Crystallization. 355 



dinary temperature nothing is obtained but calc-spar in microscopical, 

 and for the most part truncated, primitive rhombohedrons ; if, on the 

 contrary, the solution be evaporated over the water bath, arragonite is 



obtained in small six-sided prisms, mixed with 

 a few crystals of calc-spar, because the tem- 

 perature of the solution is lower at first than 

 it afterwards becomes. ( Bastian. ) The polar 

 tensions of the molecules are, without doubt, 

 due to the transmutation of heat into polar 

 energy, and it appears that different phases 

 of polar energy follow different conditions of 

 temperature ; doubtless due to the changing 

 forms of the molecules and the intermolecular 

 spaces. The changes which take place in these 

 transmutations are often spoken of as "sponta- 



FIG. 153. Different Crystal- neous. " This is because so slight a demonstra- 

 ble forms assumed by Am- ,. ,. . , ,, , .. . 

 momo-Magnesic Phosphate, tion of force is required that it is not conspicu- 

 ous. Watts says "Sometimes an amorphous solid, that is to say one which 

 has no definite structure, either crystalline or organized, passes spon- 

 taneously- into the crystalline state without previous liquefaction ; but 

 generally speaking it is in the passage of a body from the liquid or gas- 

 eous to the solid state, that the regular and symmetrical arrangement 

 of the molecules takes place, which constitutes crystallization. The 

 vapors of many substances, when they come in contact with cold sur- 

 faces, pass at once to the state of crystalline solids ; e. g. , sulphur, io- 

 dine, benzoic acid, arsenious acid, camphor, &c. It is, however, in 

 the transition from the liquid to the solid state that crystallization most 

 frequently takes place. If the body has been brought into the liquid 

 state by the action of heat alone, it may be made to crystallize by cool- 

 ing, as bismuth and sulphur, for example." Again, " The more slowly 

 the liquified body is brought back to the solid state, and the more the 

 liquid is kept at rest, the smaller the number and the greater the size 

 and regularity of the crystals ; but if the solvent be cooled or separated 

 quickly, the crystals are numerous but small and ill defined. In the 

 former case, the particles of the solidifying bod} T have time to unite 

 themselves regularly with those which separate first from the fluid and 

 form nuclei of cr} T stallization. If, on the contrary, the crystallization 

 takes place rapidly, a great number of particles solidify at the same 

 time, each forming a nucleus to which other portions attach themselves, 

 and thus we obtain a number of crystals irregularly formed and inter- 

 lacing eacli other in all directions." The transition between small, ill- 

 formed crystals and mere amorphous granules is easy to be accounted 

 for by a still greater rapidity of separation. * 



1 Watts' Chemical Dictionary. 



