MOLECULES. 41 



hedron, it will be convenient to place the octahedron 

 of fluor, which we have just obtained, in the position 

 represented in fig. 66, resting on one of its planes. 



Fig. 67. 



In this position of the crystal, if we suppose the 

 three lines a b, c d, e f^ to be drawn through the 

 centre, and parallel to the edges, of the now upper- 

 most plane, and if we apply our knife to the line a b, 

 we may cleave the crystal parallel to the plane g, 

 and may detach the portion a b g /, fig. 67. 



Fig. 68. 



By cleaving again from the lines c d, and ef, paral- 

 lel to the plane ^, and to the back plane of fig. 66, 

 and by also cleaving parallel to the plane on which 

 the figure rests, beginning at the line i k, we shall 

 obtain a regular tetrahedron as seen in fig. 69. In 

 fig. 68 this tetrahedron is exhibited in the position 

 which it occupied in the octahedron. 



