162 DISCUSSION OF EVIDENCE. 



much greater hydrating power than molecules. This conclusion was 

 confirmed by the work of Stine. It was also shown that molecules 

 in aqueous solution can combine with water, and in special cases 

 molecules may even have greater hydrating power than some ions. 



The work with the slightly hydrated potassium and ammonium 

 chlorides brought out a significant fact. These were chosen, not to 

 study the effect of one hydrated salt on the hydration of another 

 Itydrated salt, but to study the effect of change in temperature on the 

 conductivities of separate solutions of electrolytes and upon mixtures 

 of these solutions. For this purpose it was necessary to select a pair of 

 salts with small hydrating power and also which do not form double 

 salts with one another. 



If suppression of ionization were the only cause of the diminution 

 in conductivity on mixing solutions of salts, such as the above, which 

 have a common ion, then we should find the greatest dimunition where 

 the dissociation is greatest. Since dissociation is slightly greater at 

 than at 12, and slightly greater at 12 than at 25, we should expect 

 to find greater diminution in the conductivity at than at 12, and 

 greater at 12 than at 25. Exactly the reverse is true. 



Again, as the difference in dissociation between to 12 is but little 

 greater than between 12 and 25, we should expect to find that between 

 these two ranges of temperature the driving back of the conductivity 

 would be of the same order of magnitude, yet such is not the case. 



Furthermore, some of the solutions which we mixed are nearly iso- 

 hydric, and such solutions do not drive back each other's dissociation. 

 Driving back the dissociation of a salt by the addition of a second salt 

 with a common ion is, therefore, not the only cause of the diminution 

 in conductivity which results when salts with a common ion are mixed. 



It was pointed out that three other factors may come into play: 

 (1) Change in hydration giving rise to change in the size and mass of 

 the ion, w T hich probably plays a very insignificant role in the above- 

 named case, since the chlorides of ammonium and potassium are only 

 slightly hydrated. (2) Change in the number of the dissolved parts, 

 which, however, is not very large for small changes in temperature. 

 (3) Change in the viscosity of the solution with change in temperature, 

 which is undoubtedly a very prominent factor, hitherto either over- 

 looked or not given sufficient prominence in dealing with the phe- 

 nomenon in question. 



INVESTIGATIONS IN MIXED SOLVENTS. 



The study of the conductivities and dissociations in pure solvents 

 was extended here to mixed solvents. This phase of the work has now 

 been in progress continuously for a dozen years, and the results have 

 been published in monographs Nos. 80 and 180 of the Carnegie Institu- 

 tion of Washington. 



