CRYS'TALLOG'RAPHY (from Gk. p irranoc, krgsta/los, crystal ypdoetv, praphein, to write). The science which treats of crystals. A crystal is a portion of inorganic matter which has a definite molecular structure, and an out ward form hounded by plane surfaces called crys .tal faces. These bees are formed during the growth of the crystal, and have directions de pendent upon the structure of the molecule of the substance. Bence, only elementary chemical substances and definite chemical compounds form crystals. The molecular structure by which crys tal shapes are conditioned is not supposed, how ever, to be that of the chemical molecule, but a molecular grouping of a larger order involving a number of such chemical molecules. Crystals are formed either when a molten mass solidifies by cooling, or when the amount of a substance dissolved exceeds in quantity the amount which the solvent can retain in solution under the con ditions obtaining. Hence, when a solution is evaporated until supersaturated, crystals of the dissolved substance are thrown down. Solutions show, however, considerable inertness, and it is often necessary to introduce some solid sub stance—best of all, a crystal of the substance— into the solution, in order to start the process of crystallization. Exceptionally, crystals form
directly from vapors, as in the cases of iodine and chloride of ammonia.
A substance which never forms crystals is said to be amorphous. A substance which pos sesses the regular molecular structure character istic of crystals without the development of crystal faces is said to be crystalline. This con dition often exists because crystals are crowded by their neighbors. The clearest proof that the regular structure is present, even when the faces are not developed, is furnished by an examina tion of the physical properties of the substance, for in a crystalline substance these have different values (or coefficients) for the different direc tions, and these values are in accord with the symmetry of the crystal faces when they are allowed to develop. For example, a sphere cut from a crystal of quartz does not, when heated, remain spherical, as would a piece of amorphous glass, but becomes distorted into a spheroid. This is due to the fact that the coefficient of ex pansion of the quartz is different in different directions, but is distributed with a symmetry in accord with, though somewhat different from, that of the crystal's shape.