NOTE ON STELLIFORM CRYSTALS. 3 
regular octahedron on the other, will result.* These forms, moreover, 
as well as their intermediate oscillations, frequently occur in the same 
substance : red oxide of copper may be cited as an example. But 
between the cube, a square prism, and a rhombic prism, no relations 
of this kind exist. Neither are these forms related physically: for 
their optical, thermal, and other physical relations are equally 
distinct. By considerations of this sort, therefore, we are able to 
establish six (or really seven) distinct Crystal Systems. These (named 
chiefly in accordance with the relations of their axes) are enumerated 
in the annexed tabular view.t 
Crystal-axes of one) The Monometric System (including the cube, 
length. Reratin, | rhombic dodecahedron, octahedron, &c., 
single with their various combinations. 
) The Dimetrie System (including square- 
based prisms and pyramids with their 
various combinations.) 
The Hexagonal System (including regular 
hexagonal prisms and pyramids, rhom- 
J bohedrons, &c., with their combinations.) 
sy ‘| The Trimetric System (in- 
cluding right rectangular 
Axes at right- prisms and pyramids, 
angles. rhombie prisms and 
Crystal-axes of three pyramids, and combi- 
lengths. Refraction, J nations of these. 
double, with two t \ The Monoclinic System 
One axis ob-} (including oblique ree- 
lique. { tangular and rhombic 
J combinations.) 
| All the axes ) Lhe Triclinie System (in- 
cluding doubly-oblique 
combinations. ) 
Crystal-axes of two 
lengths. Refraction, 
double, with one 
neutral line or op- 
RICME GNIS et 8 
neutral lines or op- 
tical axes. 
J oblique. 
* The Law of Symmetry, in its exact acceptation, may be thus expressed : 
(1.) If an edge or angle of a crystal be modified, all the similar edges or angles must be 
equally modified. 
Or (2.) One-half or one-mth of the corresponding angles or edges, in alternate positions, 
must be equally modified. Hxample—Cube and Tetrahedron (Boracite; Arseniate of Iron.) 
Or (8.) All the similar edges or angles must be modified by one-half or one-mth the 
normal or regular number of planes. Fxample.—Cube and Pentagonal Dodecahedron 
(Iron Pyrites.) 
Conditions 2 and 3 produce hemthedrons or part-forms- 
+ See also Vol. V. of this Journal (New Series), pages 7-9. 
