infinite
number of lines, parallel to the ideal axis of the figure. The same
irregularity carried to a greater extent, frequently causes certain
faces required for the symmetry of the form, altogether to disappear.
Again, some crystals do not fill the space marked out by their outline,
holes and vacancies being left in the faces, occasionally to such an
extent that they seem little more than mere skeletons. This appearance
is very common on crystals produced artificially, as in common salt,
alum, bismuth, silver, &c. A perfect crystal can only be produced
when, during its formation, it is completely isolated, so as to have
full room to expand on every side. Hence the most perfect crystals have
been originally imbedded singly in some uniform rock mass. Next to them
in perfection are forms that grow singly on the surface of some mass of
similar or distinct composition, especially when the point of adherence
is small. An incompleteness of form, or at least a difficulty in
determining it, arises from the minuteness of some crystals, or from
their contracted dimensions in certain directions. Thus some appear
mere tabular or lamellar planes, while others run out into acicular,
needle-shaped, or capillary crystals. Amid all these modifications of
the general form of the crystal, of the condition and aspect of its
individual faces, or of its linear dimensions, one important element,
the angular measurement, remains constant. In some monoaxial crystals,
indeed, increase of temperature produces an unequal expansion in
different directions, slightly changing the relative inclination of
the faces, but so small as to be scarcely perceptible in common
measurements, and hence producing no ambiguity. More important are the
angular changes which in many species accompany slight changes in
chemical composition, particularly in the relative proportions of
certain isomorphous elements. But notwithstanding these limitations,
the great truth of the permanence of
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