the Mutual Solubility of Liquids. 183 



of the condition where the liquids become identical to the 

 critical point where a liquid and its vapour become identical. 

 This resemblance makes him suppose that any pair of liquids 

 is bound to reach a similar critical point with rise of tempera- 

 ture, in the same way as every pure substance has a critical 

 point. Rothmund agrees with Masson ; he even goes so far 

 as to admit the possibility of every combination having a lower 

 critical point of complete miscibility and a higher one, between 

 which two liquids would be possible. Or, putting it in terms 

 of the solubility-curve, this curve might be found to be a 

 closed curve, if only the limits of temperature were taken wide 

 enough. 



There is, however, another possibility with regard to the 

 solubility-curve which Masson and Rothmund seem to have 

 overlooked, viz. : — That on heating, the vapour and one of the 

 liquids may become identical in density and composition, i. e. 

 may reach a critical point, before the two liquids become 

 identical. It is often not sufficiently remembered that the 

 equilibrium between two liquids is not properly defined unless 

 the liquids are in contact with the third phase, the vapour. 

 If the vapour is not present, the composition and density of 

 the coexisting liquids depend on the pressure, as well as on 

 the temperature. As a matter of fact, in the experiments as 

 they are usually made, there is a vapour phase above the two 

 liquid layers, and the pressure which establishes itself at each 

 temperature is the true equilibrium-pressure of the three phases 

 (if we neglect the influence of the small quantity of air which 

 is usually present in the vapour). The fact, however, that 

 increase of pressure above the three-phase pressure has a very 

 small effect on the composition (and also on the density) of 

 the liquids under ordinary conditions * has led to the view, 

 which seems common among workers in this field, that the 

 influence of the vapour is of little account. Although this 

 view is correct, in so far as increase of pressure above the three- 

 phase pressure has as a rule a very small effect, it is not true 

 in so far as the pressure at which the two liquids coexist can- 

 not be less than this three-phase pressure. The importance of 

 this will be seen at once, if the case in which the temperature 

 is higher than the critical temperature of the upper liquid 

 layer and the vapour is considered. Here no such lower limit 

 for the pressure of the two remaining phases exists. The 

 pressure, and therefore also the composition and density of the 

 two phases, may be varied within wide limits ; and it depends 



* Instances will be given below where this influence was very prominent. 

 A very small pressure was sufficient to make the two liquids mix, so 

 that the rule that this influence may be neglected is not a general one. 



02 



