INTRODUCTION. 7 



equal at some point corresponding to about 65 per cent alcohol. The same phenom- 

 ena repeat themselves in mixtures of ethyl alcohol and water. With acetone and 

 water the negative viscosity coefficient again becomes apparent, this time only in 

 the and 25 per cent mixtures. In the other mixed solvents the viscosity is 

 increased by the addition of the solute. 



An examination of the literature relating to viscosity brought to light the 

 important fact that, in general, only salts of potassium, rubidium, and caesium are 

 known to lower the viscosity of water. Very few other cases of negative viscosity 

 have been found, and not all salts of these metals behave alike in this respect. For 

 instance, the sulphate, ferrocyanide, ferricyanide, and chromate of potassium give 

 positive viscosity coefficients. And it is not remarkable that, in the presence of 

 certain anions, the alkali cations do not give negative viscosity coefficients. Vis- 

 cosity must be considered to be a property which is a function of the nature of all 

 the particles in a solution ; and it is perfectly clear that here, as in conductivity, two 

 opposing influences may be operative, the potassium cation, for instance, tending 

 to lower the viscosity and the anion tending to increase it. If the algebraic sum is 

 positive, a positive viscosity results, and vice versa, the actual viscosity of the solu- 

 tion being dependent on the relative action of the two ions. 



The facts have been presented showing that in aqueous solution, or in solutions 

 containing as much as 50 per cent of water, potassium sulphocyanate produces a 

 lowering of the viscosity. What is the mechanism of this effect? 



The work of Thorpe and Rodger 1 has shown that viscosity phenomena are, in all 

 probability, dependent upon the frictional surfaces of the various particles in any 

 solution. If the total frictional surface can be diminished by any means, other 

 factors remaining constant, the viscosity will be lowered. We may actually realize 

 this by bringing into a solvent a substance which has a large molecular volume, 

 or which gives ions having large ionic volumes. The total frictional surface propor- 

 tional to the mass is thus lessened, since the surface increases as the square of the 

 diameter of the particles, while the mass increases as the cube. Potassium, rubidium, 

 and caesium salts, as we have said, lower the viscosity of water. Are their atomic 

 volumes, accordingly, larger than those of other elements? The periodic curve 

 of atomic volumes answers this question at once. The alkali metals occupy the 

 maxima of the curve, and no other metals have atomic volumes to be compared with 

 them. Moreover, that element having the greatest atomic volume caesium 

 should have the greatest negative viscosity coefficient. This point is soon tested 

 by reference to the work of Wagner. 2 If the viscosity of water is taken as unity, 

 a normal solution of caesium chloride lowers it to 0.9775, a normal solution of rubid- 

 ium chloride gives 0.9846, and potassium chloride 0.9872. Thus, the effect on vis- 

 cosity varies directly as the atomic volume of the cation; caesium having an atomic 

 volume of 74, rubidium of 57, and potassium of 47. 



It will be remembered that minima of fluidity were found in mixtures of water 

 with the alcohols or with acetone, accompanied usually by minima in conductivity. 

 The fact also came out that in those mixtures which have the minimum fluidity, 

 the temperature coefficients of conductivity are largest. The explanation of this 



iZeit. phys. Chem., 14, 361 (1894); 19, 323 (1896). 'Ibid., 5, 35 (1890). 



