In this case the freezing point of each of the components is lowered by the addition of the other, until at last a point is reached at which the liquid solution solidifies to a mixture or conglomerate of two mixed crystals.
Examples.—Curves belonging to this class have been obtained in the case of potassium and thallium nitrates[[275]] and of naphthalene and monochloracetic acid.[[276]] The data for the latter are given in the following table and represented in Fig. 56:—
| Temperature. | Liquid solution. | Solid solution. | ||
| Per cent. naphthalene. | Per cent. acid. | Per cent. naphthalene. | Per cent. acid. | |
| 62° | — | 100 | — | 100 |
| 60° | 4.0 | 96.0 | 1.7 | 98.3 |
| 55° | 21.0 | 79.0 | 2.1 | 97.9 |
| 53.5° | 29.4 | 70.0 | — | — |
| 55° | 31.3 | 68.7 | 59.6 | 40.4 |
| 60° | 42.4 | 57.6 | 80.3 | 19.7 |
| 65° | 53.3 | 46.7 | 89.2 | 10.8 |
| 70° | 69.7 | 32.3 | 95.4 | 4.6 |
| 75° | 84.4 | 15.6 | 96.6 | 3.4 |
| 79.9° | 100 | — | 100 | — |
At the eutectic point the liquid solution is in equilibrium with two different mixed crystals the composition of which is represented by D and E respectively. If, therefore, a fused mixture containing the two components A and B in the proportions represented by C is cooled down, it will, when the temperature has reached the point C, solidify completely to a conglomerate of mixed crystals, D and E.
Changes in Mixed Crystals with the Temperature.—In the case of the different types of systems represented in Fig. 49, a homogeneous liquid solution of the two components will exist at temperatures above the freezing-point curve, a homogeneous mixed crystal at temperatures below the melting-point curve, while at any point between the freezing-point and melting-point
curves the mixture will separate into a solid phase and a liquid phase. In the case, however, of the two types shown in Fig. 54 the relationships are somewhat more complicated. As before, the area above the freezing-point curve gives the conditions under which homogeneous liquid solutions can exist; but below the melting-point curve two different mixed crystals can coexist. This will be best understood from Figs. 57 and 58. D and E represent, as we have seen, the composition of two mixed crystals which are in equilibrium with the liquid solution at the temperature of the point C. These two mixed crystals represent, in the one case, a saturated solution of B in A (point D), and the other a saturated solution of A in B (point E). Just as we saw that the mutual solubility of two liquids varied with the temperature, so also in the case of two solids; as the temperature alters, the solubility of the two solid components in one another will change. This alteration is indicated diagrammatically in Figs. 57 and 58 by the dotted curve similar to the solubility curves for two mutually soluble liquids (p. [101]).
Suppose, now, that a mixed crystal of the composition x is cooled down, it will remain unchanged until, when the temperature has fallen to t′, the homogeneous mixed crystal breaks up into a conglomerate of two mixed crystals the composition of
which is represented by x′ and x″ respectively. From this, then, it can be seen that in the case of substances which form two solid solutions, the mixed crystals which are desposited from the liquid fused mass need not remain unchanged in the solid state, but may at some lower temperature lose their homogeneity. This fact is of considerable importance for the formation of alloys.[[277]]