Crystallization
The particles of many substances, when loosened either by melting, solution, or otherwise, so as to permit freedom of motion, tend to arrange themselves in regular geometrical forms, which are termed crystals. The name crystalloids is given to all substances which have this marked tendency, and to such as do not crystallize the name colloid, or glue-like, is given.
Crystalloids are represented by such substances as water, metals, acids, sugar, etc.; while albumen, jellies, etc., are examples of colloids. Crystalloids incline to take compact, angular forms, while the colloids assume rounded outlines, and are of a soft, gelatinous, and yielding nature. The former bodies predominate in the inorganic world, the latter in the organic. It is found that almost all bodies when cooled take the crystalline form, though this may not be perceptible at first. The spaces left between the crystals which form are completely filled up by the portions which solidify afterwards, so that fracture reveals only a general crystalline structure, just as we find it in cast iron, zinc, etc., when broken. Common glass, wrought iron, flint, glue, etc., are amorphous bodies, without any form of crystalline structure; these all fracture in any form or direction, are not so hard, and are more soluble than are substances of a crystalline nature. Crystalline carbon is represented in the diamond, while amorphous carbon finds its examples in common lamp-black and charcoal. A crystal is a piece of matter that by the action of molecular forces has assumed a definite geometrical form of some kind, with plain faces.
Some metals as soon as cast are tenacious and uncrystal-line, but become brittle, with traces of crystallization, when heated and cooled repeatedly. Small as the amount of freedom given to the particles by heating and cooling, yet it seems sufficient to determine a tendency towards the crystalline condition. By continuous hammering, metals are changed from a ductile to a brittle, crystalline state, as is proved by the workers in copper, who must frequently anneal the metal as they hammer it into shape; otherwise it would become so brittle that it would fly into fragments.
Owing to the fact that crystallization does take place, even in solids, if the particles are left free, bells after a time alter in tone; cannons, from constant firing, lose their strength; and thus, by the constant jar and vibrations, as well as hard blows, chains, slings, etc., made from wrought iron gradually change from the tough and fibrous into the crystalline, which weakens them and increases their liability to fracture. See AMORPHOUS; STEEL.