CRYSTALLOGRAPHY (from the Gr. Kpbo TaXXO ,

See also:

• ICE (a word common to Teutonic languages; cf. Ger. Eis)

ice, and -ypachew, to write)  , the science of the forms, properties and structure of crystals . Homogeneous solid matter, the physical and chemical properties of which are the same about every point, may be either amorphous or crystalline . In amorphous matter all the properties are the same in every direction in the mass; but in crystalline matter certain of the physical properties vary with the direction . The essential properties of crystalline matter are of two kinds, viz. the general properties, such as density, specific heat, melting-point and chemical composition, which do not vary with the direction; and the directional properties, such as cohesion and elasticity, various optical, thermal and electrical properties, as well as external form . By reason of the homogeneity of crystalline matter the directional properties are the same in all parallel directions in the mass, and there may be a certain symmetrical repetition of the directions along which the properties are the same . When the crystallization of matter takes place under conditions free from outside influences the peculiarities of internal structure are expressed in the external form of the mass, and there results a solid body bounded by plane surfaces intersecting in straight edges, the directions of which bear an intimate relation to the internal structure . Such a polyhedron Grows, many, espa, base or face) is known as a crystal . An example ofthis is sugar-candy; of which a single isolated crystal may have grown freely in a solution of sugar . Matter presenting well-defined and regular crystal forms, either as a single crystal or as a group of individual crystals, is said to be crystallized . If, on the other hand, crystallization has taken place about several centres in a confined space, the development of plane surfaces may be prevented, and a crystalline aggregate of differently orientated crystal-individuals results . Examples of this are afforded by loaf sugar and statuary marble . After a brief historical sketch, the more salient principles of the subject will be discussed under the following sections: I .

CRYSTALLINE FORM . (a) Symmetry of Crystals . (b)

Simple Forms and Combinations of Forms . (c) Law of Rational Indices . (d) Zones . (e) Projection and Drawing of Crystals . (f) Crystal Systems and Classes . i . Cubic System . 2 . Tetragonal System . 3 .

Orthorhombic System . 4 .

Monoclinic System . 5 . Anorthic System . 6 . Hexagonal System Regular Grouping of Crystals (Twinning, &c.) . Irregularities of Growth of Crystals: Characters of Faces . (x) Theories of Crystal Structure . II . PHYSICAL PROPERTIES OF CRYSTALS . (a) Elasticity and Cohesion (Cleavage, Etching, &c.) .

(b) Optical Properties (Interference figures, Pleochroism, &c.) . (c) Thermal Properties . (d) Magnetic and Electrical Properties .