74 CLASSIFICATION OF HIXEBALS. 



of oxygen, the latter the highest. Terms such as carbonate 

 of lime, sulphate of lime, indicate that the substance is com- 

 posed of an acid — carbonic acid, or sulphuric acid in the 

 instances cited, with lime. So silicate of soda is a com- 

 pound of soda and silicic acid (or silica) ; and all such com- 

 pounds are theoretically said to consist of an acid and a base — 

 lime and soda, in the cases mentioned, being bases. 



The true foundation of a species in mineralogy must be 

 derived from crystallization, as the crystallizing force is funda- 

 mental in its nature and origin : and it is now generally admit- 

 ted that identity of crystalline form and structure is evidence 

 of identity of species. This principle unites certain distinct 

 chemical compounds into the same species : — for example, a 

 silicate of magnesia and a silicate of iron crystallizing alike, 

 constitute but one species in mineralogy, though chemically 

 so different. Oxyd of iron and magnesia are themselves 

 nearly identical in molecular form and size, and on this fact 

 depends iheir power of replacing one another even in com- 

 plex compounds. They are therefore said to be isomorphous 

 (from the Greek *sos, similar, and morphe, form.) 



There are many groups of these isomorphous substances, 

 and somt. knowledge of them is necessary to enable the 

 reader to understand why different varieties of a mineral 

 species may differ so widely, as they often do, in composition. 

 Some of these groups are as follows : 



1. Alumina, peroxyd of iron, peroxyd of manganese. 



2. Lime, magnesia, protoxyds of iron, manganese and zinc. 



3. Baryta, strontia, oxyd of lead. 



4. Sulphur, selenium, tellurium. 



5. Tungsten, molydenum. 



6. Phosphoric acid, arsenic acid. 



In epidota the alumina may be replaced by peroxyd of 

 iron or manganese, and the magnesia in part or wholly by 

 lime, or the protoxyds of iron or manganese. The same is 

 true of garnet and several other minerals. The rhombohe- 

 drons of carbonate of lime, carbonate of iron, and carbonate 

 cf magnesia, are very nearly identical in angle, because the 

 baees are ismorphous. This subject is illustrated by the 

 greater- part of mineral species. 



What V3 a sulphate 1 a silicate ? What is the tes* of identity of 

 sptsicv in mineralogy ? What are isomorphous substances 1 What 

 6Se tht common groups of isomorphous substances in minerals ? Ex- 

 ^•s&m^ «xamp>s. 



