File:X rays and crystal structure (1915) (14591570707).jpg

Identifier: xrayscrystalstru00braguoft (find matches)
Title: X rays and crystal structure
Year: 1915 (1910s)
Authors: Bragg, William Henry, Sir, 1862-1942 Bragg, William Lawrence, Sir, 1890-1971
Subjects: Crystallography X-rays
Publisher: London : G. Bell
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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occurs at24°, the first (100) at 11°, the first (in) at 11.2°.Contrasted with this, we have for quartz the first(2TY) at y.S°, the first (lOo) at 10°, the first (m)at 18.5°. This is at once explained on referring ANALYSIS OF CRYSTALS 163 quartz to a hexagonal lattice, for then the prismfaces rank hig-h in importance, and we would expectto get the reflection from them at a small angle. The typical crystal of quartz consists of a six-sidedprism terminated, if perfect, by a six-sided pyramid
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Fig. 55. at each end. Some of the corners are generallymodified by small faces of other types (Fig. 53).We will suppose that the underlying lattice is suchthat the prism faces are parallel to EDde, etc., thepyramid faces to Ide. The length ID is called «,the height li, c Measurement of the quartzcrystal shows that a -.c—i : 1.10.With each point of the lattice there is associated a</3 volume c. If this lattice really underlies the 164 ANALYSIS OF CRYSTALS quartz structure, we must find the number of SiOoatoms associated with each point of it, in the waythat we have always done for other crystals. We will take the distance a, as determined bythe first spectrum of the prism face. We have 2 . -^^ , sin 3.9° = o.576 x lo^ ^ = 4.89X lo*^ cm. Therefore the mass contained in unit cell of thelattice (density of quartz = 2.65) is equal to 2.65 xrti^x I.I x^^ grams = 2.94X io~ grams. The mass of one molecule of SiO., is (28.3 + 32) X 1,64 X 10* grams = o.99 x 10 grams, and a comparision o

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