Online Encyclopedia

CRYSTALLOGRAPHY (from the Gr. Kpbo Ta...

Online Encyclopedia
Originally appearing in Volume V07, Page 569 of the 1911 Encyclopedia Britannica.
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CRYSTALLOGRAPHY (from the Gr. Kpbo TaXXO , 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. (h)
End of Article: CRYSTALLOGRAPHY (from the Gr. Kpbo TaXXO , ice, and -ypachew, to write)
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ANTON CSENGERY (1822-1880)

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