58 THE BEGINNINGS OF LIFE. 



they belong to the same system, yet exhibit such 

 differences in their corresponding angles as to render 

 it quite impossible to reduce them to the same form : 

 this was first shown by Mitscherlich, in 1823 (Ann. 

 Ch. Phys. [2] xxiv. 264). Such bodies are said to be 

 dimorphous and trimorphous. The difference of crys- 

 talline form which they exhibit is associated with 

 difference of specific gravity, hardness, colour, and 

 other properties. Whether a body shall crystallize in 

 one system or another seems to depend chiefly upon 



temperature Sometimes the form of the crystal 



varies according to the solvent from which it separates : 

 thus arsenious anhydride crystallizes from water or 

 hydrochloric acid in regular octahedrons, but from 

 alkaline solutions in trimetric prisms.' Taking some 

 other specific instances, we find that c a hot solution 

 of saltpetre yields, when slightly cooled, nothing but 

 prismatic crystals, but at ioC prismatic and rhombo- 

 hedral crystals appear together; if alcohol be added, 

 the latter are formed most abundantly ; the addition of 

 potash, nitric acid, or nitrate of sodium produces no 

 alteration.' Again, the modifying influence of tempe- 

 rature is shown by the fact that, c lf a solution of 

 carbonate of calcium in water containing carbonic 

 acid be left to evaporate at the ordinary 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 



