CRYSTALLIZATION. 



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 OP 

 SALTS. 



CRYSTALLOGRAPHY. Hauy has 

 succeeded in developing the theory of 

 crystals, so far as to shew, that in every 

 crystallized substance, whatever may be 

 the difference of figure which may arise 

 from modifying circumstances, there is in 

 all its crystals a primitive form, the nu- 

 cleus, as it were, of the crystal, invaria- 

 ble in each substance, and by various mo- 

 difications, which he points out, giving 

 rise to the numerous secondary or actual- 

 ly existing forms. 



The fact which led to these views is, 

 that crystals can be mechanically divided 

 only in certain directions, so as to afford 



