208 CONDUCTIVITY AND VISCOSITY IN MIXED SOLVENTS. 



the total surface of the molecules of the solvents is increased, i. e., the total 

 frictional surface is increased, and an increase in viscosity results. The point 

 at which the viscosity of the mixed solvents becomes a maximum, is, then, 

 the point at which the effect above mentioned of the pure solvents on each 

 other is the greatest. Thus we see that at the point where we find the maxi- 

 mum in viscosity we have the greatest number of simple molecules of the two 

 solvents present. 



According to this view of the cause of the fluidity minimum (or viscosity 

 maximum), the reason for the minimum in the conductivity curve follows 

 at once. Since the association of the solvents is a minimum at the point of 

 minimum fluidity, the dissociating power of the solvents is also at a minimum, 

 and the minimum in conductivity is a natural consequence. 



As has already been stated, it is at the minimum points of conductivity that 

 we have the phenomenon of a very small increase in molecular conductivity 

 with increase in dilution, as is shown by the conductivity curves for the different 

 dilutions approaching one another as they approach the minimum. This also 

 is a natural consequence of our theory. The association of the solvents being 

 a minimum at these points, as has already been shown, their dissociating power 

 is also a minimum, and, consequently, an increase in the amount of the solvent 

 present has little influence on the dissociation of the dissolved salt. To elabo- 

 rate this a little more fully, let us suppose that the salt were dissolved in a 

 solvent which had no dissociating power; then no matter how much of the 

 solvent were present, the dissociation of the salt and, consequently, the 

 molecular conductivity of the solution would remain zero. If, now, a solvent 

 is used which has very small dissociating power, it is evident that compara- 

 tively large amounts of it would be required to produce any very appreciable 

 effect upon the dissociation of the dissolved salt. 



In a few cases a maximum in conductivity was observed. These cases occur 

 in certain of the mixtures of acetone with the alcohols. Similar examples, 

 and in these same solvents, were first observed by Jones and Bingham, 1 who 

 suggested the explanation that these maxima are due either to an increase in 

 the number of ions present, or to a diminution in the size of the ionic spheres, 

 making possible a more rapid movement of the ions. As has been pointed out, 

 their final conclusion is that the latter explanation, i. e., a diminution in the 

 size of the ionic spheres, is the more probable. The question, however, arises: 

 Why does this change in the size of the ionic spheres take place ? 



The experimental data, obtained up to the present, are not sufficient to 

 justify a final answer to this question. We offer the following suggestion, 

 however, as probable : The increase in fluidity, in view of what has already 

 been said with regard to the cause of maxima and minima in the fluidity 



1 Loc. cit. 



