The curves representing univariant systems are of two kinds. In the one case, the two solid phases present are both binary compounds; or one is a binary compound and the other is one of the components. In the other case, either one or both solid phases are ternary compounds. Curves belonging {294} to the former class (so-called _border curves_) start from binary eutectic points, and their course is always towards lower temperatures, _e.g._ CL, EM, GN, IO. Curves belonging to the latter class (so-called _medial curves_) would, in a triangular diagram, lie entirely within the triangle. Such curves are YV, WV, VL, LM, MV, NS, ST, SO, OZ. These curves do not always run from higher to lower temperatures, but may even exhibit a point of maximum temperature. Such maxima are found, for example, at U (Fig. 116), and also on the curves ST and LV. Finally, whereas all the other ternary univariant curves run in valleys between the adjoining surfaces, we find at the point X a similar appearance to that found in the case of carnallite, as the univariant curve here rises above the surrounding surface. The point X, therefore, does not correspond with a eutectic point, but with a transition point. At this point the ternary compound 2FeCl_{3},2HCl,12H_{2}O melts with separation of 2FeCl_{3},12H_{2}O, just as carnallite melts at 168° with separation of potassium chloride. The Isothermal Curves.--A deeper insight into the behaviour of the system FeCl_{3}--HCl--H_{2}O is obtained from a study of the isothermal curves, the complete series of which, so far as they have been studied, is given in Fig. 117.[369] In this figure the lightly drawn curves represent isothermal solubility curves, the particular temperature being printed beside the curve.[370] The dark lines give the composition of the univariant systems at different temperatures. The point of intersection of a dark with a light curve gives the composition of the univariant solution at the temperature represented by the light curve; and the point of intersection of two dark lines gives the composition of the invariant solution in equilibrium with three solid phases. The dotted lines represent metastable systems, and the points P, Q, and R represent solutions of {295} the composition of the ternary salts, 2FeCl_{3},2HCl,4H_{2}O; 2FeCl_{3},2HCl,8H_{2}O; and 2FeCl_{3},2HCl,12H_{2}O. [Illustration: FIG. 117.]