FLUIDITY. 



197 



Tables 84 and 85 (fig. 98) show that potassium sulphocyanate, in mixtures 

 of methyl alcohol and water, exhibits a marked minimum in fluidity in the 

 50 per cent mixture at 25 and at 0, although in the latter case there is only 

 a slight difference between the values in the 25 per cent and the 50 per cent 

 mixtures. It is of especial importance to note here that potassium sulphocya- 

 nate shows a marked negative viscosity (or positive fluidity) in water, and that 

 the viscosity values do not become positive until a mixture about midway between 



25 <f 50$ 75 f f> 



Percentage of Methyl Alcohol 



FIG. 98. 



100 i 



Curve I, fluidities of mixture of 

 methyl alcohol and water at 0. 



Curve II, fluidities of N/10 

 potassium sulphocyanate in mix- 

 tures of methyl alcohol and water 

 atO. 



Curve III, fluidities of the 

 above solvent mixtures at 25. 



Curve IV, fluidities of N/10 

 potassium sulphocyanate in the 

 above solvent mixtures at 25. 



the 50 per cent and 75 per cent mixtures is reached. In other words, N/10 

 solution of potassium sulphocyanate in water is much less viscous (or has a 

 much greater fluidity) than pure water itself. On the other hand, a N/20 

 solution of potassium sulphocyanate in methyl alcohol has a greater viscosity 

 (or smaller fluidity) than the pure solvent itself. These two effects become 

 equal, and we have the viscosity (or fluidity) of the N/10 solution and the 

 pure solvent equal in a mixture intermediate between the 50 per cent and 75 

 per cent mixtures. The normal or general action of a salt, when dissolved 

 in water, is to increase the viscosity. Cases of negative viscosity have been 

 noted by other workers, and this subject will be discussed later in this memoir. 

 It should be noted that the difference in viscosity between the solution and 

 the pure solvent is greater in methyl alcohol than it is in water, or any of the 

 mixtures. The temperature coefficients of fluidity are greater in the pure 

 solvents than in the solutions, in the per cent, 25 per cent, and 50 per cent 

 mixtures; and vice versa in the 75 per cent and 100 per cent mixtures, and 



