In studying the fusion and solidification of those substances which exhibit the relationships of dynamic isomerism, the phenomena observed will vary somewhat according as the reversible transformation of the one form into the other takes place with measurable velocity at temperatures in the neighbourhood of the melting points, or only at some higher temperature. If the transformation is very rapid, the system will behave like a one-component system, but if the isomeric change is comparatively slow, the behaviour will be that of a two-component system. Temperature-Concentration Diagram.--The relationships which are met with here will be most readily understood with {197} the help of Fig. 59. Suppose, in the first instance, that isomeric transformation does not take place at the temperature of the melting point, then the freezing point curve will have the simple form ACB; the formation of compounds being for the present excluded. This is the simplest type of curve, and gives the composition of the solutions in equilibrium with the one modification ([alpha] modification) at different temperatures (curve AC); and of the solutions in equilibrium with the other modification ([beta] modification) at different temperatures (curve BC). C is the eutectic point at which the two solid isomerides can exist side by side in contact with the solution. [Illustration: FIG. 59.] Now, suppose that isomeric transformation takes place with measurable velocity. If the pure [alpha]-modification is heated to a temperature _t'_ above its melting point, and the liquid maintained at that temperature until equilibrium has been established, a certain amount of the [beta]-form will be present in the liquid, the composition of which will be represented by the point _x'_. The same condition of equilibrium will also be reached by starting with pure [beta]. Similarly, if the temperature of the liquid is maintained at the temperature _t"_, equilibrium will be reached, we shall suppose, when the solution has the composition _x"_. The curve DE, therefore, which passes through all the different values of _x_ corresponding to different values of _t_, will represent the change of equilibrium with the temperature. It will slope to the right (as in the figure) if the transformation of [alpha] into [beta] is accompanied by absorption of heat; to the left if the transformation is accompanied by evolution of heat, in accordance with van't Hoff's Law of movable equilibrium. If transformation occurs without heat effect, the equilibrium will be independent of the {198} temperature, and the equilibrium curve DE will therefore be perpendicular and parallel to the temperature axis.