The length of time during which the temperature remains constant at the point _c_, depends, of course, on the eutectic solution. If, therefore, we take equal amounts of solution having a different initial composition, the period of constant temperature in the cooling curve will evidently be greatest in the case of the solution having the composition of the eutectic point; and the period will become less and less as we increase the amount of one of the components. The relationship between initial composition of solution and the duration of constant temperature at the eutectic point is represented by the curve _a'c'b'_ (Fig. 77). When a compound possessing a definite melting point is formed, it behaves as a pure substance. If, therefore, the initial composition of the {231} solution is the same as that of the compound, no eutectic solution will be obtained; and therefore no line of constant temperature, such as _cd_ (Fig. 76). In such a case, if we represent graphically the relation between the initial composition of the solution and the duration of constant temperature, a diagram is obtained such as shown in Fig. 78. The two maxima on the time-composition curve represent eutectic points, and the minima, _a'_, _b'_, _e'_, pure substances. The position of _e'_ gives the composition of the compound. When a series of compounds is formed, then for each compound a minimum is found on the time-composition curve. [Illustration: FIG. 79.] If the compound formed has no definite melting point, the diagram obtained is like that shown in Fig. 79. If we start with a solution, the composition of which is represented by a point between _d_ and _b_, then, on cooling, _b_ will separate out first, and the temperature will fall until the point _d_ is reached. The temperature then remains constant until the component _b_, which has separated out, is converted into the compound. After this the temperature again falls, until it again remains constant at the eutectic point c. In the case of the first halt, the period of constant temperature is greatest when the initial composition of the solution is the same as that of the compound; and it becomes shorter and shorter with {232} increase in the amount of either component. In this way we obtain the time-composition curve _b'e"d'_, of which the maximum point _e"_ gives the composition of the compound.