solution, only when the latter has a definite composition. Lastly, the ternary eutectic point, K, represents an invariant system; three solid phases can exist in equilibrium with a ternary solution, only when the latter has one fixed composition and when the temperature has a definite value. This eutectic point, therefore, {255} has a perfectly definite position, depending only on the nature of the three components. Instead of employing the prism, the change in the composition of the ternary solutions can also be indicated by means of the _projections_ of the curves _k__{1}K, _k__{2}K, and _k__{3}K on the base of the prism, the particular temperature being written beside the different eutectic points and curves. This is shown in Fig. 96. [Illustration: FIG. 96.] The numbers which are given in this diagram refer to the eutectic points for the system bismuth--lead--tin, the data for which are as follows:--[331] -------------------------------------------------------------------- Melting point of | Percentage composition of | Temperature of binary pure metal. | binary eutectic mixture. | eutectic point. -------------------------------------------------------------------- | Bi Pb Sn | Bismuth, 268° | 55 45 -- | Bi--Pb, 127° Lead, 325° | 58 -- 42 | Bi--Sn, 133° Tin, 232° | -- 37 63 | Pb--Sn, 182° -------------------------------------------------------------------- -------------------------------------------------- Percentage composition of | Temperature of ternary ternary eutectic mixture. | eutectic point. -------------------------------------------------- Bi Pb Sn | 52 32 16 | 96° -------------------------------------------------- Formation of Compounds.--In the case just discussed, the components crystallized out from solution in the pure state. If, however, combination can take place between two of the components, the relationships will be somewhat different; the curves which are obtained in such a case being represented in Fig. 97. From the figure, we see that the two components B {256} and C form a compound, and the freezing point curve of the binary system has therefore the form shown in Fig. 64 (p. 209). Further, there are two _ternary_ eutectic points, K_{1} and K_{2}, the solid phases present being A, B, and compound, and A, C, and compound respectively. [Illustration: FIG. 97.]