Example.--As an example of this there may be taken the mixed crystals of mercuric bromide and iodide.[273] Mercuric bromide melts at 236.5°, and mercuric iodide at 255.4°. The mixed crystal of definite constant melting point (minimum point) contains 59 mols. per cent. of mercuric bromide, the melting point being 216.1°.
The numerical data are contained in the following table, and represented graphically in Fig. 52:--
----------------------------------------------------- Mols. per cent. of | | HgBr_{ 2}. | Freezing point. | Melting point. ----------------------------------------------------- 100 | 236.5° | 236° 90 | 228.8° | 226° 80 | 222.2° | 219° 70 | 217.8° | 217° 65 | 216.6° | 216° 60 | 216.1° | 215.5° 55 | 216.3° | 216° 50 | 217.3° | 216° 40 | 221.1° | 218° 30 | 227.8° | 223° 20 | 236.2° | 231° 10 | 245.5° | 242° 0 | 255.4° | 254° -----------------------------------------------------
Fractional Crystallization of Mixed Crystals.--With the help of the diagrams already given it will be possible to predict what will be the result of the fractional crystallization of a fused mixture of two substances which can form mixed crystals. Suppose, for example, a fused mixture of the composition _x_ (Fig. 53) is cooled down; then, as we have already seen, when the temperature has fallen to _a_, mixed crystals of composition, _b_, are deposited. If the temperature is allowed to fall { 189} to _x'_, and the solid then separated from the liquid, the mixed crystals so obtained will have the composition represented by e. If, now, the mixed crystals _e_ are completely fused and the fused mass allowed to cool, separation of solid will occur when the temperature has fallen to the point _f_. The mixed crystals which are deposited have now the composition represented by _g_, i.e. _they are richer in B than the original mixed crystals_. By repeating this process, the composition of the successive crops of mixed crystals which are obtained approximates more and more to that of the pure component B, while, on the other hand, the composition of the liquid phase produced tends to that of pure A. By a systematic and methodical repetition of the process of fractional crystallization, therefore, a _practically_ complete separation of the components can be effected; a perfect separation is theoretically impossible.