. Discovery. Science. DISCOVERY A MONTHLY POPULAR JOURNAL or KNOWTLLDGE No. 2. FEBRUARY 1920. PRICE «d. NET. DISCOVERY. A Monthly Popular Journal of Know- ledge. Edited by A. S. Russell, , , S Severn Road, Sheffield, to whom all Editorial Communications should be addressed. Published by John Murray, 50.^ Albemarle Street, London, \V. i, to whom all Business Communications should be addressed. Advertisement Office : 16 Regent Street, London, Single Annual Subscription 65., or post free 7s. 6d, numbers 6d. net; post free j^d. Crystal Structure By W. L. Bragg, , Nobel Laur
. Discovery. Science. DISCOVERY A MONTHLY POPULAR JOURNAL or KNOWTLLDGE No. 2. FEBRUARY 1920. PRICE «d. NET. DISCOVERY. A Monthly Popular Journal of Know- ledge. Edited by A. S. Russell, , , S Severn Road, Sheffield, to whom all Editorial Communications should be addressed. Published by John Murray, 50.^ Albemarle Street, London, \V. i, to whom all Business Communications should be addressed. Advertisement Office : 16 Regent Street, London, Single Annual Subscription 65., or post free 7s. 6d, numbers 6d. net; post free j^d. Crystal Structure By W. L. Bragg, , Nobel : Langioorlhg Professor 0/ Physics in the Universily 0/ \Ianchester When a liquid solidifies on cooling, or when a solid is deposited from a solution, the attractions between the molecules cause them to pack themselves together into a rigid structure. This can happen in two ways: the molecules may come together in any position, or they may arrange themselves in a regular pattern. In the first case the solid is said to be amorphous; in the second case it is called crystalline. The amorphous and crystalline states may be compared to a number of men forming a crowd, and to the same number drawn up in military' formation. In the great majority of cases, the molecules, when forming a solid, arrange themselves in a regular crystalline structure, and it is only under exceptional circumstances that amorphous solids are produced. The mineral constituents of the earth's crust are almost entirely crj'stalline: metals are composed of a mass of small crystals, solutions deposit the dissolved body as crj'stals. Ice consists of a mass of w-ater crystals, frost of the same crystals deposited by the condensation of vapour. When the molecules are given time, as it were, to arrange themselves regular!}', there is always a tendency to form a crystalline structure. Glass, on the other hand, is a typical example of a class of solids called " vitreous" which show no evidence of crj-sta
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