SECT. XIV. ISOMORPHISM CLEAVAGE. 109 



crystalline form. For example, the neutral phosphate of soda 

 and the arseniate of soda crystallise in the very same form, 

 contain the same quantities of acid, alkali, and water of crystal- 

 lisation ; yet they differ so far, that one contains arsenic and the 

 other an equivalent quantity of phosphorus. Substances having 

 such properties are said to be isomorphous, that is, equal in form. 

 Of these there are many groups, each group having the same 

 form, and similarity though not identity of chemical composi- 

 tion. For instance, one of the isomorphous groups is that consist- 

 ing of certain chemical substances called the protoxides of iron, 

 copper, zinc, nickel, and manganese, all of which are identical in 

 form and contain the same quantity of oxygen, but differ in the 

 respective metals they contain, which are, however, nearly in the 

 same proportion in each. All these circumstances tend to prove 

 that substances having the same crystalline form must consist of 

 ultimate atoms having the same figure and arranged in the very 

 same order ; so that the form of crystals is dependent on their 

 atomic constitution. 



All crystallised bodies have joints called cleavages, at which 

 they split more easily than in other directions ; on this property 

 the whole art of cutting diamonds depends. Each substance 

 splits in a manner and in forms peculiar to itself. For example, 

 all the hundreds of forms of carbonate of lime split into six-sided 

 figures, called rhombohedrons (N. 169), whose alternate angles 

 measure 105'55 and 75'05, however far the division may be 

 carried ; therefore the ultimata particle of carbonate of lime is 

 presumed to have that form. However this may be, it is certain 

 that all the various crystals of that mineral may be formed by 

 building up six-sided solids of the form described, in the same 

 manner as children build houses with miniature bricks. It may 

 be imagined that a wide difference may exist between the parti- 

 cles of an unformed mass and a crystal of the same substance 

 between the common shapeless limestone and the pure and 

 limpid crystal of Iceland spar; yet chemical analysis detects 

 none ; their ultimate atoms are identical, and crystallisation 

 shows that the difference arises only from the mode of aggrega- 

 tion. Besides, all substances either crystallise naturally, or may 

 be made to do so by art. Liquids crystallise in freezing, vapours 

 by sublimation (N. 170) ; and hard bodies, when fused, crystal- 

 lise in cooling. Hence it may be inferred that all substances are 



