Now this idea of structure, as produced by polar force, opens a way
for the intellect into an entirely new region, and the reason you
are asked to accompany me into this region is, that our next inquiry
relates to the action of crystals upon light. Prior to speaking of
this action, I wish you to realise intellectually the process of
crystalline architecture. Look then into a granite quarry, and spend a
few minutes in examining the rock. It is not of perfectly uniform
texture. It is rather an agglomeration of pieces, which, on
examination, present curiously defined forms. You have there what
mineralogists call quartz, you have felspar, you have mica. In a
mineralogical cabinet, where these substances are preserved
separately, you will obtain some notion of their forms. You will see
there, also, specimens of beryl, topaz, emerald, tourmaline, heavy
spar, fluor-spar, Iceland spar--possibly a full-formed diamond, as it
quitted the hand of Nature, not yet having got into the hands of the
lapidary.

[Illustration: Fig. 24.]

These crystals, you will observe, are put together according to law;
they are not chance productions; and, if you care to examine them more
minutely, you will find their architecture capable of being to some
extent revealed. They often split in certain directions before a
knife-edge, exposing smooth and shining surfaces, which are called
planes of cleavage; and by following these planes you sometimes reach
an internal form, disguised beneath the external form of the crystal.
Ponder these beautiful edifices of a hidden builder. You cannot help
asking yourself how they were built; and familiar as you now are with
the notion of a polar force, and the ability of that force to produce