FUNDAMENTAL FORMS OF CRYSTALS. 



31 



of a right rectangular prism, with the knife parallel to the ver« 

 25 -5a 26 



26a tical diagonal planes of this prism, J^i 

 ;'^TT^p>] (as is seen in fig. 26,) and a right 

 rhombic prism (fig. 26a) is the re- 

 r — Id suit. The relations of these two 



prisms is shown in figure 26 b, 

 which represents a rhombic prism 

 within a rectangular prism. R is 

 obvious on comparing these figures, that the lateral axes 

 which connect the centers of opposite faces in the rectangu- 

 lar prism, connect the centers of opposite lateral edges in 

 the rhombic prism. 



These three forms, the right rhombic prism, rhombic oc- 

 tahedron, and rectangular prism, are so closely related, that 

 one may give origin to the other, and all may occur in the 

 same mineral. This is often the case, as in the minerals 

 celestine and heavy spar. 



Again : set the right rhomboidal prism on one of its lat- 

 eral faces, and then slice off each lateral edge, (lateral, as so 

 situated,) keeping the knife parallel with the diago- 

 nal plane, and an oblique rhombic prism is obtained. 

 Figure 27 represents the process begun, and figure 

 13, as well as the interior of figure 27, the com- 

 pleted oblique rhombic prism. 



Lastly : take a rhombohedron, and after placing 

 it in position, fig. 16,) look down upon it from above, (fig. 

 19 ;) the six lateral edges are seen to form a regular six-sided 

 figure around the axis. If these edges be cut off parallel 

 \o the axis, a six-sided prism (having a three-sided pyra- 

 mid at each extremity) must, therefore, result. This pro* 

 cess is shown begun in figure 28, and completed in figurc 



How is a rhombic prism derived from a rectangular 1 What relation 

 hence between these prisms 1 How can yen make an oblique rhom- 

 bic prism from a right rhomboidal 1 How a right rhomboidal from an 

 oblique rhombic ? Explain the relation betw to. the rhombohedron aoi 

 hexagonal prism, and how one is reduced to he other. 



