It will be evident, from what has gone before, that the degree of completeness with which the different curves can be realised will depend on the velocity with which isomeric change takes place, and on the rapidity with which the determinations of the freezing point can be carried out. As the two extremes we have, on the one hand, practically instantaneous transformation, and on the other, practically infinite slowness of transformation. In the former case, only one melting and freezing point will be found, viz. the natural freezing point; in the latter case, the two isomerides will behave as two perfectly independent components, and the equilibrium curve DE will not be realised. The diagram which is obtained when isomeric transformation does not occur within measurable time at the temperature of the melting point is somewhat different from that already given in Fig. 59. In this case, the two freezing point curves AC and BC (Fig. 60) can be readily realized, as no isomeric change occurs in the liquid phase. Suppose, however, that at a higher temperature, _t'_, reversible isomeric transformation can take place, the composition of the liquid phase will alter until at the point _x'_ a condition of equilibrium is reached; and the composition of the liquid at higher temperatures will be represented by the curve _x'_F. Below the temperature _t'_ the position of the equilibrium curve is hypothetical; but as the temperature {201} falls the velocity of transformation diminishes, and at last becomes _practically_ zero. The equilibrium curve can therefore be regarded as dividing into two branches _x'_G and _x'_H. At temperatures between G and _t'_ the [alpha] modification can undergo isomeric change leading to a point on the curve G_x'_; and the [beta] modification can undergo change leading to a point on the curve H_x'_. The same condition of equilibrium is therefore not reached from each side, and we are therefore dealing not with true but with false equilibrium (p. 5). Below the temperatures G and H, isomeric transformation does not occur in measurable time. We shall not, however, enter into a detailed discussion of the equilibria in such systems, more especially as they are not systems in true equilibrium, and as the temperature at which true equilibrium can be established with appreciable velocity alters under the influence of catalytic agents.[283] Examples of such systems will no doubt be found in the case of optically active substances, where both isomerides are apparently quite stable at the melting point. In the case of such substances, also, the action of catalytic agents in producing isomeric transformation (racemisation) is well known. [Illustration: FIG. 60.]