BASE, ^AND ITS MODIFICATIONS. 137 



Class o. Edges of the base replaced by two planes. 

 The new figures would be octahedrons, more acute 

 than the primary. 





The character, and number, of the modifications of 

 this, and some other of the following classes of pri- 

 mary forms, arise from the dissimilarity between the 

 edges and angles of the summits, and those of the 

 base, in the octahedrons ; and between the angles of 

 the terminal planes, or the terminal and the lateral 

 edges of the prisms, and sometimes between the la- 

 teral edges themselves. 



The primary crystals belonging to this class of 

 primary forms, are distinguishable from regular octa- 

 hedrons by the unequal inclinations of the plane P 

 on P', and P on P'' ; and the secondary forms may be 

 distinguished by the modifications taking place on 

 some only of the edges or angles, and not on all, as 

 they do on the regular octahedron. If two of its 

 edges measure over the summit more than 90, the 

 octahedron of this class is called obtuse ; if less than 

 90, it is called acute. In the regular octahedron the 

 edges measure exactly 90 over the summit. 



The secondary octahedrons belonging to this class, 

 have, like the primary, square bases. 



