TERMINOLOGY. . 



H, O, R, Z, the edges of that regular six-sided prism, in 

 whose planes are situated the lateral edges CB, C'B' of two 

 rhombohedrons. These lateral edges, intersecting each 

 other at the points M, N, turn as it were round these, and 

 consequently the faces of the rhombohedron turn round the 

 line MN, in a ratio, dependent upon the length of the axis. 

 If the axis becomes infinitely long, the lateral edges of the 

 rhombohedron assume the situation C"B" ; the rhombohe- 

 dron is transformed into a regular six-sided prism, upon the 

 faces of which are drawn the horizontal lines MN, ML, 

 &c. ; and these lines, the sides of the transverse section of 

 the rhombohedron, are therefore likewise the sides of the 

 transverse section of the prism, whose position is thus de- 

 termined in respect to the rhombohedrons of the series, and 

 to their horizontal projection. 



As to the opposite limit, it is evident, that if the axis 

 becomes infinitely small, all the faces of the rhombohedron 

 coincide in a single plane, and that this form is therefore 

 changed into a regular hexagon, equal and similar to the 

 horizontal projection of the fundamental form. 



The crystallographic signs of the limits are R + co and 

 R 03, those of the series between its limits, 

 R - co ... R -f n ... R + os. 



Many examples are found in nature, illustrative of this 

 series. Thus, rhombohedral Lime-haloide presents five 

 consecutive members, and both the limits ; in rhombohe- 

 dral Tourmaline and rhombohedral Ruby-blende, four con- 

 secutive members and both limits have been observed ; 

 two or three consecutive members occur in many species ; 

 and in most of those, affecting forms which are in connexion 

 with the rhombohedron, we likewise frequently meet with 

 the limits on either side of the series. 



. 112. DERIVATION OF SCALENE SIX-SIDED PYRA- 

 MIDS. 



From the members of the series in the preced- 

 ing paragraph, several scalene six-sided pyramids 



