PRIMARY FORMS. 85 



recourse to a cube of fluor. Let fig. 114 represent 

 this cube. If we apply a knife to the diagonal a ft, 

 and detach successively the two solid angles a b c, 

 a b d, and then place the crystal with the edge a ft, 

 downwards, and remove the solid angles c d a, c d ft, 

 the figure we shall obtain will be the regular tetra- 

 hedron. 



Each of the classes of primary forms contained in 

 page 6 to 11, except the cube, the regular tetrahedron, 

 octahedron, and rhombic dodecahedron, comprehends 

 many individual forms belonging to as many species of 

 minerals; which individual forms differ from each other 

 in some of their angles, or in the relative lengths of 

 some of their adjacent edges. 









