skins
in a pearl of that size, the divisions could not mark either years,
seasons, or breeding periods. In some experiments made by Mr. Stross,
he found that borings made to the interior of a living mollusk's shell
were closed by a film of hard nacre in two days.
The
known facts about a pearl are these. It is composed of about ninety-two
per cent, carbonate of lime, about six per cent, organic matter and a
little over two per cent, water in combination almost identical with
the lining of the shell in which it grows and similar to the mineral
aragonite. In construction it is usually a series of layers, which can
sometimes be peeled off entirely, each one successively enveloping its
predecessors apparently as an independent structure though itself
composed of a number of thin lapping waves. Upon cutting through these
layers the divisions appear as a series of rings and the intervals,
though composed of many thin waves, appear compact. It grows
spherically or with such modifications as the exigencies of position in
the shell would reasonably account for. These facts seem to justify
the hypothesis that a foreign substance upon
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