11. In the formation of solid bodies, one of the most remarkable phenomena is that of crystallization, which consists of the regular form which certain substances appropriate in their passage from the liquid or gaseous state to a solid condition. This form, which varies according to the nature of the substance, is generally that of geometrical solids, such as the prism, the rhomboid, cube, and pyramid. Everyone has seen the crystals of sugar candy, — rock crystals, or crystallized silica, which are prisms with six sides terminated by a pyramid equally hexagonal. The diamond is pure carbon, or crystallized coal. The designs which are produced upon window-panes in winter are due to the crystallization of the vapor from water under the form of prismatic needles.
The regular disposition of the crystals belongs to the particular form of the molecules of each body. These infinitely small particles occupy, nevertheless, a certain space, have been drawn toward one another by molecular attraction; they are arranged and in juxtaposition to one another, according to the exigency of form, in such a way that each one takes its place around the nucleus, or first center of attraction, and forms a symmetrical whole.
Crystallization only operates under the empire of certain favorable circumstances, without which it cannot take place. A right degree of temperature with repose is an essential condition. Too much heat, keeping the molecules separated, would prevent condensation; and, as agitation is opposed to their symmetrical arrangement, they would form only a confused and irregular mass under its influence, which is consequently not crystallization in the true sense of the word.