In this connection, also, one other point may be emphasised. For the production of the equilibrium between a solid and a liquid, time is necessary, and this time not only varies with the state of division of the solid and the efficiency of the stirring, but is also dependent on the nature of the substance.[182] Considerable care must therefore be taken that sufficient time is allowed for equilibrium to be established. Such care is more especially needful when changes may occur in the solid phase, and neglect of it has greatly diminished the value of many of the older determinations of solubility. Form of the Solubility Curve.--The solubility curve--that {109} is, the curve representing the change of concentration of the components in the solution with the temperature--differs markedly from the curve of vapour pressure (p. 63), in that it possesses no general form, but may vary in the most diverse manner. Not only may the curve have an almost straight and horizontal course, or slope or curve upwards at varying angles; but it may even slope downwards, corresponding to a decrease in the solubility with rise of temperature; may exhibit maxima or minima of solubility, or may, as in the case of some hydrated salts, pass through a point of maximum temperature. In the latter case the salt may possess two values of solubility at the same temperature. We shall consider these cases in the following chapter. [Illustration: FIG. 26.] The great variety of form shown by solubility curves is at once apparent from Fig. 26, in which the solubility curves of various substances (not, however, drawn to scale) are reproduced.[183] Varied as is the form of the solubility curve, its _direction_, nevertheless, can be predicted by means of the theorem of van't Hoff and Le Chatelier; for in accordance with that theorem (p. 57) increase of solubility with the temperature must occur in those cases where the process of solution is accompanied by an _absorption_ of heat; and a decrease in the solubility with rise of temperature will be found in cases where solution occurs with _evolution_ of heat. Where there is no heat effect accompanying solution, {110} change of temperature will be without influence on the solubility; and if the sign of the heat of solution changes, the direction of the solubility curve must also change, _i.e._ must show a maximum or minimum point. This has in all cases been verified by experiment.[184]