the critical temperature K is 92°, and the composition at this ternary critical point is-- Water 59 per cent. Acetone 12 " Phenol 29 " [Illustration: FIG. 88.] The difference between the two classes of systems just mentioned, is seen very clearly by a glance at the Figs. 88 and 89, which show the projection of the isothermals on the base of the prism. In Fig. 88, the projections yield paraboloid curves, the two branches of which are cut by one side of the triangle; and the critical point is represented by a point on {249} this side. In the second case (Fig. 89), however, the projections of the isothermals form ellipsoidal curves surrounding the supreme critical point, which now lies _inside the triangle_. At lower temperatures, these isothermal boundary curves are cut by a side of the triangle; at the critical temperature, _k"_, of the binary system AB, the boundary curve _touches_ the side AB, while at still higher temperatures the boundary curve comes to lie entirely within the triangle. At any given temperature, therefore, between the critical point of the binary system (_k"_), and the supreme critical point of the ternary system (K), each pair of the three components are miscible with one another in all proportions; for the region of heterogeneous systems is now bounded by a closed curve lying entirely within the triangle. Outside this curve only homogeneous systems are found. Binary mixtures, therefore, represented by any point on one of the sides of the triangle must be homogeneous, for they all lie outside the boundary curve for heterogeneous states. [Illustration: FIG. 89.] 2. _The three components can form two pairs of partially miscible liquids._ In the case of the three components water, alcohol, and succinic nitrile, water and alcohol are miscible in all proportions, but not so water and succinic nitrile, or alcohol and succinic nitrile. [Illustration: FIG. 90.] [Illustration: FIG. 91.]