-------------------------------------------------- Heavier alloy. | Lighter alloy. -------------------------------------------------- Percentage amount of | Percentage amount of Silver. | Lead. | Zinc. | Silver. | Lead. | Zinc. -------------------------------------------------- 1.25 | 96.69 | 2.06 | 38.91 | 3.12 | 57.97 1.71 | 96.43 | 1.86 | 45.01 | 3.37 | 51.62 5.55 | 93.16 | 1.29 | 54.93 | 4.21 | 40.86 -------------------------------------------------- The numbers in the same horizontal row give the composition of the conjugate alloys, and it is evident that the upper layer consists almost entirely of silver and zinc. On allowing the mixture to cool slightly, the upper layer solidifies first, and can be separated from the still molten lead layer. It is on this behaviour of silver towards a mixture of molten lead and zinc that the Parkes's method for the desilverization of lead depends.[325] If aluminium is also added, a still larger proportion of silver passes into the lighter layer, and the desilverization of the lead is more complete.[326] [Illustration: FIG. 86.] [Illustration: FIG. 87.] The Influence of Temperature.--As has already been said, a ternary system existing in three phases possesses two degrees of freedom; and the state of the system is therefore dependent not only on the relative concentration of the components, but also on the temperature. As the temperature changes, therefore, the boundary curve of the heterogeneous system will also alter; and in order to represent this alteration we shall make use of the right prism, in which the temperature is measured upwards. In this way the boundary curve passes into a boundary surface (called a dineric surface), as shown in Fig. 86. In this figure the curve _akb_ is the isothermal for the ternary system; the curve _a_K_b_ shows the change in the _binary_ system AB with the temperature, with {248} a critical point at K. This curve has the same meaning as those given in Chapter VI. The curve _k_K is a critical curve joining together the critical points of the different isothermals. In such a case as is shown in Fig. 86, there does not exist any real critical temperature for the ternary system, for as the temperature is raised, the amount of C in the "critical" solution becomes less and less, and at K only two components, A and B, are present. In the case, however, represented in Fig. 87, a real ternary critical point is found. In this figure _ak'b_ is an isothermal, _ak"_ is the curve for the binary system, and K is the ternary critical point. All points outside the helmet-shaped boundary surface represent homogeneous ternary solutions, while all points within the surface belong to heterogeneous systems. Above the temperature of the point K, the three components are miscible in all proportions. An example of a ternary system yielding such a boundary surface is that consisting of phenol, water, and acetone.[327] In this case