For the reader will often find that crystals
of a mineral determined to be the same by physical characters, crystalline form,
and optical properties, have been declared by skilful analysers to be composed
of distinct elements. This disagreement seemed at first subversive of the atomic
theory, or the doctrine that there is a fixed and constant relation between the
crystalline form and structure of a mineral and its chemical composition. The
apparent anomaly, however, which threatened to throw the whole science of
mineralogy into confusion, was reconciled to fixed principles by the discoveries
of Professor Mitscherlich at Berlin, who ascertained that the composition of the
minerals which had appeared so variable was governed by a general law, to which
he gave the name of ISOMORPHISM (from isos, equal, and morphe, form). According
to this law, the ingredients of a given species of mineral are not absolutely
fixed as to their kind and quality; but one ingredient may be replaced by an
equivalent portion of some analogous ingredient. Thus, in augite, the lime may
be in part replaced by portions of protoxide of iron, or of manganese, while the
form of the crystal, and the angle of its cleavage planes, remain the same.
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