ON WATKi; AND ITS COMPOUNDS 89 



the chemical atlinitics acting in solutions of solids becomes evident 

 from those multifarious methods by \vhich their solutions are drcum 

 jiowd, whether they be saturated or not. On heating (absorption of 

 heat), on cooling, and by internal forces alone, aqueous solutions in 

 many cases separate into their components or their definite com- 

 pounds. The water contained in solutions is removed from them 

 as vapour, or, by freezing, in the form of ice, 46 but the tension of the 

 rn/iour of water 47 held in solution is less than that of water in a free 



just as the velocity of the movement of different gaseous molecules may be far from 

 being the same (see Note 34, p. 80). 



It is, therefore, very probable that the reactions taking place in solution vary both 

 quantitatively and qualitatively with the mass of water in the solution, and the great 

 difficulty in arriving at a lasting decision on the question as to the nature of the chemical 

 relations which take place in the process of solution will be understood, and if besides 

 this the existence of a physical process, like the sliding between and interpenetration of 

 two homogeneous liquids, be also recognised in solution, then the complexity of the 

 problem as to the actual nature of solutions, which is now to the fore, appears in its 

 true light. However, the efforts which are now being applied to the solution of this 

 problem are so numerous and of such varied aspect that they will offer the coming 

 investigators a vast mass of material towards the construction of a complete theory of 

 solution. 



For my part, I think that the study of the physical properties of solutions (and 

 especially of weak ones) which now reigns, cannot give any fundamental and complete 

 solution of the problem whatever (although it should add much to both the provinces of 

 physics and chemistry), but that, parallel with it, should be undertaken the study of the 

 influence of temperature, and especially of low temperatures, the application to solu- 

 tions of the mechanical theory of heat, and the comparative study of the chemical pro- 

 perties of solutions. The beginning of all this is already established, but it is impossible 

 to consider in so short an exposition of chemistry the further efforts of this kind which 

 have been made up to the present date. 



46 If solutions are regarded as being in a state of dissociation (see footnote 19, p. 64) it 

 would be expected that they would contain free molecules of water, which form one of the 

 products of the decomposition of those definite compounds whose formation is the cause 

 of solution. In separating as ice or vapour, water makes, with a solution, a heteroge- 

 neous system (made up of substances in different physical states) similar, for instance, 

 to the formation of a precipitate or volatile substance in reactions of double decom- 

 position. 



47 If the substance dissolved is non-volatile (like salt or sugar), or only slightly volatile, 

 then the whole of the tension of the vapour given off belongs to the water, but if a 

 solution of a volatile substance for instance, a gas or a volatile liquid evaporates, then 

 only a proportion of the pressure belongs to the water, and the whole pressure observed 

 consists of the sum of the pressures of the vapours of the water and of the substance 

 dissolved. The majority of researches bear on the first case, which will be spoken of 

 presently, and the observations of D. P. Konovoloff (1881) refer to the second case. He 

 showed that in the case of two volatile liquids, mutually soluble in each other, forming 

 two layers of saturated solutions (for example, ether and water, note 20, p. 66), both solu- 

 tions have an equal vapour tension (in the case in point the tension of both is equal to 

 481 mm. of mercury at 19'8). Further, he found that for solutions which are formed 

 in all proportions, the tension is either greater (solutions of alcohol and water) or lesa 

 (solutions of formic acid) than that which answers to the rectilinear change (proportional 

 to the composition) from the tension of water to the tension of the substance dis- 

 solved ; thus the tension, for example, of a 70 p.c. solution of formic acid is less, at all 



