44 



MOLECULES. 



Grey copper, whose primary form is a tetrahedron, 

 occurs under the forms of the cube, octahedron, and 

 rhombic dodecahedron. 



Blende is found sometimes, though rarely, crystal- 

 lized in cubes, sometimes in octahedrons, tetrahedrons, 

 and rhombic dodecahedrons. 



Fig. 70. 



If we attempt to fracture a cube of blende, we find 

 it will split in directions parallel only to its diagonal 

 planes. These cleavages will truncate the edges of 

 the cube, and if continued until all the edges are 

 removed, and the face of the cube disappear, a rhom- 

 bic dodecahedron will be produced, which has been 

 considered the primary crystal of blende. 



If a cube of blende, fig. 70, be cleaved in directions 

 parallel to its diagonal planes, beginning at the lines 

 a b, c d, fig. 71 will be produced. 



Fig. 71. 



If fig. 71 be further cleaved in directions corres- 

 ponding to a b c d e, so as to remove all the perpen- 

 dicular edges, and to obliterate the remainder of the 

 perpendicular planes of the cube, fig. 72 will remain. 



