to employ a solution having a composition near to the line OE. Formation of Mixed Crystals.--If the two single salts A and B do not crystallize out pure from solution, but form an unbroken series of mixed crystals, it is evident that an invariant system cannot be produced. The solubility curve will therefore be continuous from A to B; the liquid solutions of varying composition being in equilibrium with solid solutions also of varying composition. If, however, the series of mixed crystals is not continuous, there will be a break in the solubility curve at which two solid solutions of different composition will be in equilibrium with liquid solution. This, of course, will constitute an invariant system, and the point will correspond to the point C in Fig. 108. A full discussion of these systems would, however, lead us too far, and the above indication of the behaviour must suffice.[362] {282} Application to the Characterization of Racemates.--The form of the isothermal solubility curves is also of great value for determining whether an inactive substance is a racemic compound or a conglomerate of equal proportions of the optical antipodes.[363] As has already been pointed out, the formation of racemic compounds from the two enantiomorphous isomerides, is analogous to the formation of double salts. The isothermal solubility curves, also, have a similar form. In the case of the latter, indeed, the relationships are simplified by the fact that the two enantiomorphous forms have identical solubility, and the solubility curves are therefore symmetrical to the line bisecting the angle of the co-ordinates. Further, with the exception of the partially racemic compounds to be mentioned later, there is no transition interval. In Fig. 111, are given diagrammatically two isothermal solubility curves for optically active substances. From what has been said in the immediately preceding pages, the figure ought really to explain itself. The upper isothermal _acb_ represents the solubility relations when the formation of a racemic compound is excluded, as, _e.g._ in the case of rubidium _d_- and _l_-tartrates above the transition point (p. 265). The solution at the point _c_ is, of course, inactive, and _is unaffected by addition of either the _d_- or _l_- form_. The lower isothermal, on the other hand, would be obtained at a temperature at which the racemic compound could be formed. The curve _a'e_ is the solubility curve for the _l_- form; _b'f_, that for the _d_- form; and _edf_, that for the racemic compound in presence of solutions of varying concentration. The point _d_ corresponds to saturation for the pure racemic compound. [Illustration: FIG. 111.]