78
PHYSICAL PROPERTIES OF MINERALS.
Double refraction increases from this axis, where it is 0, to a planeat right angles with it.
In some instances, the extraordinary ray is situated between theordinary ray .and the perpendicular; in others, it is exterior to thisray. The former, possess a greater index of refraction for theextraordinary than for the ordinary ray, and the axis is called apositive axis of double refraction. The latter have a less indexof refraction for the extraordinary than ordinary ray, and thereforea negative axis of double refraction.
All crystals possess the doubly refracting structure, except-ing monometric solids, that is. the cube, octahedron, &c., &c.Some crystals, unlike calcareous spar, contain two axes of doublerefraction, or two directions in which the ordinary and extraordi-nary rays are coincident, and where, therefore, double refraction isnot seen.
We remark, preliminary to an explanation of this distinction ofcrystals, into those with one axis, and those with two, that themolecule of a right rectangular prism has three principal sections ;one through the vertical axis and the longer horizontal, anotherthrough the vertical and shorter horizontal, and a third throughthe two horizontal axes. These three planes of section intersectat right angles, and are called the axial planes of the ellipsoid, twoof which are vertical, and one horizontal. With this explanation,we proceed.
If the molecule is an ellipsoid of revolution, in which the twovertical axial planes are equal ellipses , there is but one axis ofdouble refraction.
If the molecule is an ellipsoid, not of revolution, in which thetwo vertical axial planes are unequal ellipses, there are two axesof double refraction.
The crystals of one axis, are included in the classes Dimetricaand Tetraxona; those of two axes, in the classes Trimetrica, Mono-el inata, Diclinata, and Triclinata.
There are a few exceptions to this remark. The most promi-nent is the crystal of water, which, though secondary to a rightrhombic prism, possesses but one axis of double refraction. Butobserving that some rhombohedrons possess positive, some nega-tive axes, and that, consequently, double refraction is not depend-ent on form merely, but probably, also, on the nature of the attrac-tion in the molecular axes, and also the structure of the molecule,we must allow the possibility of this exception, and not so violateall principles of Crystallography as to attempt to derive its formsfrom any dinictric or tetraxoncil solid.
94. Phosphoresence, or the emission of light by minerals, maybe produced in different ways : by friction, by heat, or by exposureto the light of the sun.