30 J. G. MACGREGOE ON THE 



regarded as a system of atoms or atomic groui»s, bound together by finite forces and 

 moviug relatively to one another, it may occasionally, or indeed frequently, happen that 

 the perturbations of surrounding water or salt molecules may be sufhcient to separate 

 the atoms or atomic groups, i. e., to dissociate the molecules. "We must now ask, there- 

 fore, how elevation of temperature and increase of concentration will affect the relative 

 numbers of dissociated and undissociated molecules in a solution. 



The elevation of the temperature of a given solution, involving increase of the velo- 

 city of all its constituent parts, will obviously both facilitate the dissociation of the salt 

 molecules and impede the recombination of those constituents of the dissociated molecules 

 which happen to meet. A larger number of salt molecules per second will undergo dis- 

 sociation at the high temperature, and a smaller proportion of the meetings which may 

 occvxr of the constituents of dissociated molecules will result in recombination. The fre- 

 quency of these meetings, however, may be increased. For although the general expansion 

 of the solution increases the distances which must be traversed by the constituents of the 

 dissociated molecules before meeting with congenial partners, the increased number of 

 dissociated molecules in the solution will diminish those distances, while the diffusion of 

 the molecules will go on more rapidly at the high than at the low temperature. Eleva- 

 tion of temperature will, therefore, in two respects tend to increase the number of dis- 

 sociated molecules in a solution, while in one respect it may tend to diminish the number. 

 These opposing tendencies cannot be estimated quantitatively. But the two which make 

 for increase of dissociation would seem to be more important than the one against it ; and 

 we may therefore conclude that in general elcAi-ation of temperature will result in increased 

 dissociation. In certain cases, however, the balance of oppoising tendencies maybe on the 

 other side, and elevation of temperature may result in diminished dissociation And 

 these remarks apply obviously both to dilute and to strong solutions. 



What the effect of increase of concentration will be is not so easy to determine directly. 

 But it may be learned hypothetically through the medium of experiment. For there are 

 various properties of saline solutions, such as the differences in the values of the electrical 

 conductivity and of the " molecular lowering " of the vapour tension and the freezing 

 point, both in solutions of different salts and in solutions of different strengths of the 

 same salt, which may be coordinated in a beautifully consistent manner, as has been 

 shown by Arrhenius, Van 't Hoff, Ostwald and others, if it be assumed that these differ- 

 ences depend upon differences in the degree of dissociation of the salts in solution ; and 

 the very considerable degree of dissociation requisite on this assumption to produce the 

 differences referred to may then be determined. As it has been found that the degrees of 

 dissociation determined for any given solution by studying the various properties referred 

 to, are the same, the hypothesis has established for itself a certain probability. And we 

 may therefore accept the deduction which has been made from it as to the influence of 

 increase of concentration on the state of dissociation of the salt in a solution, viz., that it 

 diminishes the ratio of the number of dissociated molecules to the total number of mole- 

 cules in solution, as probably correct. 



If, therefore, the dissolved salt in a solution be partially dissociated, elevation of tem- 

 perature and increase of concentration will in general produce opposite changes in its 

 degree of dissociation. 



The assumption of a comparatively high degree of dissociation in salt solutions is 



