TERMINOLOGY. . 150. 



whose halves they are. Combinations, into which forms of 

 this kind enter, are termed hemi-pyramidal combinations 

 of parallel faces. 



But a scalene eight-sided pyramid may also be resolved 

 according to the process, applied in . 146. to the di-pyramid, 

 for reproducing the simple forms which it contains. This 

 is effected by enlarging the alternate pairs of faces which 

 meet in the acute terminal edges of the one, and those of the 

 other apex which are not contiguous to them. The result- 

 ing forms are contained under eight equal and similar sca- 

 lene triangles, all their faces being inclined to one another, 

 Figs. Co. 66. ; hence they likewise change those pyramidal 

 combinations in which they are contained, into hemi-pyra- 

 midal ones of inclined faces. These forms are in respect to 

 the eight-sided pyramids, what the forms analogous to the 

 tetrahedron considered above are to the four-sided pyra- 

 mids, and occur along with them in the same combinations. 



Hemi-pyramidal combinations of parallel faces occur in 

 pyramidal Scheelium-baryte, and hemi-pyramidal combina- 

 tions of inclined faces in pyramidal Copper-pyrites. No ex- 

 ample has yet been found in nature of di-pyramidal combi- 

 nations, or such as would contain one and the same form of 

 the pyramidal system in two different positions ; nor have 

 we any reason to suspect their existence, because no ra- 

 tional number of derivation can produce from any pyrami- 

 dal form, another diagonally situated, and equal and similar 

 to that pyramidal form. 



Hemi-pyramidal forms, like the hemi-rhombohedral ones, 

 are designated by adding the divisor 2 to the crystallogra- 

 phic signs of the entire forms. The situation of those faces 

 which occur in the combination, are moreover indicated by 

 the signs 4- and , or r and 1. 



The following signs refer to the figures 61 66 : 



+ LJ? Fig. 61. ; - Lt^ Fig. 62. ; 

 _L 



Fig.C3.; Fig. 64.; 



_ Fig . 65 . ; - _- Fi>. 66. 



