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46
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A POPULAR TREATISE ON GEMS.
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lateral edges lie in one plane, and form a rhomb. They
have eight polar edges,—four acute and four more obtuse,
—and four lateral edges, and six rhombic angles, the most
acute at the extremities of the longest axis. (2d.) The
rhombic sphenoids (fig. 49)
are bounded by four scalene
triangles with their lateral
edges not in one plane ;
and are a hemihedric form
of the rhombic pyramid of
unfrequent occurrence. The
open forms again are,—(3d.)
Rhombic prisms bounded
by four planes parallel to
one of the axes which is
indefinitely extended. They are divided into upright and
horizontal prisms, according as either the principal or one of
the lateral axes is supposed to become infinite. For the
latter form the name doma or dome has been used ; and
two kinds, the macrodome and the braehydome, have been
distinguished. Rhombic pinacoids also arise when one axis
becomes=0, and the two others are indefinitely extended.
In
deriving these forms from a primary, a particular rhombic pyramid Ρ is
chosen, and its dimensions determined either from the angular
measurement of two of its edges, or by the linear proportion of its
axes a: b: c; the greater lateral axis h being assumed
equal to 1. To the greater lateral axis the name macrodiagonal is
frequently given ; to the shorter, that of brachydiagonal ; and the two
principal sections are in like manner named macrodiagonal and
brachydiagonal, according to the axis they intersect. The same terms
are applied throughout all the derived forms, where they consequently
mark only the position of the faces in respect to the axes of the
fundamental crystal,
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