292 DEDUCTION FROM THE GASEOUS THEORY OF SOLUTION. 



rises tlie liquid diminishes in total quantity, tlie vapor increasing; but 

 the specific volume of the liquid increases, while that of the vapor de- 

 creases. The residual liquid is, in fact, constantly encroaching on the 

 space of its vapor, just as the residual substance A in the A layer is 

 constantly absorbing the solvent B from the B layer. Finally, in either 

 case, the specific volume of the substance will become identical in both 

 layers, which means that the layers themselves will become homogene- 

 ous and indistinguishable. Our system will then have reached' its 

 critical temperature — the temperature of infinite volatility in the one 

 case and of infinite solubility in the other. 



So much for hypothesis. Are there any facts in support of it? Well, 

 in the first place the hypothesis demands that (in the absence of chem- 

 ical change) increase of solubility with rise of temperature shall be as 

 genera] a law as increase of vapor pressure, and we find that this agrees 

 with the known facts, more especially since Tilden and Shenstone {Phil. 

 Trans., 1884) cleared up certain doubtful cases. Secondly, the hypothesis 

 seems to demand some connection between the true melting points of 

 salts and the rates of their increase of solubility; and such a relation 

 has in a general way been established by the same observers. Thirdly, 

 we have the fact, in complete accordance with the hypothesis, that while 

 no case is known of a solid body having, as such, infinite solubility in 

 any simple solvent, several cases are knowai of liquids of infinite solu- 

 bility, and also of solids which, after they have melted in presence of 

 their own solution, become at some higher temperature infinitely soluble. 

 This last statement refers to the cases described by Alexeeft" ( ^Vicde- 

 maiui's Annalen, 1886), of which 1 must say a good deal more directly. 

 It would seem to apply also to the case of silver nitrate, which Tilden 

 and Shenstone described as dissolving in water to the extent of 18.25 

 parts to one at so low a temperature as 130° C. The true melting-point 

 of the salt is 217'^, and I have seen it stated (but have been unable to 

 find the published account) that Shenstone has himself shown it to be 

 fusible in water, and of infinite solubility at quite reachable temper- 

 atures. 



With regard to substances that are liquid under ordinary conditions, 

 we have the well-known fact that some pairs are infinitely soluble in 

 one another, while others exhibit the phenomenon of only i)artial solu- 

 bihty. The hypothesis would draw no hard and fast distinction 

 between these cases, except the practically important one that such a. 

 mixture as that of ether and alcohol, which belougsHo the first class, 

 is usually above its critical solution point, while such a one as ether 

 and water, which belongs to the second class, is usually below it. It 

 should be possible, according to the hypothesis, to cool mixtures of 

 ether and alcohol sufficiently to cause separation into two layers, simi- 

 lar to those observed at the ordinary temperature in the case of ether 

 and water; but I do not know that this has yet been put to the test of 

 experiment. 



