ATTRACTION. 141 



(G.) CRYSTALLIZATION is THE RESULT OF THE ATTRACTION OF 



AGGREGATION. 



(h.) A crystal is a symmetrical solid, produced by the union of in- 

 tegrant particles.* 



(i.) Natural crystals are numerous, and art produces many more; 

 every good mineral cabinet exhibits great numbers of the former, and 

 every good chemical collection of the latter. 



(j.) Destruction or great diminution of the power of cohesion 

 is an indispensable preliminary. This is effected either by so- 

 lution in a fluid, or by the aid of heat producing fluidity or the state 

 of vapor. In the former case, it is necessary to drive off part of the 

 solvent by heat ; in the latter, merely to allow the fluid to cool, or the 

 vapor to be condensed, in order that crystals may be formed. Cer- 

 tain circumstances are, however, necessary to be attended to in order 

 to success. If the solvent be very rapidly expelled by the aid of a 

 high temperature, or, if the fused body be suddenly exposed to an in- 

 tense cold, either a shapeless mass will be formed, or only confused 

 and irregular crystals. In general, fine crystals are obtained only by 

 slow evaporation and by slow cooling. Water and most of the metals 

 are examples of bodies that crystallize by a mere reduction of tem- 

 perature. A saturated solution of sulphate of soda, boiled and cork- 

 ed in that state, does not become solid on cooling, but on letting in 

 the air ; agitating it by a jerk or jar, or dropping in a crystal, it con- 

 geals and heat is evolved, sufficient to melt it again. If a string or 

 mark be placed on the neck of the vessel, it will be seen that the mass 

 has been expanded by the crystallization. It does not appear that it 

 is the mere pressure of the air, as was formerly supposed, that pro- 

 duces the crystallization ; the air seems to act as a disturbing force, 

 or perhaps by the introduction with it, of some foreign body, which 

 may serve as a nucleus. f A gentle waving motion does not cause it 

 to congeal. The salts are crystallized generally by diminishing the 

 quantity of the solvent, that is, by evaporation, or by conjoining both, 

 diminishing the solvent by evaporation and reducing the tem- 

 perature ; or, when a particular portion of a salt has been sus- 

 pended by the aid of an elevated temperature, a simple reduction 

 of temperature is sufficient, without evaporation. For, an elevated 

 temperature increases the power of most solvents. Common salt, 

 however, being dissolved in nearly equal quantities by cold as by hot 



* That is of particles of the same kind, but these particles may be chemically, 

 either simple or compound. 



t A point or almost any solid frequently determines incipient crystallization ; 

 so a jar or sudden vibratory motion brings the particles into such a position, that 

 their polar attractions become effectual, and the negative pole of the galvanic series 

 produces crystallization, while the positive pole counteracts it. Light also causes 

 camphor to crystallize from its alcoholic solution, and it is rcdi^sojvcd in a dark dav 

 Dr. Ure. 



