44 CKYSTALS AND LIVING UNITS 



cases, the assumption or not of such a form is entirely dependent 

 upon the conditions under which the separation takes place. 



Many substances which, in the chemist's laboratory, are only 

 seen in the form of insoluble precipitates made up of amorphous 

 granules, could have been procured in a crystalline condition, 

 if the same decomposition which had given rise to the amorphous 

 precipitate had been allowed to take place more slowly. If 

 instead of pouring a certain amount of a solution of sulphate of 

 potassium into one of chloride of barium, we allow the mixture to 

 take place gradually by means of dialysis, then crystals of sulphate 

 of barium are formed rather than an amorphous precipitate. It 

 has, in fact, been ascertained by Fremy' that insoluble compounds 

 generally, which appear in the laboratory as a result of double 

 decomposition in the form of amorphous precipitates, can almost 

 invariably be obtained in a crystalline condition when the chemical 

 reaction is allowed to take place very slowly. This may be brought 

 about by making the saline solutions mix after osmosis through 

 membranes, wooden vessels, or porous porcelain. By one or 

 other of these methods, he obtained many very insoluble salts in 

 the crystalline condition — such as the sulphates of baryta, strontia, 

 and lead, the carbonates of baryta and lead, oxalate of lime, 

 chromate of baryta, and several sulphides. 



Variation in the ' conditions ' under which the crystallisation of 

 any particular substance occurs, moreover, often gives rise to the 

 most marked variation in its crystalline form. Thus, referring 

 to the article 'Dimorphism' in Watts's " Dictionary of Chemistry," 

 we find the following statements : — " Many substances, both simple 

 and compound, crystallise in forms which belong to two or three 

 different systems of crystallisation, or which, even if 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 crystaUine form which they 

 exhibit is associated with difference of specific gravity, hardness, 

 colour, and other properties. Whether a body shall crystallise 

 in one system or another seems to depend chieily upon tempera- 

 ture. . . . Sometimes the form of the crystal varies according 

 to the solvent from which it separates : thus arsenious anhydride 



' " Compt. Rend." t. Ixiii. p. 714. 



