Apart from their hardness, the chief mechanical property of metals is that of neither fracturing nor cleaving when over stressed, but deforming by the slipping of atoms along the glide planes, with consequent increase in hardness. This enables metals to be successfully worked, rolled, drawn, etc. Pure metals form glide planes very easily, but the presence of another metal in solid solution interferes with the regularity of the lattice and makes gliding much more difficult, thus increasing hardness. The effect of alloying is greater, the more highly charged the alloying ion. Carbon has more effect, for instance, on iron than the same amount of cobalt. A piece of metal when cast has its small crystals of which it is composed oriented at random towards each other.
On rolling or drawing, the crys tals are not only elongated but they tend to take up positions with some crystallographic axis in the direction of the rolling.
This is shown on a Debye dia gram by the splitting up of the uniform rings into patches indi cative of these preferred direc tions. On heating a rolled or a drawn metal, it recrystallises and the random orientation of the particles is restored. But at the same time, the hardness is lost.
The explanation of these pro cesses of hardening and annealing is being energetically sought, largely by X-ray methods, and already this has led to important technical results. It is clear that the practical importance of X-ray
analysis has only begun to be felt and will, in time, prove to be of immense significance.
BIBLIOGRAPHY.--General: Sir W. H. Bragg and W. L. Bragg, X Rays and Crystal Structure (1915, 4th ed. 1924) ; Sir W. H. Bragg, An Introduction to Crystal Analysis (1928) ; R. W. Wyckoff, The Struc ture of Crystals (1924) ; F. Rinne, Crystals and the Fine Structure of Matter (trans., 1924) ; P. P. Ewald, Krystalle and Rontgenstrahlen (1923) ; H. Mark, Die Verwendung der Rontgenstrahlen in Chemie and Technik ; Maugin, La Structure des Crystaux (1924). Structure Theory: H. Hilton, Mathematical Crystallography (Oxford, 1903) ; Schoenflies, Theorie der Krystallstructur (Leipzig, 1925) ; P. Niggli, Geometrisches Krystallographie des Disco-ntinuums (Leipzig, 1919) ; Structurtheoretische Grundbegrifie; Wiener Harms, Handbuch der Experimentalische Physik, Teil VII. (I), 1928. Space Group Tables: R. W. Wyckoff, Analytical Theory of Space Groups (Washington, 1922) ; W. T. Astbury and K. Yardley, Philosophical Transactions of the Royal Society, V. 224, p. 221. Crystal Physics: Born, Atom