ISOMORPHISM. 13/ 



at which he arrived is as follows : The same number of atoms 

 combined in the same way produce the same crystalline form ; 

 and crystalline form is independent of the chemical nature of 

 the. atoms, and determined only by their number and relative 

 position. 



This law has not been established in all its generality, but 

 perhaps no fact is certainly known which is inconsistent with 

 it, while an indisposition which certain classes of elements 

 have to form compounds at all similar in composition to those 

 formed by other classes, limits the cases for comparison, and 

 makes it impossible to trace the law, throughout the whole 

 range of the elements, in the present state of our knowledge 

 respecting them. 



The relation of isomorphism is most frequently observed 

 between salts, from their superior aptitude to form good 

 crystals. Thus the arseniate and phosphate of soda are ob- 

 tained in the same form, and are exactly alike in composition, 

 each salt containing one proportion of acid, two of soda and 

 one of water as bases, together with twenty-four atoms of 

 water of crystallization. With a different proportion of water 

 of crystallization, namely, with fourteen atoms, and the other 

 constituents unchanged, the crystalline form is totally different, 

 but is again the same in both salts. For every arseniate, there 

 is a phosphate corresponding in composition and identical in 

 form ; the isomorphism of these two classes of salts, is indeed 

 perfect. The arsenic and phosphoric acids, contain each five 

 proportions of oxygen to one of arsenic and phosphorus res- 

 pectively, and are supposed to be themselves isomorphous, 

 although the fact cannot be demonstrated, as the acids do not 

 crystallize. The elements, phosphorus and arsenic, are also 

 presumed to be isomorphous ; and the isomorphism of their 

 acids and salts is referred to the isomorphism of the elements 

 themselves ; isomorphous compounds in general appearing to 

 arise from isomorphous elements uniting in the same manner 

 with the same substance. The isomorphism of the sulphate, 

 seleniate, chromate and manganate of the same base is like- 

 wise always clear and easily observed ; each of the acids in 

 these cases containing three proportions of oxygen to one of 

 selenium, sulphur, chromium and manganese, themselves pre- 

 sumed to be isomorphous. Of bases, the isomorphism of the 

 class consisting of magnesia, oxide of zinc, oxide of cadmium 



