The third type of curve, Fig. 65, can be distinguished in a similar manner from the ordinary eutectic curve, Fig. 63, I., to which it bears a certain resemblance. Whereas in the case of the latter, the eutectic point is the temperature of complete solidification of all solutions, the point of minimum temperature in the case of the formation of mixed crystals, is the solidification point only of solutions having one particular composition; that, namely, of the minimum point. For all other solutions, the temperature of complete solidification is different. Whereas, also, in the case of the simple eutectic curve, the solid which separates out from the solutions represented by either curve remains the same throughout the whole extent of that curve, the composition of the mixed crystal varies with variation of the composition of the liquid phase, and the relative proportions of the two components in the solid and the liquid phase are reversed on passing through the minimum.[290] In a similar manner, type IV., Fig. 65, can be distinguished from type II., Fig. 64, by the fact that it does not exhibit a {212} eutectic point, and that the composition of the solid phase undergoes continuous variation with variation of the liquid phase on either side of the transition point. Lastly, type V., which does exhibit a eutectic point, differs from the eutectic curve of Fig. 63, in that the eutectic point does not constitute the point of complete solidification for all solutions, and that the composition of the solid phase varies with the composition of the liquid phase. Such, then, are the chief general types of equilibrium curves for two-components; they are the pattern curves with which other curves, experimentally determined, can be compared; and from the comparison it will be possible to draw conclusions as to the nature of the equilibria between the two components under investigation. 1. _Organic Compounds._ [Illustration: FIG. 66.] The principles of the Phase Rule have been applied to the investigation of the equilibria between organic compounds, and Figs. 66-69 reproduce some of the results which have been obtained.[291] {213} Fig. 66, the freezing point curve (curve of equilibrium) for _o_-nitrophenol and _p_-toluidine, shows a curve of the simplest type[292] (type I., Fig. 63), in which two branches meet at an eutectic point. The solid phase in equilibrium with solutions represented by the left-hand branch of the curve was _o_-nitrophenol (m.p. 44.1°); that in equilibrium with the solutions represented by the right-hand branch, was _p_-toluidine (m.p. 43.3°). At the eutectic point (15.6°), these two solid phases could co-exist with the liquid phase. This equilibrium curve, therefore, shows that _o_-nitrophenol and _p_-toluidine do not combine with one another.