For most cases of practical importance, liquid metal alloys occur as a single homogeneous liquid phase. To simplify the present discussion, we shall treat a special case—a pure metallic element with only a single solid phase. Figure 14.1 is a schematic phase diagram of a single-component system. From what we have previously learned, we know that the solid is a crystalline phase in which the atoms are aligned in space in definite patterns over long distances. The regularity of crystal lattices makes it easy to study their structures with the aid of X-ray diffraction and electron microscopy; therefore, a great deal is known about the internal arrangements of atoms in metal crystals. At the same time, the uniformity of the structure of crystals makes it possible to employ mathematics in the study of their properties. On the other hand, the gas phase represents the other extreme from the solid phase where the structure is one of almost complete randomness, or disorder, instead of almost complete order. Here, in most cases, the atoms can be assumed to be removed far enough from each other that metallic gases can be treated as ideal gases. The physical properties of metallic gases, like those of metallic solids, are therefore capable of mathematical analysis. While the solid crystalline phase is pictured as a completely ordered arrangement of atoms (neglecting defects such as dislocations and vacancies) and the gas phase as a state of random disorder (ideal gas), no simple picture has as yet been devised to represent the structure of the liquid phase. The principal trouble is the difficulty of the problem. The liquid phase possesses neither the long-range order of the solid nor the lack of interaction between atoms characteristic of the gas phase. It is, therefore, essentially an indeterminant structure. Actually, in a liquid, the average separation between atoms is very close to that in the solid. This fact is shown by the small change in density on melting, which for closepacked metals amounts to 2 to 6 percent only, where part of this density change is probably associated with the formation of additional structural defects in the liquid phase. A compilation of physical properties of pure metals at elevated temperatures and those of liquid metals can be found in Reference 1. Further, the latent heat of fusion released