CRYSTALLIZATION. 



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 finst 

 long and slender spiculae form 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 heat, 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. 



ID general it holds true, that the slower 

 the formation of a crystal, the more per- 



fect 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 acrystalbe 

 introduced into the solution ; crystalliza- 

 tion immediately commences from it, if the 

 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 obtaininglarge 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. 



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 



