students of mineralogy to understand how it was
that substances of the same composition might yet have what seemed to
be different crystalline forms. Calcspar, for instance, might be found
crystallized in forms, apparently, quite at variance with one another.

By his studies, however, Hauey was able to determine that whenever
substances of the same composition crystallized, even though the
external form of the crystals seemed to be different, all of them were
found to have the same internal nucleus. Whenever the mineral under
observation was chemically different from another, then the nucleus
also had a distinctive character; and so there came the law that all
substances of the same kind crystallized in the same way,
notwithstanding apparent differences. Indeed, one of the first results
of this law was the recognition of the fact that when the crystalline
forms of two minerals were essentially different, then, no matter how
similar they might be, there was sure to be some chemical difference.
This enabled Hauey to make certain prophecies with regard to the
composition of minerals.

A number of different kinds of crystals had been classed together
under the name of {180} heavyspar. Some of these could not, by the
splitting process, be made to produce _nuclei_ of similar forms, and
the angles of the crystals were quite different. Hauey insisted that,
in spite of close resemblances, there was an essential distinction in
the chemical composition of these two different crystalline
formations; and before long careful investigation showed that, while
many of the specimens called heavyspar contain barium, some of them
contain a new substance--strontium--which had been very little
studied heretofore. This principle did not prove to be absolute in its
application; but the amount of truth in it attracted attention to the
subject of crystallography because of the help which that science
would afford in the easy recognition of the general chemical
composition of mineral substances. The most important part of Hauey's
work was the annunciation of the law of symmetry. He emphasized the
fact that the forms of crystals are not irregular or capricious, but
are very constant and definite, and founded on absolutely fixed and
ascertainable laws. He even showed that, while from certain
crystalline nuclei sundry secondary forms may be derived, there are
other forms that cannot by any possibility occur. Any change of
crystalline form noticed