446 SCIENTIFIC THOUGHT. 



lich discovered another crystalline property of certain * 

 chemically pure substances. He found that some sub- 

 stances can crystallise in more than one distinct and 

 definite form. The alums and vitriols are typical of iso- 

 44. morphism. As typical of the second property, which was 

 phism. termed by him dimorphism or polymorphism, we have" 

 the well-known mineral calc-spar, which is dimorphous 

 with aragonite, both having the same chemical constitution 

 and properties. A typical example of dimorphism is the 

 mineral rutile, which is chemically the same substance as 

 the mineral anatase, both being chemically pure titanic 

 oxide. Among the elements, pure sulphur crystallises in 

 two different forms. The property of dimorphism seemed 

 at first to contradict the inference which Mitscherlich had 

 drawn from his first discovery viz., that the crystalline 

 shape is expressive of the number and chemical connec- 

 tion of the smallest particles or atoms ; but the further 

 discovery, that if of two isomorphous bodies one is dimor- 

 phous, the other is likewise so, gave again a great support 

 to the geometrical conception of atomic complexes i.e., 

 to the idea that chemical individuality is ultimately to be 

 explained not only by the number, but also by the mutual 

 fixed position and shape, of the atoms. And yet it seemed 

 a long way, and is a long way still, from the external, 

 visible, and well-marked shape of a crystal, with its 

 peculiar and well-defined geometrical, elastic, optical, and 

 thermal properties, to the primitive molecule, made up of 

 still more simple atoms, in the form, number, and arrange- 

 ment of which we are again and again tempted to see the 

 nature of chemical or qualitative individuality. To obtain 

 a clear view in this way would be to work our way from 



