6
CRYSTALLOGRAPHY.
A cube, fig. 1, is one of the primaries; the same with its anglesremoved, fig. 2, is a secondary to the cube, and the planes a, a, onthe angles, are called the secondary planes. The cube, with itsedges removed, fig. 5, is another secondary to this solid; and asabove, the planes e are secondary planes, and P, the remainingparts of the primary.
The occurrence of these forms is governed by two importantlaws, on which the value of the science of Crystallography to themineralogist mainly depends: 1. that the same mineral presentsuniversally the same primary form, and always, when crystallized,exhibits either this primary or some secondary to it; 2. that aparticular' primary is invariable in its interfacial angles, and theinterfacial angles of its similar secondary planes.
Thus galena always crystallizes in cubes, or secondaries to thisprimary; calcareous spar, in oblique prisms of constant angles, orsome modification of this form, produced by a simple law, whichwill be given in a future section.
We are thus enabled, by a determination of one or two angles,to arrive with certainty at the names of most of the mineral species,when they occur in regular crystals.
CHAPTER I.
PRIMARY FORMS.
COMPARATIVE VIEW OF THE PRIMARY FORMS.
6. The primary solids are fourteen in number, and may be dis-tributed as follows : 1. Prisms ; 2. Octahedrons ; 3. Dodecahedrons .
The prisms have either a six-sided base, or a four-sided base.
Of the former there is but one instance in the mineral king-dom. Its name, derived from the nature of the base, a regularhexagon, is the Hexagonal Prism, fig. 114.
The prisms with tetragonal bases are either right or oblique,(§ 3, l.) ;md are named according to their bases.
1. Right Prisms.
Base a square; lateral planes equal to basal.Base a square; lateral planes not equal to basal.Base a rectangle, (§ 5, e.)
Base a rhomb, (§ 5, e.)
Base a rhomboid, (5 5, e.)