HEXAGONAL SECTION OE HEXAGONAI SFSTEM. 



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lateral axes connect the centres of the opposite lateral faces. 

 This prism is represented in fig. 5. 



The lateral edges of the hexagonal prisms occur sometimea 

 with two similar planes on each edge, and these planes, when 

 extended to the obliteration of the hexagonal prism, make 

 % twelve-sided prism. These two 

 planes are seen in fig. 8, along 

 with the planes J of the hexago- 

 nal prism, and 1 of a double six- 

 sided pyramid, besides the basal 

 plane 0. 



Double pyramids. The double 

 pyramids are of three kinds: (1) 



A series of six-sided, whose planes belong to the same verti- 

 cal zone with the planes I. The planes of two such pyramids 

 (lettered 1, 2) are shown in figs. I and 2, three of them in fig. 

 3 (lettered ^-, 1, 2), and one in fig. 7, and one such double 

 pyramid, without combination with other planes, in fig. 6. 

 (2) A series of six-sided double pyramids, whose planes are in 

 the same vertical zone with *-2, examples of which occur on fig. 

 2 (plane 2-2)— on fig. 3 (planes 1-2, 2-2, 4-2). The form of this 

 double pyramid is like that represented in fig. 6, but the lateral 

 axes connect the centres of the basal edges. The double six- 

 sided pyramid is sometimes called a quartzoid, because it occurs 

 in quartz. (3) Twelve-sided double pyramids. Two planes of 

 such a pyramid are shown on a hexagonal prism in fig. 9, also in 



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fig. 2 (the planes 3-f ), and the simple form consisting of such 

 planes in fig. 10 — a form called a berylloid, as the planes are 

 common in beryl. In fig. 1 1 the planes 1 bel mg to a double 

 six-sided pyramid ; and those next below (of which three are 

 lettered W) to a double twelve-sided pyramid. 



