-------------+-----------------+---------------- Temperature. | C_{1} per cent. | C_{2} per cent. -------------+-----------------+---------------- 70° | 1.6 | -- 50° | 2.9 | -- 30° | 5.6 | 96 25° | 7.3 | 95.5 20° | 15.5 | 73 ±18.5° | ±30 | ±30 -------------+-----------------+---------------- General Form of Concentration-Temperature Curve.--From the preceding figures it will be seen that the general {102} form of the solubility curve is somewhat parabolic in shape; in the case of triethylamine and water, the closed end of the curve is very flat. Since for all liquids there is a point (critical point) at which the liquid and gaseous states become identical, and since all gases are miscible in all proportions, it follows that there must be some temperature at which the liquids become perfectly miscible. In the case of triethylamine and water, which has just been considered, there must therefore be an upper critical solution temperature, so that the complete solubility relations would be represented by a closed curve of an ellipsoidal aspect. An example of such a curve is furnished by nicotine and water. At temperatures below 60° and above 210°, nicotine and water mix in all proportions.[173] Although it is possible that this is the general form of the curve for all pairs of liquids, there are as yet insufficient data to prove it. With regard to the closed end of the curve it may be said that it is continuous; the critical solution point is not the intersection of two curves, for such a break in the continuity of the curve could occur only if there were some discontinuity in one of the phases. No such discontinuity exists. The curve is, therefore, not to be considered as two solubility curves cutting at a point; it is a curve of equilibrium between two components, and so long as the phases undergo continuous change, the curve representing the equilibrium must also be continuous. As has already been emphasized, a distinction between solvent and solute is merely conventional (p. 93). Pressure-Concentration Diagram.--In considering the pressure-concentration diagram of a system of two liquid components, a distinction must be drawn between the total pressure of the system and the partial pressures of the components. On studying the total pressure of a system, it is found that two cases can be obtained.[174]