* * * * *
THE PEARL IN THE OYSTER.
Cowper eloquently says
There is glory in the grass, and splendour in the flower;
and the imagery might have been extended to the irridescent pearl
within the rudely-formed shell of the oyster. Poets have feigned that
pearls are
Rain from the sky,
Which turns into pearls as it falls in the sea;
we need scarcely add that science has exploded this imaginative
fertility.
Pearl is, in fact, a calcareous secretion by the fish of bivalve
shells; and principally by such as inhabit shells of foliated
structure, as sea and fresh water muscles, oysters, &c. A pearl
consists of carbonate of lime, in the form of nacre, and animal matter
arranged in concentric layers around a nucleus; the solution
indicating no trace of any phosphate of lime. To this lamellar
structure the irridescence is to be ascribed. Each layer is _presumed_
to be annual; so that a pearl must be of slow growth, and those of
large size can only be found in full-grown oysters. The finest and
largest are produced from the Meleagrina margaratifera, (_Lamarck_,) a
native of the sea, and of various coasts. A considerable number are
likewise taken from the Unio margaratifera, which inhabits the rivers
of Europe; and, it is singular, as remarked by Humboldt, that though
several species of this genus abound in the rivers of South America,
no pearls are ever found in them.
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