652 On the Mutual SoluUlity of Lit/uids. 



which occurs on the liquids being mixed '^, but an explanation 

 b}^ molecular considerations cannot be given until the phe- 

 nomenon of partial miscibility itself has been more fully- 

 explained. 



Assuming that the abnormality diminishes as the tempera- 

 ture rises, it looks at first sight as if the influence of heating 

 ought always to be in the direction of increasing the mutual 

 solubility, as it actually is in by far the greatest number of 

 cases ; but there are exceptions such as with water and tri- 

 ethylamine. Here it is that the general point of view, taken 

 by van der Waals in his theory, allows us to bring apparent 

 exceptions better into line with the other cases. The appear- 

 ance or disappearance of the two layers does not depend on 

 the liquid phases only, but also on the vapour in equilibrium 

 with the liquids ; if on cooling a mixture of water and tri- 

 ethylamine the layers mix at 18°*3 C, this does not mean 

 that the liquids are now soluble at all densities, but it is due 

 to the fact that the liquid which is in equilibrium wdth the 

 vapour at this moment begins to be denser than the liquids of 

 the two-liquid equilibrium ; if at temperatures below 18°*3 

 the pressure were lowered below the vapour-pressure, it would 

 undoubtedly be found that in the metastable region the two 

 layers made their appearance, in the same way as we proved 

 to be the case with mixtures of propane and methyl- alcohol 

 under similar conditions. At higher temperatures the liquid 

 saturation-curve will shrink and disappear in this case as in 

 all the others. From the point of view^ of the theory the 

 existence of a liquid-curve is the principal matter ; and the 

 appearance or disappearance of the two layers under the 

 pressure of the saturated vapour is of secondary importance. 



In conclusion let us classify the various cases which have 

 been discovered w^ith respect to the effect both of temperature 

 and pressure, excepting those like ethane and methyl-alcohol, 

 where the critical temperature of the upper liquid and the 

 vapour is reached before that of the two liquids. A priori 

 we find the following four combinations to be possible : — 



1. Upper critical point, pressure mixes liquids. Example: 



propane and methyl- alcohol. 



2. Upper critical point, pressure separates liquids. Ex- 



ample : phenol and water, hexane or pentane with 

 methyl-alcohol, &c. 



3. Lower critical point, pressure mixes liquids. Example : 



ethane and ethyl-, propyl-, &c. alcohol, triethylamine 

 and water. 



4. Lower critical point, pressure separates liquids. 



* This question we hope to discuss in a suhsequeut paper. 



