PKESIDENTIAL ADDRESS — SECTION B. 91 



the theory of gases, may fairly play a part in the gaseous 

 theory of solution : the abnormal behaviour of electrolytic 

 solutions can thus be explained : for other reasons Clausius 

 has already taught us to believe that at least a small percent- 

 age of the molecules of an electrolyte must be dissociated into 

 its ions at any given moment : assume therefore that this 

 percentage is large enough, and all difficulties disappear. Take, 

 for instance, the case of potassium nitrate. A dilute solution 

 of this salt gives practically twice the osmotic pressm-e, twice 

 the lowering of vapour-pressure, twice the depression of 

 freezing-point, that it should give by comparison with non- 

 electrolytes on the assumption that its molecule is KNO.,. 

 During electrolj'sis we know that this salt is split into the 

 ions K and NO.,. If this dissociation pre-exists in the solution, 

 and if it be practically complete, there are in reality two 

 molecules where we reckoned on only one, and the results 

 obtained no longer contradict the law, but support it. So also 

 with other electrolytes. The extent of dissociation is by no 

 means always the same ; and it is easy to see how, if the 

 hypothesis be accepted, the exact extent — the ratio of mole- 

 cules decomposed to molecules intact — may be calculated from 

 the degree of departure from normal osmotic effects. Now, 

 this has been proved to vary with concentration and tempera- 

 ture, for the most part just as would be expected from the 

 influence of pressure and temperature in ordinary cases of 

 dissociation, so that a great support is here afforded to the 

 hypothesis. Again, a number of facts are accounted for that 

 it is very difficult to understand without the dissociation 

 theory — facts which show that an electrolyte in solution owes 

 many of its properties to two independent factors, one sup- 

 plied by and characteristic of the positive ion, the other due 

 to the negative ion. Arrhenius and Ostwald have made 

 these arguments clear ; and the latter has called attention 

 especially to the following one. The heat of neutralsiation 

 of an acid by a base in dilute solution has been shown by 

 Thomsen to be practically independent of the nature of either, 

 so long as only strong acids and bases are considered. This, 

 according to old ideas, is extraordinary ; for why should the 

 actions 



HCl -H KOH = KCl -f H.O 

 and 



HNO3 + NaOH = NaNOs + H,0 



be of equal thermal value, different salts being formed V By 

 the new ideas, on the other hand, we could expect nothing 

 else, for the dissociation of such strong acids and alkalis is 

 practically complete in dilute solution ; and why should the 

 actions 



