Professor Plücker has ascertained that certain crystals, in particular
the cyanite, “point very well to the north by the magnetic power of the
earth only. It is a true compass-needle; and more than that, you may
obtain its declination.” Upon this Mr. Hunt remarks: “We must remember
that this crystal, the cyanite, is a compound of silica and alumina
only. This is the amount of experimental evidence which science has
afforded in explanation of the conditions under which nature pursues
her wondrous work of crystal formation. We see just sufficient of
the operation to be convinced that the luminous star which shines in
the brightness of heaven, and the cavern-secreted gem, are equally
the result of forces which are known to us in only a few of their
modifications.”--_Poetry of Science._
Gay Lussac first made the remark, that a crystal of potash-alum,
transferred to a solution of ammonia-alum, continued to increase
without its form being modified, and might thus be covered with
alternate layers of the two alums, preserving its regularity and proper
crystalline figure. M. Beudant afterwards observed that other bodies,
such as the sulphates of iron and copper, might present themselves
in crystals of the same form and angles, although the form was not a
simple one, like that of alum. But M. Mitscherlich first recognised
this correspondence in a sufficient number of cases to prove that it
was a general consequence of similarity of composition in different
bodies.--_Graham’s Elements of Chemistry._