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
structural arrangement, your inevitable answer will be, that those
crystals are built by the play of polar forces with which their
molecules are endowed. In virtue of these forces, molecule lays
itself to molecule in a perfectly definite way, the final visible form
of the crystal depending upon this play of its ultimate particles.

Everywhere in Nature we observe this tendency to run into definite
forms, and nothing is easier than to give scope to this tendency by
artificial arrangements. Dissolve nitre in water, and allow the water
slowly to evaporate; the nitre remains and the solution soon becomes
so concentrated that the liquid condition can no longer be preserved.
The nitre-molecules approach each other, and come at length within the
range of their polar forces. They arrange themselves in obedience to
these forces, a minute crystal of nitre being at first produced. On
this crystal the molecules continue to deposit themselves from the
surrounding liquid. The crystal grows, and finally we have large
prisms of nitre, each of a perfectly definite shape. Alum crystallizes
with the utmost ease in this fashion. The resultant crystal is,
however, different in shape from that of nitre, because the poles of
the molecules are differently disposed. When they are _nurs