[Illustration: FIG. 74.] {223} A curve belonging to the same type, but more complicated, is obtained with gold and aluminium;[306] in this case, several compounds are formed, some of which have a definite melting point, while others exhibit only a transition point. The chief compound is AuAl_{2}, which has practically the same melting point as pure gold. 3. _The two metals form mixed crystals (solid solutions)._ The simplest case in which the metals crystallize out together is found in silver and gold.[307] The freezing-point curve in this case is an almost straight line joining the freezing points of the pure metals (_cf._ curve I., Fig. 65, p. 210). These two metals, therefore, can form an unbroken series of mixed crystals. In some cases, however, the two metals do not form an unbroken series of mixed crystals. In the case of zinc and silver,[308] for example, the addition of silver _raises_ the freezing point of the mixture, until a transition point is reached. This corresponds with curve IV., Fig. 65. Silver and copper, and gold and copper, on the other hand, do not form unbroken series of mixed crystals, but the freezing-point curve exhibits an eutectic point, as in curve V., Fig. 65. Not only may there be these three different types of curves, but there may also be combinations of these. Thus the two metals may not only form compounds, but one of the metals may not separate out in the pure state at all, but form mixed crystals. In this case the freezing point may rise (as in the case of silver and zinc), and one of the eutectic points will be absent. Iron-Carbon Alloys.--Of all the different binary alloys, probably the most important are those formed by iron and carbon: alloys consisting not of two metals, but of a metal and a non-metal. On account of the importance of these alloys, an attempt will be made to describe in brief some of the most important relationships met with. Before proceeding to discuss the applications of the Phase Rule to the study of the iron-carbon alloys, however, the main {224} facts with which we have to deal may be stated very briefly. With regard to the metal itself, it is known to exist in three different allotropic modifications, called [alpha]-, [beta]-, and [gamma]-ferrite respectively. Like the two modifications of sulphur and of tin, these different forms exhibit transition points at which the relative stability of the forms changes. Thus the transition point for [alpha]- and [beta]-ferrite is about 780°; and below this temperature the [alpha]- form, above it the [beta]- form is stable. For [beta]- and [gamma]-ferrite, the transition point is about 870°, the [gamma]- form being the stable modification above this temperature.