PKESIDENTIAL ADDRESS — SECTION B. 9S 



acids and other much -dissociated compounds (as judged by 

 osmotic phenomena) are the best electrolytes. This question 

 has been investigated with most important results, due chiefly 

 to Arrhenius and Ostwald. The molecular conductivity of a 

 salt was comparatively long since shown by Kohlrausch to be 

 one of those properties which are composed by addition, as I 

 have already mentioned, of two factors due to the two ions. 

 Now, only the dissociated ions contribute to this addition, so 

 that alteration in the molecular conductivity depends entirely 

 on the proportionate amounts of dissociation when different 

 solutions of the same electrolyte are compared. The value of 

 the molecular conductivity is found to increase more or less 

 rapidly, according to the nature of the salt, as more and more 

 dilute solutions are employed ; but it attains a maximum, 

 or practically so, within experimental limits. That maximum 

 corresponds to complete dissociation. By comparing, then, the 

 molecular conductivity of a given solution with that obtained 

 for the same electrolyte in (practically) infinite dilution, we get 

 a measure of the extent of the dissociation in the first ; for 



X = , where /x and /aoo represent the molecular conducti- 

 vities of the two solutions, and x the proportion of dissociated 

 molecules in the stronger one. The results obtained by this 

 mode of measurement confirm those arrived at by the osmotic 

 methods ; only they are very much more exact. 



Finally, Ostwald has shown that from this value of x for 

 any electrolyte can be calculated the coefficient of affinity k, 



by the formula k — =—- -, a value which is constant for 



^ V(1-£C) 



all dilutions. This coefficient is of the very utmost im- 

 portance, for it is found in the case of acids to be practically 

 identical with the coefficients of affinity as determined from 

 Thomsen's thermo-chemical data and by the purely chemical 

 methods. By determinations of conductivity, then, it is pos- 

 sible to ascertain that all-important coefficient of a substance 

 which expresses its chemical activity, which regulates its in- 

 fluence on every chemical action in which it can play a part. 

 And this is so far explained by, and is one of the greatest 

 results of, the hypothesis of the dissociation of electrolytes, 

 which is itself an essential feature of the gaseous theory of 

 solution. 



I must not allow myself to convey a false impression. 

 Although this theory, from its power of explaining so many 

 facts consistently, and from the great results it has achieved, 

 has gained much ground with chemists and physicists, there 

 are not wanting advocates of the older hydrate theory of 

 solution, who reject its fundamental doctrines uncompro- 



