MODIFICATIONS OF CRYSTALS. 



37 



cube may have ojily the alternate angles replaced ; or only 

 one of the two beveling planes shown in figure 32 may occui 

 on each edge ; or three of the six on each angle in figure 35. 

 The following are examples ; and each figure in the lower 

 line, represents the completed form, produced by extending 

 the secondary planes in the figure above, to the obliteration 

 of the primaries, as explained on the preceding pages. 

 40 41 42 43 









( 



P 



7 

 P 



v> 



— i/ 



44 





The replacement begun in figure 40, continued to the oblit- 

 eration of the Ps, produces figure 44, which is a tetrahedron, 

 or three-sided pyramid. So the planes a in figure 41. give 

 rise to fig. 45 ; the planes e in 42, to figure 46, which is a 

 pentagonal dodecahedron, so called because it has twelve 

 pentagonal (or five-sided) faces. The forms represented in 

 figures 40 and 41 are common in boracite, and those of figures 

 42, 43, in iron-pyrites. These forms with half the full num- 

 ber of planes are called hemihedral forms, from the Greek 

 words for half and face. 



The tetrahedron is sometimes placed among the primary 

 forms ; but it is properly a secondary form, derived from the 

 cube, in the manner here explained, or from the octahedron 

 by the extension of four faces to the obliteration of the other 

 four. (Compare figs. 2 and 44.) 



In the right square prism, the basal edges being unequal 

 to the vertical, (because thef prism, unlike the cube, is higher 

 than broad,) these two kinds of edges are not replaced by 

 similar planes, and the basal 'may be modified when the 

 lateral are not modified, (figs. 48, 49.) The lateral edges 

 may be truncated, because their including planes are equal ; 



Explain the second law. What are the resulting forms called ? 



What i3 said of the tetrahedi 



