CRYSTALLIZATION. When the at traction of aggregation has been weak ened, either by the application of heat, or of a chemical affinity, and is suffered to resume its force more slowly or equally, the particles are not united indiscrimi nately, but in uniting assume a particular arrangement ; and thus form masses of regular figures, bounded by plain sur faces and determinate angles. When aggregation is exer'ed in this manner, and with this result, the operation is named crystallization, and the regularly figured masses are denominated crys tals.
Crystallization is of two kinds : first, as it takes place from the reduction of tem perature in a body which has had flui dity communicated to it by the operation of heat ; and, secondly, as it proceeds from the diminution of the solvent power of a fluid, which has communicated flui dity to a solid by having combined with it.
Of the first kind of crystallization, wa ter affords an example in passing into ice by a reduction of its temperature. At fillet long and slender spiculze tbrm in the fluid, and from these others shoot out at a cer tain angle, and this continues till the in terstices are filled with the crystals, and the whole becomes a solid transparent mass. We have also examples of it in the metals, which, when melted and cooled slowly, assume symmetrical forms. Some inflammables, as sulphur, crystallize in a similar way.
Of the second kind of crystallization, the principal examples are derived from the order of salts, and a few other solids, soluble in water ; and with regard to this, several facts of importance require to be stated.
The solution of a solid in a fluid is in almost every case increased by beat, which weakens cohesion : hence a larger quanti ty of the solid is kept in solution at a high than a low temperature. If, there fore, we prepare a solution of salt in hot water, the solution being saturated, or the fluid having dissolved as much of the salt as it can do, on allowing it to cool, the portion of the salt which the heat enabled the fluid to dissolve will separate ; and unless the cooling of the solution has been very rapid, the particles of the solid, in approaching to each other, will pass into those regular arrangements which consti tute crystals.
The same result will be obtained by withdrawing parts of the fluid in which the solid is dissolved. If this be done slowly, or by spontaneous evaporation, the particles will obey the law of attrac tion, which unites them in regular forms ; the crystals are in this way formed fre quently more regular, and of a larger size than by the former method of reducing the temperature of the solution: some can be crystallized only in this method.
In both cases the fluid in which the crystals form is still a saturated solution of the solid, and by a farther evaporation, joined sometimes with subsequent cooling, will again crystallize.
Ip general it holds true, that the slower the formation of a crystal, the more per feet is its symmetrical arrangement ; it is also larger, harder, and more transparent: whereas, when the process is too rapid, or is disturbed by agitation, or other causes, the arrangement is less regular and the form incomplete. Hence the crystals formed by nature are so much more per fect than those produced by artificial pro cesses.
Crystallization is promoted by affording a nucleus, or solid point, at which it may commence, and still more so, if a crystal be introduced into the solution ; crystalliza tion immediately commences from it, ifthe solution be a saturated one, and it is even capable of causing part of the solid to be separated, if the temperature at which it takes place could have retained it in solu tion. Even the regularity of the figure of this crystal seems to have an effect in rendering the crystallization more or less regular ; and on this Le Blank has found ed a method of obtaining large and perfect crystals. It consists in selecting very re gular crystals of a salt that have been newly formed, and putting them into a saturated solution of the same salt. They increase in size : and as the side which is in contact with the vessel receives no in crease, they are to be turned daily, to pre serve their regularity. After some time, the largest and most regular of these crys tals are to be selected, and the same pro cess repeated on them ; and thus crystals much larger and more regular than are usually formed in a solution may be ob tained.