14 PROCEEDINGS OF THE AMERICAN ACADEMY. 



cobalt, uickel, zinc, and magnesium sulphates,* but undoubtedly others 

 exist. In a large majority of cases when an electrolyte is dissolved in 

 water, the sura of the volumes of salt and of the solvent taken together 

 considerably exceeds the volume of the solution. This contraction is 

 usually ascribed wholly to the dissolved substance in dilute solutions,! 

 but it seems to me that the behavior of the salts named above proves the 

 falsity of this method of calculation. The water as well as the salt must 

 contract when a salt is dissolved. So many complications are concerned 

 in the act of the solution of an electrolyte that it is difficult to unravel 

 the tangled clues ; but the wide deviations exhibited by different sub- 

 stances seem to indicate that there are present overlapping contractions 

 and expansions, the resultant of which is a smaller quantity than some 

 of the individual influences. Such contractions and expansions are just 

 what one would expect to find in a readjustment of affinities. 



In considering the simpler case of solid non-electrolytes, one usually 

 finds here also a contraction upon solution, although less marked than in 

 the extreme cases named above. For this reason, one is inclined to 

 ascribe the act of solution of all kinds primarily to the affinity of the 

 solvent for the dissolved substance. The solution tension of a metal or 

 salt becomes simply a balance or ratio of attractions, — the separating 

 tendency of heat upon the dissolving phase is much assisted by the 

 attraction from outside. This is of course no new idea. The possible 

 method of treating mathematically these balanced influences is suggested 

 in a recent paper on the "driving tendency " of reaction. $ 



That electi'olytic separation also should be assisted by the outside 

 attraction for the solvent is almost a foregone conclusion. This may be 

 inferred from the contraction shown by most electrolytes on dissolving. 

 Hence may arise the various contact-potentials exhibited by the same 

 substance in different solvents ; for different solvents must possess differ- 

 ent affinities. Hence also one would expect to find a much greater 

 potential needed for the dissociation of gases than for that of dissolved 

 substances. 



The mechanism of electrolytic dissociation in gases is now usually 



* Thomsen, Thermochemische Untersuchungen, I. 45 (1882). MacGregor, 

 Trans. Eoy. Soc. Canada, 1890, p. 19; 1891, p. 15; Trans. Nova Scotia Inst. Nat. 

 Sc, 7, 368 (1890). 



t Van't Hoff, Vorlesung. phys. theoret. Cliem., III. p. 41 (1900). Drude and 

 Nernst (Z. pliys. Cliera., 15, 79 (1896)) ascribe this contraction to "Electro- 

 striction." 



t Ricliards, Jour. Phys. Chem., 4, 385 (1900). See specially p. 391. 



