MODIFICATIONS OF CRYSTALS. 



57 



cube may have only the alternate angles replaced ; or only 

 vne of the two beveling planes shown in figure 32 may occur 

 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 



/ 



The replacement begun in figur/ 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, jsb 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 As sometimes placed among the primary 

 forms , but it is pr/perly 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 the prism, unlike the cube, is higher 

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

 similar pjanes, and the basal may be modified when the 

 lateral ai-e 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 is said of the tetrahedron ? 



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