28 J. G. MACGEEGOE ON THE 



tion on crystallizing, seems to imply that the attraction between salt and water molecules 

 is greater than that between water molecules themselves. Finally, that the molecular 

 attractions must be assumed to diminish rapidly with the distance between the attracting 

 molecules, is obvious from the fact that the molecules must be brought very close together 

 before the phenomena occur which suggest the assumption of the attractions between 

 them ; while once they are brought into sufficiently close contiguity the resulting pheno- 

 mena are such that, if mutual attractions be assumed to account for them, they must be 

 assumed to be great. 



An aqueous saline solution, according to the molecular theory of the constitution of 

 liquids, consists of a mixture of molecules of water and salt, all being in motion, but 

 having free paths which are comparatively short. According to the kinetic theory of 

 heat the mean velocity of the molecules determines the temperature, the temperature 

 rising as the mean velocity increases. According to the above additional hypotheses, the 

 various molecules attract one another, the attractions between unlike molecules being 

 greater than those between like molecules, at equal distances, and all diminishing rapidly 

 as the distance between the attracting molecules increases. 



From this conception of a solution we may deduce its properties with more or less 

 exactness. I have found it useful in coordinating such phenomena as the contraction of 

 solutions, the solubility of salts, saturation and supersaturation, the lowering of the vapour 

 tension and the freezing point, the relation of the " osmotic pressure " to the specific volume 

 of the dissolved salt and the temperature of the solution, etc. In this paper, however, I wish 

 to deduce from it merely the relative effects of elevation of temperature and of increase of 

 concentration on the selective absorption of light, and to test the conclusion reached. 



Let us think first of a dilute solution of a salt so constituted that its molecules will 

 not undergo dissociation. In such a solution the salt molecules will not move among the 

 water molecules independently, but will form small groups or systems, each consisting of 

 a salt molecule and of more or fewer of the more slowly moving of the neighbouring water 

 molecviles, which, under the strong attraction of the salt molecule, will be kept revolving 

 round it. The water molecules of such a system may occasionally, or indeed frequently, 

 be carried beyond the sphere of the practical influence of the salt molecule by the " per- 

 turbations " of neighbouring molecules ; but if so, other water molecules will quickly 

 take their places from outside. The attractive forces involved diminishing rapidly with 

 the distance, the sphere surrounding the salt molecule, within which the water molecules 

 will be ke-i^t revolving round it, must be small, so that, although even a single salt molecule 

 placed in a body of water must make its influence felt throughout the whole volume (so 

 far, for example, as contraction is concerned), it will be able to include in its own system, 

 and to carry with it as it moves, only a limited number of the surrounding water mole- 

 cules. "What the magnitude of the sphere will be, and what the number of water 

 molecules which on the average it will enclose, will, for a given salt, depend upon the 

 mean velocity of the particles of the system, i. e., upon the temperatui-e. But at a definite 

 temperature, in a given solution, there will be on the aA'erage a definite number of 

 water molecules revolving about each salt molecule and following it in its motion through 

 the water. Such a system may be appropriately called a hydrate of the salt, though it 

 does not coincide with the chemical conception of the hydrate as I understand that con- 

 ception. To distinguish it therefrom we may call it a physical hydrate, 



