50 CRYSTALLOGRAPHY. 
rhombic; and instead of having twelve rectangular edges, they 
have six obtuse edges and six acute. ; 
2. Scalenohedrons, fig. 7. Scalenohedrons are really double 
six-sided pyramids; but the six equal faces of each extremity 
14. 1d. 

FIGURES OF CRYSTALS OF CALCITE. 
of the crystals ave scalene triangles, and are arranged in three 
pairs; moreover the pairs: above ‘alter nate with the pairs below ; 
the edges in which the pairs above and below meet—that is the 
basal edges—make a zig-zag around the crystal. 
3. Hexagonal prisins, J, fig. 8. These are regular hexagonal 
prisms, having angles between their faces of 120°. 
A rhombohedron has two of its solid angles made up of three 
equal plane angles. When in position the apex of one of these 
solid angles is directly over that of the other, as in figs. 1 to 6, 
and also in fig. 18, and the line connecting the apices of these 
angles (fig. 18) is called the vertical axis. In this position 
18. 19. 20. 21. 

the rhombohedron has six terminal edges, three above and 
three below, and six lateral edges. As these lateral edges 
are synimetrically situated around the centre of the crystal, the 
three lines connecting the centres of opposite basal edges will 
evoss at angles of 60°. These lines are the /ateral axes of tha 
