CKYSTALLI^E FOEMS. 



97 



operation, the latter is converted into the former. In this 

 last case, the solid angles of the rhombic octohedron must be 



FIG. 50. 



TIG. 51. 



so placed as to bisect the edges of the base of the rectangular 

 octohedron. 



The rhombohedron (fig. 44) and the six-sided or hexagonal 

 prisms (fig. 45) are allied to each other. If tangent planes 

 are laid on the two solid angles -of the rhombohedron, 

 and the six, solid, lateral angles marked 5, or the edges 

 between them, are replaced by equal planes perpen- 

 dicular to the former, a six-sided prism results; the six- 

 sided prism may be reconverted into the rhombohedron, by 

 replacing all its alternate solid angles by equal and similar 

 rhombic planes. 



The six-sided prism is often associated in nature (for 

 instance in quartz) with a six-sided pyramid, formed by all 

 its terminal planes, being replaced by isosceles triangles 

 (fig. 9). If the faces of the prism disappear, the double six- 

 sided pyramid results. 



Those crystalline forms which have an intimate geometrical 

 connexion with each other, are considered by crystallo- 

 graphers, as constituting certain groups, 

 which are termed systems of crystallo- 

 graphy. 



CLASSIFICATION OF MOHS. This ar- 

 rangement apportions the fifteen primary 



forms as follows : 



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I. The Tessular system comprises the 

 cube (fig. 52), the tetrahedron (fig. 32), the regular octo- 

 hedron (fig. 33), and the rhombic dodecahedron (fig. 34). 



II. The Pyramidal includes the octohedron, with a square 

 base (fig. 35), and the right square prism (fig. 38). 



