If no compound is formed between the two components, {209} the general form of the equilibrium curve will be that of curve I. or II., Fig. 63. Type I. is the simplest form of curve found, and consists, as the diagram shows, of only two branches, AC and BC, meeting at the point C, _which lies below the melting point of either component_. The solid phase which is in equilibrium with the solutions AC is pure A; that in equilibrium with BC, pure B. C is the eutectic point. Although at the eutectic point the solution solidifies entirely without change of temperature, the solid which is deposited is not a homogeneous solid phase, but a mixture, or conglomerate of the two components. _The eutectic point, therefore, represents the melting or freezing point, not of a compound, but of a mixture_ (p. 119). Curve II., Fig. 63, is obtained when two liquid phases are formed. C is an eutectic point, D and F are transition points at which there can co-exist the four phases--solid, two liquid phases, vapour. DEF represents the change in the composition of the two liquid phases with rise of temperature; the curve might also have the reversed form with the critical solution point below the transition points D and F. [Illustration: FIG. 64.] In the second class of systems (Fig. 64), that in which combination between the components occurs, there are again two types according as the compound formed has a definite melting point (_i.e._ can exist in equilibrium with a solution of the same composition), or undergoes only partial fusion; that is, exhibits a transition point. If a compound possessing a definite melting point is formed, the equilibrium curve will have the general form shown by curve I., Fig. 64. A, B, and D are the melting points of pure A, pure B, and of the compound A_{x}B_{y} respectively. AC {210} is the freezing point curve of A in presence of B; BE that of B in presence of A; and DC and DE the freezing point curves of the compound in presence of a solution containing excess of one of the components. C and E are eutectic points at which mixtures of A and A_{x}B_{y}, or B and A_{x}B_{y} can co-exist in contact with solution. The curve CDE may be large or small, and the melting point of the compound, D, may lie above or below that of each of the components, or may have an intermediate position. If more than one compound can be formed, a series of curves similar to CDE will be obtained (_cf._ p. 152).