DEFINITIONS. 



11 



Fig. 22. 



A doubly oblique prism, fig. 22, whose bases and 

 whose lateral planes are general!?/ oblique-angled paral- 

 lelograms. The only equality subsisting among these 

 planes, is between each pair of opposite or parallel 

 ones. 



Fig. 23. 



The rhomboid, fig. 23, a solid contained within six 

 equal rhombic planes, and having two of its solid 

 angles, and only two, as a, b, composed each of three 

 equal plane angles ; these are sometimes called the 

 summits. 



Fig. 24. 



The regular hexagonal prism, fig. 24, or right prism 

 whose bases are regular hexagons. 



The secondary forms of crystals consist of all those 

 varieties belonging to each species of mineral, which 

 differ from the primary form. 



These, although extremely numerous, may be re- 

 duced to a few principal classes, as will appear in the 

 sequel. 



B 2 



