Pliny gravely tells us, that the oyster produces
pearls from feeding upon heavenly
dew. Our own early writers entertained
the same notion; and Boece, speaking ot the
pearl mussel of the Scottish rivers, says,
that "these mussels, early in the morning,
when the sky is clear and temperate, open
their mouths a little above the water, and
most greedily swallow the dew of heaven;
and after the measure and quantity of the dew
which they swallow, they conceive and breed
the pearl." Harrison says, that the pearls
are also sought for in the latter end of
August, a little before which time the sweetness
of the dew is most convenient for that
kind of fish which doth engender and conceive
them. In the East, the belief is equally
common, that these precious gems are

Rain from the sky,
Which turns into pearls as it falls in the sea.

"But,alas!" as Doctor Baird justly says, "alas!
for poetry and romance! the science of chemistry,
which has with its sledge hammer of
matter-of-fact converted the all-glorious diamond
into vulgar charcoal, has also pronounced
the precious pearl to be composed of concentrate
layers of membrane and carbonate of
lime!" This being admitted, the question
then arises as to the cause of a substance so
dissimilar in appearance to the shell in which
it exists, and why it should be present in
some shells, and absent in others.

In all cases it appears that the ultimate
cause of the animal's forming this beautiful
substance, is to get rid of a source of
irritation. Sometimes this happens to be a
grain of sand, or some such small foreign
substance, which has got between the mantle
of the oyster and the shell; and, proving a
great annoyance, the animal covers it with a
smooth coat of membrane and a layer of
nacre, forming a projection on the interior,
generally more brilliant than the rest of the
shell. At other times it is caused by some
enemy of the inhabitant of the shell
perforating it from the outside, to get within
reach of its prey. The animal, therefore,
immediately plugs up the opening made,
with a coat of nacre; and, shutting out the
intruder, balks it of its nefarious design. In
both these cases the pearl is usually found
adhering to the internal surface of the shell.
The most valuable specimens, are generally
found loose in the muscles or other soft parts
of the animal. This source of irritation is
proved, according to the observations of Sir
Everard Home, to be an ovum, or egg of the
animal; which, instead of becoming ripe, proves
abortive, and is not thrown out by the mother
along with the others, but remains behind in
the capsule in which the ova are originally
contained. This capsule, being still supplied
with blood-vessels from the parent animal,
goes on increasing in size for another year,
and then receives a covering of nacre, the
same as the animal spreads over the internal
surface of the shell. As long ago as sixteen
hundred and seventy-three, Sandius communicated
the same fact to the Royal Society of
London; he was led to it while investigating
the mode of breeding of the fresh-water
mussel, by generally finding in the
ovarium, round hard bodies, too small to be
noticed by the naked eye, having exactly
the appearance of seed-pearls, as they are
called. On further examination into the
structure of pearls, he ascertained that all split
pearls possessed a small central cell, which
surprised him by its extreme brightness of
polish; and, in comparing the size of this cell
with that of the ovum when ready to drop
off from its pedicle, he found it sufficiently
large to inclose it. He came thus to the
conclusion, that these abortive eggs are the
commencement or nuclei of the pearl. When
once formed, the animal continues to increase
its size by the addition of fresh coats of
nacre; adding, it is said, a new layer every
year. Those pearls found in the substance
of the animal are generally round; but
occasionally they are found pear-shaped, from
the pedicle by which the animal is attached
having received a coat of nacre as well as
itself. When the pear-shaped pearls are
perfect, they are the most valuable, as they
are in great demand for ear-drops. The true
pearl is remarkable, as it is well known, for
its beautiful lustre; which cannot be imitated.
According to Sir Everard Home, this peculiar
lustre arises from the central cell being
lined with a highly polished coat of nacre;
and, the substance of the pearl itself being
transparent, the rays of light easily pervade
it.

Mother-of-pearl is the inside lining of the
nacrous shells, which, like the pearl itself,
is composed of alternate layers of very thin
membrane and carbonate of lime; but this
alone does not give the pearly lustre, which
appears to depend on minute undulations or
waves of the layers. This lustre, it is said,
has been successfully imitated on engraved
steel buttons. Sir David Brewster, in the
Edinburgh Encyclopaedia, tells us that the
iridescence of the inside of the pearl-oyster
arises from the circumstance, that we find
in all mother-of-pearl a grooved structure
upon its surface, resembling very closely the
delicate texture of the skin at the top of an
infant's finger, or the minute corrugations
which are often seen on surfaces covered
with varnish or with oil-paint. Similar appearances
are to be seen in the structure of
pearls. In these, the coloured images are
crowded into a small space round the common
image, partly on account of the spherical
form of the pearl; and the various hues are
thus blended into a white uniform light,
which gives to this substance its high value
as an ornament. Pearls, however—at least,
the most valuable—are not perfectly solid;
and in a split pearl the transparency is
considerable. If a split pearl is set in a