18 TERMINOLOGY. 



. 31. The different crystalline forms belonging to each 

 species may be conceived to be derived, by certain laws, 

 from one type, or fundamental form. 



This derivation is based upon the obvious relations presented by 

 the different forms of the same mineral species, (. 30.) as well as 

 upon certain peculiarities visible in their internal structure, to be 

 developed hereafter. 



To become acquainted with the almost innumerable forms of crys- 

 tals, without the aid of some arrangement would be very difficult. 

 The preliminary study of the fundamental forms therefore, from 

 which nature appears to have produced her numerous second a- 

 ries, is no less necessary, than it is natural. And it cannot be too 

 strongly insisted upon, that the student in Mineralogy should estab- 

 lish in his mind as clearly as possible, the precise grounds of discrim- 

 ination among these few important solids, and familiarize himself with 

 the language adapted to denote the peculiar properties of each ; 

 since without a perfect familiarity with these points, he will con- 

 stantly find himself embarrassed in the systematic study of minerals. 



CONSIDERATION OF FUNDAMENTAL OR PRIMARY FORMS AND 

 SOME OF THEIR GEOMETRICAL RELATIONS. 



. 32. PRIMARY FORMS. 



The primary form is assumed to be that particular form 

 from which all the different crystals of a mineral species 

 are derived, in consequence of certain symmetrical changes, 

 it may be supposed to have undergone, during their forma- 

 tion. The primary forms are fifteen in number; 1, the 

 Cube; 2, the regular Tetrahedron ; 3, the regular Octahe- 

 dron; 4, the rhombic Dodecahedron ; 5, the Octahedron 

 with a square base; 6, the Octahedron with a rectangular 

 base ; 7, the Octahedron with a rhombic base ; 8, the right 

 square Prism; 9, the right rectangular Prism ; 10, the right 

 rhombic Prism ; 1 1 , the right oblique angled Prism; 12, 



