96 Mr. W. W. J. Nicol on the 



number of specific-gravity determinations. I hope soon to 

 be able to investigate this point more fully. 



The next evidence I have to bring forward is that based on 

 the volume-alteration attending the solution of salts in water 

 and the dilution of these solutions at various temperatures ; 

 but before doing so it is necessary to have a clear understand- 

 ing of what is meant by saying that one salt is more soluble 

 in water than another, and that two solutions of different salts 

 are of the same strength. As solution, however it may be 

 explained, is evidently a molecular process, I will in the fol- 

 lowing pages express the solubility of a salt in water in terms of 

 salt-molecules to a fixed number of water-molecules (100H 2 O), 

 and consider, as indeed my experiments show, that salt-solu- 

 tions are of the same strength, and are therefore comparable, 

 when they contain an equal number of salt-molecules dissolved 

 in each case in 100 molecules of water. 



When a salt is dissolved in water, the volume of the solu- 

 tion is usually less than the sum of the volumes of the salt 

 and the water before solution takes place. This has long been 

 stated as a general rule, to which, among anhydrous salts, 

 ammonium chloride formed the only exception. I have re- 

 cently shown* that this property is shared by other ammonium 

 salts ; and I believe that, on further investigation, it will be 

 found that contraction is by no means the necessary or even 

 the usual concomitant of solution. But in cases where such 

 contraction does occur, it is found that the amount increases 

 as the amount of salt in solution increases, but that the 

 amount of contraction produced by the solution of each suc- 

 cessive molecule is less than that produced by the preceding 

 one, as stated by Gerlachj : — 



" Die Contraction wachststetig in alien Fallen, wenn in 

 einer constantbleibenden Wassermenge eine stetig wachsende 

 Anzahl gleicher Salzatome gelost wird. Die Zunahme der 

 Contraction steht indess keineswegs in einfachem Verh'altniss 

 mit der Zunahme geloster Salzatome ; ist letztere eine stets 

 gleiche, so ist erstere eine stetig abnehmende." 



This is shown by Table II., which gives the specific gravity 

 of molecular solutions of sodium chloride, ranging from 1 mo- 

 lecule in 200 of water up to 10*99 molecules in 100 of water 

 (saturated solution). 



This is entirely in accordance with my statement of the 

 nature of solution : — the greater the amount of salt already 

 in solution the less the result of the attraction of dissimilar 

 molecules and the greater that of similar molecules. 



* Proc. Roy. Soc. Edinb. July 1882. 



f Spec. Gewicht d. Salztesungen, 1859, p. 68. 



