual breaking-down of the associated molecules into 

 smaller particles with greater frictional surfaces; the difference in the 

 sizes of the particles at different points on the dilution curve is clearly 

 seen from the effect produced by salts of different molecular volumes. 



The apparent transition from negative to positive viscosity with rise 

 in temperature, noted in certain instances, would seem to indicate 

 either a polymerization of the salt or else a solvent envelope which is 



