308 report— 1859. 



which each salt experimented upon can produce in raising the boiling-point ; 

 secondly, to determine, as Legrand did for a great number of salts, how much 

 of each salt would be required to raise the temperature of boiling by one 

 degree Cent, successively from 100° to the maximum which the particular 

 salt can raise it to ; and thirdly, the effect of mixture in the laws established 

 for the pure salt. 



The study of the tension of vapours from saline solutions promises to be a 

 very useful test of molecular changes. Regnault believes that the vapour from 

 boiling saline solutions has the same temperature as the solution, and that the 

 study of the tension of the vapours of such solutions might determine the im- 

 portant problem, whether salts were still joined to their crystalline water when 

 in solution, and may in this way prove as useful a test as polarized light ; 

 and also whether double salts exist in solution, or are only formed at the 

 moment of crystallization. Rudberg concluded from his experiments that 

 the temperature of vapour arising from saline solutions was independent of 

 the temperature of the solution. Von Babo has found* that the vapour from 

 a boiling saline solution has less tension than steam of a corresponding tem- 

 perature. Pliicker has also foundf that the vapour tension of saline solutions 

 is less than that of pure water, and that the difference of tension may answer 

 as a relative measure of the active molecular force between the salt and 

 water, and of the law of its increase with the increasing quantity of salt. 

 He states that the diminution of tension of the vapour is so directly propor- 

 tional to the increase of salt, that the determination of this tension at 100° 

 Cent, will give the per-centage of salt to solution, at least as well as the finest 

 areometer. WullnerJ has also found that the differences of tension are directly 

 as the quantities of salts dissolved. The differences of tension are repre- 

 sented by a differently expressed function of the temperature for each salt 

 and mixture of salts. He also finds that no relationship can be established 

 between the differences of tension and any of the other properties of the 

 salts, especially the solubility ; which fully bears out a conclusion which I 

 came to above upon different grounds, that the force with which a salt may 

 be held by the solvent, does not seem to bear much relation to its solubility. 

 He also states that salts which cannot act chemically upon one another, 

 modify each other's attraction for the water, when they occur together in 

 solution. As it is doubtful whether any such salts exist, this would mean 

 that all salts modify each other's affinity for water, when their solutions are 

 mixed. Wiillner also considers the differences of tension to be a measure 

 of the actual attractive force between the water aud salt. It appears to me 

 that the difference of tension can serve as a measure of the amount of 

 decomposition which takes place on mixing two soluble salts which do not 

 give a precipitate, and also of the influence of mass and of form in such 

 decompositions at different temperatures. I accordingly propose to use them 

 for that purpose, as well as also in conjunction with the experiments which I 

 am making en the removal of salts from solutions by porous substances not 

 acting chemically upon them, as a measure of the force by which salts are 

 held in solution. 



Pliicker has also observed some very remarkable affinity relations which 

 admit of being numerically determined when to a mixture of two fluids a 

 third is added, or when a salt is dissolved in the mixture — which is indeed 

 the case upon which he lays most stress, and the tension of the vapour de- 

 termined, and from this the composition calculated. Thus, for example, 



* Ann. der Chern. und Pharra. vol. Ixvii. p. 356. 

 t Poggendorff's Annalen, vol. xcii. p. 19-3. 

 J Ibid. vol. ciii. 529 ; anil vol. cv. p. 85. 



