Since the critical solution point represents the highest temperature at which two liquid phases consisting of phenol and {100} water can exist together, these two substances can be brought together in any amount whatever at temperatures higher than 68.4°, without the formation of two layers. It will therefore be possible to pass from a system represented by _x_ to one represented by _x'_, without at any time two liquid phases appearing. Starting with _x_, the temperature is first raised above the critical solution temperature; phenol is then added until the concentration reaches the point _x__{2}. On allowing the temperature to fall, the system will then pass into the condition represented by _x'_. [Illustration: FIG. 23.] Methylethylketone and Water.--In the case just described, the solubility of each component in the other increased continuously with the temperature. There are, however, cases where a maximum or minimum of solubility is found, _e.g._ methylethylketone and water. The curve which represents the equilibria between these two substances is given in Fig. 23, the concentration values being contained in the following table:[171]-- METHYLETHYLKETONE AND WATER. --------------+-----------------+----------------- Temperature. | C_{1} per cent. | C_{2} per cent. --------------+-----------------+----------------- -10° | 34.5 | 89.7 +10° | 26.1 | 90.0 30° | 21.9 | 89.9 50° | 17.5 | 89.0 70° | 16.2 | 85.7 90° | 16.1 | 84.8 110° | 17.7 | 80.0 130° | 21.8 | 71.9 140° | 26.0 | 64.0 151.8° | 44.2 | 44.2 --------------+-----------------+----------------- {101} These numbers and Fig. 23 show clearly the occurrence of a minimum in the solubility of the ketone in water, and also a minimum (at about 10°) in the solubility of water in methylethylketone. Minima of solubility have also been found in other cases. [Illustration: FIG. 24.] Triethylamine and Water.--Although in most of the cases studied the solubility of one liquid in another increases with rise of temperature, this is not so in all cases. Thus, at temperatures below 18°, triethylamine and water mix together in all proportions; but, on raising the temperature, the homogeneous solution becomes turbid and separates into two layers. In this case, therefore, the critical solution temperature is found in the direction of lower temperature, not in the direction of higher.[172] This behaviour is clearly shown by the graphic representation in Fig. 24, and also by the numbers in the following table:-- TRIETHYLAMINE AND WATER.