8 CRYSTALLOGRAPIIY. 
2. DESCRIPTIONS OF CRYSTALS. 
In describing crystals there are two subjects for considera- 
tion: First, Form; and secondly, SrructurRr. 
A. Form.—Under form come up for description, not only 
the general forms of crystals, but also— 
(1.) The systems of crystallization, that is, the relations ot 
all crystalline forms, and their classification. 
(2.) The mutual relations of the planes of a crystal as ascer- 
tained through their positions and the angles between them. 
(3.) The distortions of crystals. The perfection of symmetry 
exhibited in the figures of crystals, in which ail similar planes 
are represented as having the same size and form, is seldom 
found in nature, and the true form is often greatly disguised by 
this means. The facts on this point, and the methods of avoid- 
ing wrong conclusions need to be understood, and these are 
given beyond. With all such imperfections the angles of crys- 
tals remain essentially constunt. There are irregularities also 
from other sources. , 
(4.) Twin or compound crystals. With some species twins 
are more common than regular crystals. 
(5.) Crystalline aggregates, or combinations of imperfect 
crystals, or of crystalline grains. 
Explanations of Terms. 
The following are explanations of a few terms used in connection 
with this subject: 
1. Octahedron.—A solid bounded by eéght equal triangles. They are 
equal equilateral triangles in the regular octahedron (Fig. 2, p. 17) ; 
equal isosceles triangles in the square octahedron (Fig. 17, p. 82) ; equal 
inequilateral triangles in the rhombic octahedron (Fig. 8, p. 37). 
2. Doudle six-sided pyram’ds. Double eight-sided pyramids. Double 
twelve-sided pyramids.—Solids made of two equal equilateral six-sided, 
or eight-sided, or twelve-sided, pyramids placed base to base (Fig. 20, 
p. 82, and 6, 10, pp. 46, 47). 
3. Right prisms. Oblique prisms.—Right prisms are those that are 
erect, all their sides being at right angles to the base. When inclined, 
they are called oblique prisms. 
4. Interfacial angle.-—Angle of inclination between two faces or planes. 
5. Similar planes. Similar angles.—The lateral faces of a square 
prism (Fig. 2, p. 14) are equal and have like relations to the axes, and 
hence they are said to be similar. Solid angles are similar when the 
plane angles are equal each for each, and the enclosing planes are sev- 
erally similar in their relations to the axes. 
6. Truncated. Bevelled.—An edge of acrystal is said to be truncited 
when it is replaced by a plane equally inclined to the enclosing planes, 
as in Fig. 18, p. 19; and it is bevelled when replaced by two planes 
