The access of the air has an important influence on this process. If a saturated solution of salt, when hot, be put into a vessel from which the air is excluded, it does not crystallize even when cold. But if the air be admitted, the crystallization immediately commences, and proceeds with rapidity. It has been shewn by Dr. Higgins, that any pressure, equivalent to that of the atmosphere, as the pressure of a column of mercury, has the same ef fect.
During crystallization a quantity of heat is rendered sensible. In many cases the volume of the substance crystallizing is enlarged, as in the example of water, of iron, and of the greater number of salts ; but in others the volume is diminished. Quicksilver, in congealing, contracts about one twenty-third of its whole bulk, yet it exhibits the crystalline texture; and when the congelation is partial, the crys talline figure can even be discovered.
Crystals deposited from water always contain a part of it, which is retained by the affinity of the solid, and has passed with it into the concrete form. It is term ed water of crystallization. Its quantity is very various ; sometimes it equals or ex ceeds the weight of the solid, and some times it amounts only to a few parts in the hundred. Much of.the cold produced du ring.the solution of salts in water is owing to this water of crystallization passing in to the fluid state : hence crystallized salts generally produce more cold than when they are uncrystallized.' If the water of crystallization be expelled from a crystal, it loses its transparency, and at length its form. Crystals which part with their water of crystallization when exposed to the atmosphere are said to effloresce, and to deliquesce when they attract water and become humid.
Some substances have so strong an affi nity for the fluids in which they are dis solved, or so little tendency for cohesion, that they do not crystallize. In some cases their crystallization may be effected by adding to the solution a substance exert ing an affinity to the fluid, and of course weakening its affinity for the solid it dis solved.
As different bodies require very differ ent quantities of water for their solution, it is possible, when two such bodies are dissolved in one fluid, to obtain them separate by crystallization, the one which is least soluble, or most disposed to crys tallize, first passing into the solid form ; and by farther evaporation the other is obtained. A fact on this subject, some what singular, is noticed by Mr. Kir wan. If into a saturated solution of two salts in water, a crystal of either be put, that salt crystallizes in preference to the other.
By crystallization, also, salts, the solu bility of which is unequally promoted by heat, may be obtained separately from the same solution. Thus, if one salt be much more soluble in hot than in cold water, and another be equally soluble, or nearly so, at any temperature, on eva porating the solution sufficiently, the lat ter salt will crystallize while the solution is hot ; on cooling, the other will shoot into crystals ; and by alternate evapora tion and cooling, the two may be obtain ed uncombined, though generally with a little intermixture of each other.
Sometimes, however, when two salts are in solution in the same fluid, and have even different tendencies to crys tallization, their mutual affinity leads them to crystallize in one mass, and even to assume a form different from that in which separately they would have crystallized.
In other cases this mutual affinity, be tween substances in solution, is sufficient to resist their crystallization, or to render it more difficult.
Crystallization sometimes takes place, when bodies in the gaseous form become subject to the attraction of aggregation, as in sublimates ; and even solids sepa rated from a liquid by chemical action, in some instances at the moment of their separation, assume a crystallized form.
Every substance in crystallizing is dis posed to assume a particular figure. Thus, sea-salt crystallizes in the form of a cube ; nitre in that of a hexaedral prism; sugar in that of a four or six sided prism, with triedral terminations. The crystalline figure in any substance, however, is not invariable, but may be altered by circumstances affecting the crystallization : and we find the same substance crystallized under a variety of forms. Sea-salt crystallizes, not only in cubes, but also in octaedrons ; and car bonate of lime is found in nature in the form of an hexaedral prism, an hexaedral and a triedral pyramid.
The effect of light upon the act of crystallization is very remarkable. It is found in general, that the crystals of salts are larger and better formed in the dark than when light falls upon the solution. But this relates only to such crystals as are formed in the fluid. In many, and in deed most salts, there are crystals form ed, during the spontaneous evaporation of the solution, which rise above the sur face into the air, either in contact with the sides of the vessel, or supported by their own structure. This phenomenon is very striking and curious, and it ap pears to have been well determined, be experiments of Chaptal and others, that it does not take place without the presence of light. See VEGETATION or Slams.