ON THE SOLUBILITY OF SALTS. 295 



Although fluids whose molecular motions are not incompatible may be 

 able to mix in the same indefinite proportion as gases, true chemical combi- 

 nation may also take place between them. It is very easy to prove this in 

 some cases, as, for example, when we add 27 parts of water to 63 parts of 

 HO, N0 5 ; because in this case we have heat evolved, and the mixture has 

 a fixed boiling-point, and may be distilled without decomposition. In other 

 cases it is not possible to prove it. Change of volume, accompanied by the 

 evolution of heat, can scarcely be considered a proof, inasmuch as we would 

 assume as true the very thing we have to prove, namely, that the conden- 

 sation is due to chemical combination. In the case of a mixture of alcohol 

 and water, there is a considerable condensation, which may, as some suppose, 

 be due to true chemical combination. M. Vergnette Lamotte, for instance, 

 states that in congealed wine the alcohol and water are in definite proportions 

 in the frozen part, an opinion not, however, adopted by M. Boussingault and 

 others. This condensation may perhaps be accounted for in another way, 

 which, however, it would take too much space to develope fully here, but 

 which I shall have a more fitting opportunity of doing hereafter. 



As the molecules of solids have a determinate position of equilibrium, 

 their solution in any medium implies the exertion of a certain amount of 

 force by the molecules of the liquid upon the solid. There is thus a distinc- 

 tion between the solution of solids and of liquids ; the former partakes more 

 of the character of chemical comb ination. It may be that when a solid is 

 dissolved in water, it forms in the first instance a definite chemical compound 

 which is liquid, and may therefore mingle with the uncombined water. If 

 this hypothesis were correct, it would account for the absence of stoichio- 

 metrical relations between the quantity of salt and of the water in which it is 

 dissolved, for a saline solution would be a compound of water and salt mole- 

 cules mixed with n quantity of water. Some salts may possess the property 

 of forming several such liquid compounds, others, perhaps, only one. There 

 is considerable analogy between solution in the ordinary sense, and alloys, 

 especially amalgams. Mercury appears to be capable of forming an endless 

 number of compounds with several metals, some of which are solid and others 

 liquid. The solid alloys freely dissolve in mercury. In the solution thus 

 formed have we the original alloy simply dissolved in the mercury, or has a 

 new compound been formed ? If we cool the mixture crystals often sepa- 

 rate, which may or may not be the alloy originally dissolved, according as 

 the conditions under which they are formed are the same or different from 

 those under which the alloy was first formed ; exactly as when a salt sepa- 

 rates from its aqueous solution at different temperatures with different pro- 

 portions of hydrated water. The proportions in which mercury combines 

 with metals are so numerous, and so unlike those which we meet with among 

 the compounds of the metalloids, that we are perhaps justified in concluding 

 that a metal dissolved in mercury is always in chemical combination, and 

 that when the constituents of an amalgam or of other liquid alloys are not in 

 definite proportion, we may account for it by supposing that it consists of 

 some liquid alloy mingled "with n quantity of mercury. As an example of 

 the proportions in which mercury combines with the metals, I may mention 

 the alloys made by Crookewitt* :— KHg 20 ; KHg^ ; Cd 2 Hg 5 ; PbHg ; BiHg ; 

 Ag s Hg lB ; AgHg 2 ; AgHg 3 ; AgHg 4 ; AuHg 4 . 



Among the other arguments which may be urged in favour of solution 



being reckoned as a case of chemical combination, I may mention — I, that 



saline solutions in HO have a fixed boiling-point, a point of congelation, 



and, like water itself, a point of maximum density; and 2, that solutions are 



* Ann. der Chem. und Pharm. vol. lxviii. p. 259. 



