of Crystalline Combinations. 403 
RHOMBOHEDRAL SYSTEM. 
Relations of Edges, &c. 
The forms are in a transverse position. 
The first truncates the edges of the second (C). 
The Edges of Combination are parallel to one another; to the terminal edges of the 
‘acute rhombohedron; to the inclined diagonals of the obtuse one (C). 
The forms are in a parallel position. 
E. C. are horizontal (A). 
A face perpendicular to the axis. 
E. C. horizontal. 
The second form is a regular six-sided prism. 
E. C, of its alternate faces with upper faces of pR, are horizontal (A); 
and of remaining alternate faces, with lower faces of pR. 
remaining E. C. are inclined. 
cos. hor. edge = 2 cos. incl. edge. 
E. C. parallel to edges of rhombohedron pR (B); 
and to lateral edges of pyramid p Rm. 
Faces of rhombohedron are rhombs contiguous to the apex. 
E. C. horizontal. 
The face (OR) perpendicular to axis, is a hexagon with alternate angles equal. 
The faces of the prism ©R are irregular tetragons divisible by a horizontal line into 
two isosceles triangles of unequal heights. These haye their more acute angles 
alternately upwards and downwards. The heights of the two isosceles triangles 
are as m—1:m-+1. 
The forms are in a transverse position (being derived from R’ and R). 
Faces of the rhombohedron replace acute terminal edges of the pyramid. (S. 7’.) 
E. C. parallel to each other, 
oral aed» he .. to acute terminal edges of pyramid, 
..+eee.. to inclined diagonals of rhombohedron. 
The lateral edges (e) of the pyramid are inclined in a direction opposite to the edges 
of intersection (f) of the pyramid and rhombohedron. 
Vol. II, Part IL. 3F 
