50 



CRYSTALLOGRAPHY. 



rhombic; and instead of having twelve rectangalar edges, thej 

 have six obtuse edges and six acute. 



2. Scalenohedrons, fig. 7. Scalenohedrons are really double 

 six-sided pyramids ; but the six equal faces of eaoh extremity 



14. 



15. 



FIGURES OF CRYSTALS OF CALCITE. 



of the crystals are scalene triangles, and are arranged in three 

 pairs ; moreover the pairs above alternate 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 prisms, 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 tlrree 

 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. 



the rhombohedron has six terminal edges, three above and 



tnree below, and six 



lateral edges. As these lateral edges 



are symmetrically situated around the centre of the crystal, the 

 three lines connecting the centres of opposite basal edges will 

 cross at angles of 60°. These lines are tho lateral axes of the 



