208 TERMINOLOGY. . 157. 



eight equilateral triangles, originating from the octahedron, 

 and twelve isosceles triangles originating from the hexahe- 

 dral pentagonal-dodecahedron. This form has been called 

 the Icosahedron of Mineralogy. The icosahedron, however, 

 is not a simple form ; and, on this account, it receives no 

 particular name in the systematic nomenclature of forms 



( > 



The other is the combination of the hexahedron and the 



trigrammic tetragonal-icositetrahedron. The faces of the 

 hexahedron appear in the figure of rhombs in the place of 

 the prismatic solid angles of the icositetrahedron ; and if 

 they are enlarged till all the faces limiting the combination 

 become tetragons, the result is that form which has been 

 called the Trlacontahedron of Mineralogy. This form is con- 

 tained under six rhombs and twenty-four trapezoidal faces, 

 the one and the other equal and similar among themselves. 

 It is not a simple form ; and therefore as little entitled to 

 a peculiar systematic name as the icosahedron. 



If two forms occur at the same time in a combination, 

 we must attend to their position, whether they are both in 

 the normal position, or whether one of them is in the nor- 

 mal while the other is in the inverse position ; for the ge- 

 neral appearance of the combination is very much influ- 

 enced by this difference. 



The two semi-tessular combinations of parallel faces, 

 Figs. 75. 76., contain the same halves ; with this difference 

 only, that one of them in Fig. 76. is the inverse of the 



same in Fig. 7^., the former being the combination '- (c). 



2 ii 



(f), and the latter the combination (c'\ JL 1 (/) : 

 2 ii 2 ii 2 ii 



and although the general appearance of the two forms is 

 very different, yet an accurate comparison of the faces 

 will easily shew their identity. Both of them occur in 

 hexahe&lral Iron-pyrites. 



The semi-tessular combinations of inclined faces, Figs. 

 77- 78., likewise contain similar halves, but Fig. 77- will be 



