Pearl

(pürl.) [Fr. perle.] (Nat. Hist.) Ahard, white, shining body, usually ofa globular, but sometimes ofa pyriform shape. It is formed by certain bivalve mollusks, belonging to the fam. Aviculide. The Oriental pearls of commerce are obtained from the Avicula mar gar it if era, and other species of shell-fish called"pearl oysters." Pearls consist of concentric layers of a fine compact nacre, or substance identical with that which lines the inside of the shell, the layers being alternatively membrane and carbonate of lime. It is this structure which occasions the play of light called pearly •iridescence. They are sometimes found free, and detached from the lobes of the mantle; but are most usually adherent to the nacrous coat of the shell, which, on that account, is termed mother-of-pearl. They are the consequence of a disease in the fish,caused by the introduction of foreign bodies within the shells. Pearls were held in the highest estimation by the ancients, with whom they were of an enormous price. In modern times their value is greatly le ened-probably on account of the beautiful imitations which may be obtained at a trifling cost. When pearls are very small, they are termed seed-pearls. The seas about the East Indies and America yield pearl-fish in great abundance. In the east, the coasts of the island of Ceylon and the Persian Gulf are the localities most celebrated for pearl fisheries; and in the west, the coast of Terra-Firma and the Gulf of Mexico. The worth of a pearl is in proportion to its magnitude, roundne of form, polish, and clear lustre. Sometimes, but very rarely, ap earl is found as large as a nutmeg.-Artificial pearls are small globules or pear shaped spheroids of thin gla , perforated with two opposite holes, through which they are strung, and mounted into necklaces, ., like real pearl ornaments. The liquor employed to imitate the pearly lustre is called e ence d'Orient, which is prepared by throwing into water of ammonia the brilliant lam ellæ separated, by washing and friction, from the scales of the bleak, a small river-fish. The ammonia renders them sufficiently soft and flexible to adhere closely to the inner surface of the gla , and pa es off by the drying. The French are particularly succe ful in this manufacture. There are various other methods of imitating pearls, in which also the French are said to excel. [s. 691]