From this it will also be clear that on cooling a fused mixture of two substances capable of forming mixed crystals, {185} the temperature of solidification will not remain constant during the separation of the solid; nor, on the other hand, will the temperature of liquefaction of the solid solution be constant. Thus, for example, if a liquid solution of two components, A and B, having the composition represented by the point _x_ (Fig. 50), is allowed to cool, the system will pass along the line _xx'_. At the temperature of the point _a_, mixed crystals will be deposited, the composition of which will be that represented by b. As the temperature continues to fall, more and more solid will be deposited; and since the solid phase is relatively rich in the component B, the liquid will become relatively poorer in this. The composition of the liquid solution will therefore pass along the curve _ad_, the composition of the solid solution at the same time passing along the curve _bc_; at the point _c_ the liquid will solidify completely.[271] [Illustration: FIG. 50.] Conversely, if mixed crystals of the composition and at the temperature _x'_ are heated, liquefaction will begin at the temperature _c_, yielding a liquid of the composition d. On continuing to add heat, the temperature of the mass will rise, more of the solid will melt, and the composition of the two phases will change as represented by the curves _da_ and _cb_. When the temperature has risen to _a_, complete liquefaction will have occurred. The process of solidification or of liquefaction is therefore extended over a temperature interval _ac_. Even when the freezing-point curve is a straight line joining {186} the melting points of the pure components, the melting-point curve will not necessarily coincide with the freezing-point curve, although it may approach very near to it; complete coincidence can take place only when the melting points of the two components are identical. An example of this will be given later (Chap. XII.). (_b_) _The freezing-point curve passes through a maximum_ (Curve II., Fig. 49). [Illustration: FIG. 51.] This curve exhibits the greatest degree of contrast to the freezing-point curve which is obtained when the pure components crystallize out. For, since the curve passes through a maximum, it is evident that the freezing point of each of the components must be _raised_ by the addition of the other component. Example.--Very few cases belonging to this type are known. The best example is found in the freezing-point curve of mixtures of _d_- and _l_-carvoxime[272] (C_{10}H_{14}N.OH). The freezing points and melting points of the different mixtures of _d_- and _l_-carvoxime are given in the following table, and represented graphically in Fig. 51:--