CRYSTALLINE FORMS. 



93 



\vill hereafter be denominated the base of the octohedron, is 

 a square (fig. 33). 



TIG. 31. 



FIG. 32. 



4. The rhombic dodecahedron, contained between twelve 

 equal rhombic planes, having six solid angles, consisting 

 each of four acute plane angles, the two opposite ones, as a 

 and &, being sometimes called the summits, and eight solid 

 angles or corners, consisting each of three obtuse plane 

 angles (fig. 34). 



5. The octohedron with a square base, which is bounded 

 by eight faces which are similar isosceles triangles. The base, 

 #, 6, b, b, is always a square, and is the only part of the figure 

 which is constant (fig. 35). 



FIG. 36. 



6. The rectangular octohedron, which is limited by eight 



