m 
Mohs' System of Crystallography 
the preceding. Such are those that possess the greatest, some- 
times even a geometrical regularity in their simple forms, and 
the highest degree of symmetry in their combinations. They 
can all be derived from the hexahedron (cube), by placing it 
upright, and considering the position of a moveable plane with 
regard to its faces and edges. Forms of this sort never assum- 
ing infinite directions, have no limit like the preceding. Some 
of them, however, have constant dimensions, while the dimen- 
sions of others are variable. The former may be viewed as 
limits of the latter. 
55. Their hinds and differences. — ^The forms now under 
consideration are, 1 . The hexahedron ; 2. The octahedron, in- 
cluding the tetrahedron ; 3. The dodecahedron ; 4. The icosite- 
trahedron. The first two have neither variable dimensions nor 
different species, the rest have both these properties. The spe- 
cies belonging to the dodecahedron are the rhomboidal dodecahe- 
dron, (whose dimensions are constant,) and the pentagonal do- 
decahedron, of which sort we already know several that differ in 
their dimensions. The species included under the icositetrahe- 
dron, are the trigonal and the tetragonal icositetrahedron. The 
first results from enlarging the faces /, Fig. 37., in lead-glance ; 
it is bounded by twenty-four equal and similar isosceles tri- 
angles : Those belonging to the other species consist of twenty- 
four some of which have two equal plane angles, 
and give rise to the first subspecies of the tetragonal-icositetrahe- 
dron. Two of its varieties are known, Fig. 1 91 • of Analcime re- 
presents the one, the other results from enlarging the faces r, 
Fig. 104. of Pleonaste. The second subspecies of tetragonal-ico- 
sitetrahedron has not two equal plane angles ; three of its varieties 
are known, one of which arises from enlarging the faces f, 
(Fig. 148.) of pyrites, the other from enlarging the faces s or o, 
and n {Tableau Comparatif, PI. iv. Fig. 60.) of the same mi- 
neral. It will be convenient to omit the more extensive of these 
species. 
56. Combinations. Well-defined Limits, separating the Forms 
hitherto considered. — The forms that arise from the hexahedron 
produce among themselves various combinations ; but they ad- 
mit into them no form, which either is a rhomboid, or a Jhur- 
fided pyramid with a square base, or an oblique fbur^sided 
