By X-ray spectrometry we can not only determine the exact manner in which the atoms of a given type of crystal are arranged in space, but also identify the presence of a particular type of crystal in an alloy or mixture. For the latter purpose, however, the method is much less sensi tive than microscopy or the other methods already mentioned, but has, on the other hand, the great advantage of affording a means of definitely identifying crystals of a particular type. Much more important, however, than its use in connection with equilibrium diagrams, is the application of X-ray crystal analysis to the study of the inner structure of metallic crystals. Not only has this method of examination confirmed the earlier conclusions derived from microscopic evidence, that metals are essentially crystalline aggregates, but it has also shown that they remain crystalline even after the most severe plastic deformation. The proof, however, only shows that a good deal of crystalline metal is present even in severely deformed material, but does not show that the metal is entirely or even mainly crystalline in that condi tion. (Consult articles on "Strength of Materials" and "Fatigue.") When a previously polished and etched specimen of a pure or nearly pure metal is plastically deformed, the surfaces of the crystals seen in the section become cross-hatched with fine black lines. These lines, known as "slip bands," are illustrated in fig.
9, Plate 1. It has been shown that these fine lines are in reality little steps in the surface which have been produced by minute slips occurring on some of the crystal planes. It is by successive slips of this kind, occurring within each of the crystals of a piece of metal, that the material takes up the new shape imposed upon it when it undergoes plastic deformation, as—for instance—when it is stretched when drawn out into wire or flattened during rolling. As a result of the slipping process, the individual crystals become elongated in the direction in which the metal has been extended. On subsequent heating to a sufficiently high temperature (without melting, however) the metal "recrystallises" and the .crystals resume their normal shape of approximately equal length in all directions. In some of the softer metals, such as lead and cad mium, the recrystallisation process occurs gradually at ordinary temperatures but in iron and steel there is neither "crystallisation" nor "recrystallisation" at room temperatures, whether the metal be at rest or exposed to violent vibration. (W. RN.)