PHYSICAL PROPERTIES OF MINERALS.. 89
allel
lines, passing through every atom of the crystal. It is also important
to remark that this property divides the systems of crystallization
into three precise groups,—the tesseral, with single refraction ; the
tetragonal and hexagonal, with double refraction, and monoaxial; the
other three systems also double, but binaxial. It is therefore of use
to determine the system to which a mineral belongs, but is not of great
value as a character for distinguishing species.
Polarization of Light.—Intimately
connected with this property is that of the polarization of light,
which being more easily and precisely observable than double
refraction, is in many cases of higher value as a mineralogical
character. By this term is meant a peculiar modification which a ray of
light undergoes, in consequence of which its capability of being
transmitted or reflected towards particular sides is either wholly or
partially destroyed. Thus, if from a transparent prism of tourmaline
two thin plates are cut parallel to its axis, they will transmit light,
as well as the prism it self when they are placed above each other with
the chief axis of both in the same direction. But when the one slip of
tourmaline is turned at right angles to the other, either no light at
all or verv little is transmitted, and the plates consequently appear
black. Hence, in passing through the first slip the rays of light have
acquired a peculiar property, which renders them incapable of being
transmitted through the second, except in a parallel posi-tion,
and they are then said to be polarized. The same property is acquired
by a ray of light when reflected, at an angle of 35 1/2° (or angle of
incidence 54 1/2°), from a plate of glass, one side of which is
blackened, or from some other non-metallic body. When such a ray falls
on a second similar mirror at an equal angle, but so that the plane of
reflection in the second is at right angles to that in the first, it is
no longer reflected, but wholly absorbed.
