HZ PRIMARY FORMS. 



edges or angles, and tend equally to the extinction of 

 all its planes, while the modifications of the tetra- 

 hedron, regulated by the same law, would affect only 

 some of the edges or angles of a crystal containing all 

 the octahedral planes. 



It may be observed by examining the crystals of 

 spinelle and red oxide of copper, that the solid angles 

 are sometimes replaced by four planes resting on the 

 primary planes. 



This change of figure results from a single modifica- 

 tion of the octahedron belonging to class b. See tables. 



But in order to produce four similar planes on each 

 of the solid angles of the octahedron, regarded as a 

 secondary form of the tetrahedron, two modifications of 

 the tetrahedron must concur. We must suppose each 

 of the solid angles of the tetrahedron to be replaced by 

 three planes resting on its edges, at the same time that 

 its edges are bevilled. 



The octahedron will therefore, under the definition 

 I have given of a primary form, be adopted as the 

 primary form of spinelle and red oxide of copper, and 

 of such other minerals as present secondary forms of 

 a similar character. 



I shall cite only one other instance of the insuffi- 

 ciency of cleavage alone to determine the primary 

 form of a mineral, although many more might be 

 adduced. 



Fig. 112. 



The petalite has two sets of cleavages, one in the 

 direction of a b, c d, fig. 112, at right angles to each 



