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300 PRECIOUS STONES.
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the
" mother-of-Pearl," or nacreous material which lines the interior of
the shell. It consists of that form of carbonate of lime which is known
as " aragonite," being harder, and heavier than " calcite," the other,
and commoner kind. Pearls are secreted by the mantle of the mollusc in
the same way as that by which the shell is formed. It may be asked, "
How does the detached Pearl first come into being ? " Its occurrence is
the outcome of irritation to the mantle caused by the intrusion of some
foreign body: which foreign body is frequently a minute parasitic
animal (a Cestode larva).
The
Chinese take advantage of such action, introducing for their purpose
some such a foreign body within the bivalve shells of a fresh-water
mussel (Dipsas plicata). They keep the mussels in a tank, and
insert between the shell and the body of the mollusc rounded bits of
mother-of-pearl, or little metal images of Buddha. In either case the
object introduced becomes slowly coated, and cemented to the shell. A
specimen of this may be seen in the Shell Gallery of the Natural
History Museum, at Kensington.
"Pearls,"
wrote Leonardus (London, 1750), "have physical virtues exceeding the
commodiousness of Ornament : being boil'd in meat they cure the Quartan
Ague; bruised, and given with milk, they heal putrid Ulcers; and, being
so taken, wonderfully clear the voice. If mixed with Sugar they yield
help in Pestilential Fevers ; and they render him who carries them
chaste."
The
" oneirocritics," or interpreters of dreams, were wont to draw their
interpretations from Pearls. Among the medicines given to Charles VI.
of France, for trying
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