MOSAIC

( Mosaïque , Fr.; Mosaisch , Germ.) There are several kinds of mosaic, but all of them consist in imbedding fragments of different coloured substances, usually gla or stones, in a cement, so as to produce the effect of a picture. The beautiful chapel of Saint Lawrence in Florence, which contains the tombs of the Medici, has been greatly admired by artists, on account of the vast multitude of precious marbles, jaspers, agates, avanturines, malachites, ., applied in mosaic upon its walls. The detailed discu ion of this subject belongs to a treatise upon the fine arts. MOTHER OF PEARL ( Nacre de Perles , Fr.; Perlen mutter , Germ.); is the hard, silvery, brilliant internal layer of several kinds of shells, particularly oysters, which is often variegated with changing purple and azure colours. The large oysters of the Indian seas alone secrete this coat of sufficient thickne to render their shells available to the purposes of manufactures. The genus of shell fish called pentad inæ furnishes the finest pearls, as well as mother of pearl; it is found in greatest perfection round the coasts of Ceylon, near Ormus in the Persian Gulf, at Cape Com or in, and among some of the Australian seas. The brilliant hues of mother of pearl, do not depend upon the nature of the substance, but upon its structure. The microscopic wrinkles or furrows which run acro the surface of every slice, act upon the reflected light in such a way as to produce the chromatic effect; for Sir David Brewster has shown, that if we take, with very fine black wax, or with the fusible alloy of D’Arcet, an impre ion of mother of pearl, it will po e the iridescent appearance. Mother of pearl is very delicate to work, but it may be fashioned by saws, files, and drills, with the aid sometimes of a corrosive acid, such as the dilute sulphuric or muriatic; and it is polished by colcothar of vitriol.