Sect. l.J
CRYSTALLIZATION AND CRYSTALS.
5
18. Crystallization, in the most limited extent of the term, is thatprocess, by which the particles of bodies unite in such manner, as toproduce determinate and regular solids. But it is equally true, thatthose minerals, which possess a foliated or fibrous structure, are theproducts of crystallization under circumstances, which have renderedthe process more or less imperfect, and prevented the appearance ofdistinct and regular forms.
Let a quantity of muriate of soda (common salt) be dissolved inwater, and permit the solution to evaporate by a moderate heat; theparticles of the salt will separate from the water, unite, and form lit-tle cubes, which float on the surface, till their increased weight causesthem to fall through the liquid. These cubes are called crystals. Oth-er substances, when permitted to crystallize, also exhibit regular sol-ids, but of a different form. Thus the emerald presents the form of ahexaedral prism, and the garnet that of a dodecaedron with rhombicfaces.
19. The ancients believed crystallized quartz (rock crystal) to be
water, congealed by exposure to intense cold ; and accordingly appli-ed to it the term which signified ice. Hence the etymol-
ogy of the word crystal. Now, as a beautiful regularity of form is oneof the most striking properties of crystallized quartz, t’ e name crys-tal has been extended to all mineral and other inorganic substances,which exhibit themselves under the form of regular, geometrical solids.
20. A crystal may therefore be defined an inorganic body, which,by the operation of affinity, has assumed the form of a regular solid,terminated by a certain number of plane and polished faces. The cor-responding faces of all crystals, which possess the same variety ofform and belong to the same substance, are inclined to each other inangles of a constant quantity. This constancy of angles remains evenin those cases, where the faces themselves, from some accidental caus-es, have changed their dimensions or number of sides. Transparen-cy, though many crystals possess it in a greater or less degree, is nota necessary property. But plane surfaces, bounded by right lines, areso essential to the crystalline form, that their absence decidedly indi-cates imperfection in the process of crystallization. The lustre andsmoothness of the faces may also be diminished by accidental causes.
21. The property of crystallizing is by no means confined to asmall number of bodies. Nearly all the different species of simpleminerals, and some inorganic bodies of vegetable and even animal or-igin, such as sugar, camphor, and spermaceti have been seen in a crys-tallized state. Most of the aforementioned substances are alsocapa-