142 ATTRACTION. 



water, no advantage is gained by the aid of heat, except in speed, nor 

 does a reduction of temperature cause it to crystallize. The only 

 method in which this can be effected, is by diminishing the solvent by 

 evaporation. It is found that crystallization is much facilitated by 

 supplying a nucleus ; and Le Blanc, a Parisian apothecary, has even 

 founded upon it a method of obtaining large and beautiful crystals, by 

 selecting the best, replacing them in the solution, and turning them 

 daily j as the lower side does not increase. 



(k.) An increase of bulk is commonly an effect of crystallization, 

 but sometimes the bulk is diminished, as in the case of mercury. 

 Substances which have been deposited from an aqueous solution, 

 generally retain, intimately combined, a portion of water, which is 

 called their water of crystallization. The efficacy of freezing mix- 

 tures is owing, in a considerable degree, to this water of crystalliza- 

 tion, which, by becoming fluid, absorbs caloric ; when, with the aid 

 of heat, it causes the salt to become fluid, the salt is said to suffer the 

 aqueous fusion. When it escapes spontaneously, into the atmosphere, 

 the salt is said to effloresce, for the crystalline form is destroyed, and 

 it falls into powder. When the salt attracts water from the air, and 

 becomes more or less fluid, it is said to deliquesce* When it splits 

 and crackles by heat, it is said to decrepitate. 



(I.) All bodies, in crystallizing, assume a determinate form. Thus 

 the crystal of alum is an octahedron ; that of common salt a cube ; 

 of the beryl, a hexahedral prism, &c. It must not be understood, 

 however, that these forms are invariable. The same substance will 

 sometimes assume one form, sometimes another, according to cir- 

 cumstances. But, to this apparent caprice there is a limit, for a 

 given substance will always crystallize in one of a given number of 

 forms, which are appropriate to it. 



Prisms and pyramids are among the most common forms of crys- 

 tals, but they admit of great diversity. 



(m.) Ml the forms of crystals are reducible either by dissection or 

 by calculation, to six primitive forms, namely, the hexahedron, includ- 

 ing the cube, parallelopipedon and rhomboid ; the regular octahedron ; 

 the prism of six sides ; the regular tetrahedron ; the dodecahedron 

 with rhomboidal faces, and the dodecahedron with isosceles triangu- 

 lar faces. This very curious subject has been developed by the suc- 

 cessive labors of Rome de L'Isle, Gahn, Bergman, Bournon, and 

 Haiiy. Haiiy completed what Bergman had begun, by extracting 

 the primitive form of calcareous spar in the following manner. 



* Sometimes portions of the fluid from which crystals have been precipitated, are 

 lodged mechanically between the plates, and it may be even a portion of a fluid con- 

 taining a different substance, if other salt* or compounds were present in the solution. 



