Crystallization is a species of precipita tion, in which the particles ofthe solvend, on separating from the solution, assume certain determinate forms. The condi tions necessary for crystallization are, that the integrant particles have a tendency to arrange themselves in a determinate manner, when acted on by the attraction of aggregation; that they be disaggre gated, at least so far as to possess suffi cient mobility to assume their peculiar arrangement; and that the causes dis aggregating them he slowly and gradually removed.
Notwithstanding the immense variety in the forms of crystals, AI. Hauy has rendered it probable that there are only three forms of the integrant particles ; the parallelopiped, the triangular prism, and the tetrahedron. But as these particles may unite in different ways, either by their faces or edges, they will compose crystals of various forms.
The primitive forms have been reduced to six; the parallelopiped, the regular tetrahedron, the octahedron with triangu lar faces, the six-sided prism, the dode cahedron terminated by rhombs, the do decahedron with isosceles triangular faces.
Almost all substances on crystallizing retain a portion of water combined with them, which is essential to their existence as crystals, and is therefore denominated water of crystallization. Its quantity varies very much in different crystallized sub stances.
The means by which the particles of bodies are disaggregated, so as to admit of crystallization, are solution, fusion, vaporization, or mechanical division and suspension in a fluid medium. The means by which the disaggregating causes are removed are evaporation, reduction of temperature, and rest.
When bodies are merely suspended in a state of extreme mechanical division, nothing but rest is necessary for their crystallization. When they are disaggre gated by fusion or vaporization, the regu larity of their crystals depends on the slowness with which' their temperature is reduced; for if cooled too quickly, their particles have no time to arrange them selves, and are converted at once into a confused or unvaried solid mass. Thus glass, which when cooled quickly is so perfectly uniform in its appearance, when cooled slowly has a crystalline texture. But in order to obtain crystals by means of fusion, it is often necessary, after the substance has begun to crystallize, to remove the part which remains fluid; for otherwise it would fill up the interstices among the crystals first formed, and give the whole the appearance of one solid mass. Thus, after a crust has formed on
the top of melted sulphur, by pouring off the still fluid part we obtain regular crystals.
The means by which bodies which have been disaggregated by solution are made to crystallize most regularly, vary according to the habitudes of the bodies with their solvents and caloric.
Some saline substances are much more soluble in hot than in cold water. There fore a boiling saturated solution of any of these will deposit, on cooling, the excess of salt, which it is unable to dissolve when cold. These salts commonly con tain much water of crystallization. Other salts are scarcely, if at all, more soluble in hot than in cold water; and, therefore, their solutions must be evaporated either by heat or spontaneously. These salts commonly contain little water of crystal lization. The beauty and size of the crystals depend upon the purity of the solution, its quantity, and the mode of conducting the evaporation and cooling.
When the salt is not more soluble in hot than in cold water, by means of gentle evaporation a succession of pel licles are formed on the top of the solu tion, which either are removed or per mitted to sink to the bottom by their own weight; and the evaporation is continued until the crystallization be completed. But when the salt is capable of crystalli zing on cooling, the evaporation is only continued until a drop of the solution, placed upon some cold body, shews a disposition to crystallize, or at furthest only until the first appearance of a pelli cle. The solution is then covered up, and set aside to cool, and the more slowly it cools the more regular are the crystals. The mother-water, or solution which re mains after the crystals are formed, may be repeatedly treated in the same way, as long as it is capable of furnishing any more salt.
When very large and beautiful crystals are wanted, they may be obtained by lay ing well-formed crystals in a saturated solution of the same salt, and turning them every day. In this way their size may be considerably increased, though not with out limitation, for after a certain time they grow smaller instead of larger.