CBYSTALLLNE POEMS. 



95 



13. The oblique rhomboidal prism, sometimes called the 

 doubly oblique prism, differs from the preceding in the ter- 

 minal planes, a, a } being rhomboids (fig. 43). 



FJG. 42. 



14. The rhombohedron, which is bounded by six prisms 

 which are exactly of the same size and form (fig. 44). 



FIG. 44. 



FIG. 45. 



15. The regular hexagonal prism, which is bounded by six 

 perpendicular, and two horizontal or terminal planes, which 

 are at right angles to the former. Like the regular hexagon 

 of geometry, the lateral planes incline to each other at angles 

 of 120 degrees. If these angles are not of 120 degrees, the 

 prism is irregular (fig. 45). 



These are considered as the primary forms, many of which 

 are geometrically allied to each other. Thus, as we have 



FIG. 46. 



FIG. 47. 



already observed, if the six solid angles of the octohedron 

 are replaced by tangent planes, and these are enlarged till 



