Tension and Complex Molecules. 393 



both layers is in this ease the essential condition of equi- 

 librium. All the above considerations must be true for 

 almost immiscible liquids if they are true in the case of the 

 liquids with finite solubility. 



The experiment may fail in proving the correctness of the 

 above law for almost immiscible liquids, and yet it will not 

 convince me that it does not hold true. 



For that reason I do not attribute much importance to the 

 remark by W. B. Hardy * that it is not a general law, not 

 being applicable in the case of immiscible liquids. He does 

 not give any examples nor figures. It would also be im- 

 portant to know how the figures were obtained, and whether 

 all the necessary conditions to maintain equilibrium were 

 satisfied. 



§ 4. The Existence of Complex Molecules in 

 the Solutions. 



In the preceding paragraph we admitted that the equality 

 of molecular concentration of two coexisting liquid phases 

 must be an essential condition of equilibrium from the stand- 

 point of the above theory, only under those conditions two 

 layers of different composition may coexist permanently 

 without having the tendencj- to mix with one another by 

 diffusion. 



However absurd this may appear at first sight, the 

 assumption may be demonstrated to be quite necessary in 

 the following way: — 



It is known that the two superposed liquid layers in equi- 

 librium boil at the same temperature and they have the same 

 freezing-point. For the systems forming two layers the 

 following types of freezing curves are known (see figs. 4 

 and 5)t? where the concentrations are plotted along the 

 abscissse and the freezing-points along the ordinates. (By 

 the freezing-point is understood the temperature at which 

 the solution can coexist with verj^ small quantity of its 

 ice.) 



Consider first the curve of fig, 4. 



Point A gives the freezing-point of substance A, and B, 

 that of B. Between A and C the substance A freezes out, 

 and between (J and B the substance B, C being the so-called 

 eutectic point where both A and B fall out simultaneously. 



* Proc. Boy. Soc. lxxxviii. 1913, A, p. 325. 



t The dotted curve shows the limits of solubility of one liquid in 

 another, inside which there are two layers. 



