6 



DEFINITIONS. 



AM minerals which are composed of similar ele- 

 mentary particles combined in equal proportions, and 

 whose molecules are similar in form, are said to 

 belong to one species. 



The same species of mineral is frequently observed 

 to crystallize in a great variety of forms. 



From among the variety of crystalline forms under 

 which any species of mineral may present itself, some 

 one is selected as the primary , and the remainder are 

 termed secondary forms. 



A primary form is that parent or derivative form, 

 from which all the secondary forms of the mineral 

 species to which it belongs, may be conceived to be 

 derived according to certain laws. 



The primary forms are at present supposed to con- 

 sist of only the following classes. 



Fig. 10. 





The cube^ fig. 10, contained within six square 

 planes. 



Fig. 11. 



The regular tetrahedron, fig. 11, contained within 

 four equilateral triangular planes. The solid angle 

 at #, is sometimes called its summit. 



