In the present chapter we shall consider only the fusion curves, the solubility curves being discussed in the next section on three-component systems. The rules which are hereby obtained, have reference only to the nature of the inactive substance in the neighbourhood of the melting points. I. _The inactive substance is a _dl_-mixture._ In this case the fusion curves will have the simple form shown in type I, Fig. 63. A and B are the melting points of the two optical isomerides, and C the eutectic point at which the inactive mixture consisting of equal amounts of d- and l-form melts. Owing to the similar effect of the one form on the freezing point of the other, the figure is symmetrical. No example of this simple case has been investigated. II. _The two components form a racemic compound._ In this case there will be three melting point curves as in Fig. 64, type I. In this case also the figure must be symmetrical. Examples.--As examples of this, may be taken dimethyl tartrate and mandelic acid, the freezing point curves of which are given in Figs. 70 and 71.[299] As can be seen, the curve for the racemic tartrate occupies a large part of the diagram, {218} while that for racemic mandelic acid is much smaller. In the case of dimethyldiacetyl tartrate, this middle portion is still less. [Illustration: FIG. 70.] [Illustration: FIG. 71.] [Illustration: FIG. 72.] Active dimethyl tartrate melts at 43.3°; racemic dimethyl tartrate at 89.4°. Active mandelic acid melts at 132.8°; the racemic acid at 118.0°. In the one case, therefore, the racemic compound has a higher, in the other a lower melting point than the active forms. {219} In the case of partially racemic compounds (_i.e._ the compound of a racemate with an optically active substance) the type of curve will be the same, but the figure will no longer be symmetrical. Such a curve has been found in the case of the l-menthyl esters of d- and l-mandelic acid (Fig. 72).[300] The freezing point of l-menthyl d-mandelate is 97.2°, of l-menthyl l-mandelate 77.6°, and of l-menthyl r-mandelate 83.7.° It will be observed that the summit of the curve for the partially racemic mandelate is very flat, indicating that the compound is largely dissociated into its components at the temperature of fusion. III. _The inactive substance is a pseudo-racemic mixed crystal._ In cases where the active components can form mixed crystals, the freezing-point curve will exhibit one of the forms given in Fig. 65. The inactive mixed crystal containing 50 per cent. of the dextro and laevo compound, is known as a pseudo-racemic mixed crystal.[301] So far, only curves of the types I. and II. have been obtained.