On passing to the ternary systems, the composition of the solutions must be represented by points or curves situated _between_ the two planes. We shall now turn to the consideration of these. BD and CD are ternary eutectic curves (p. 284). They give the composition of solutions in equilibrium with ice and magnesium chloride dodecahydrate (BD), and with ice and potassium chloride (CD). D is a _ternary cryohydric point_. If the temperature is raised and the ice allowed to disappear, we shall pass to the solubility curve for MgCl_{2},12H_{2}O + KCl (curve DE). At E carnallite is formed and the potassium chloride disappears; EFG is then the solubility curve for MgCl_{2},12H_{2}O + carnallite (KMgCl_{3},6H_{2}O). This curve also shows a point of maximum temperature (F) and a retroflex portion. GH and HJ represent the solubility curves of carnallite + MgCl_{2},8H_{2}O and carnallite + MgCl_{2},6H_{2}O, G and H being transition points. JK is the solubility curve for carnallite + MgCl_{2},4H_{2}O. At the point K we have the _highest temperature at which carnallite can exist with magnesium chloride in contact with solution_. Above this temperature decomposition takes place and potassium chloride separates out. If at the point E, at which the two single salts and the double salt are present, excess of potassium chloride is added, the magnesium chloride will all disappear owing to the formation of carnallite, and there will be left carnallite and potassium chloride. The solubility curve for a mixture of these two salts is represented by EMK; a simple curve exhibiting, however, a temperature maximum at M. This maximum point corresponds with the fact that dry carnallite melts at this temperature with separation of potassium chloride. _At all temperatures {287} above this point, the formation of double salt is impossible_. The retroflex portion of the curve represents solutions in equilibrium with carnallite and potassium chloride, but in which the ratio MgCl_{2} : KCl is greater than in the double salt. Throughout its whole course, _the curve EMK represents solutions in which the ratio of MgCl_{2} : KCl is greater than in the double salt_. As this is a point of some importance, it will be well, perhaps, to make it clearer by giving one of the isothermal curves, _e.g._ the curve for 10°, which is represented diagrammatically in Fig. 114. E and F here represent solutions saturated for carnallite plus magnesium chloride hydrate, and for carnallite plus potassium chloride. As is evident, the point F lies above the line representing equimolecular proportions of the salts (OD). [Illustration: FIG. 114.] Summary and Numerical Data.--We may now sum up the different systems which can be formed, and give the numerical data from which the model is constructed.[366] I. _Bivariant Systems._