The ancient metallic mirrors, which were in use before the present mirrors, or the discovery of glass, and the mode of applying to its surface an amalgam of tin, were composed of two parts of copper and one part of tin. Mr. Mudge asserts, that the best proportion for mirrors is 32 parts of copper and 14.5 parts of tin. Klaproth found a specimen of ancient mirror to consist of 32 of tin, 8 of lead, and 62 of copper. The alloys of copper and tin may be decomposed by dissolving them in an acid, the muriatic for instance, and immersing a sheet of iron, which will precipitate the copper. The tin may then be separated by immersing a plate of lead, or zinc, by either of which metals, it will be precipitated. Bronze, being a mixed metal, in which the copper forms the principal ingredient, is sometimes used in fire-works, in lieu of copper or brass; for its effects are similar. By the combustion of bronze filings, we have an oxide of copper and an oxide of tin. _Sec. XLV. Of Mosaic Gold._ This name, or _aurum musivum_, was given to a preparation of tin, composed of tin and sulphur. It is considered to be a persulphuret of tin. Several methods are recommended for preparing this substance. The oldest process is to sublime a mixture of 12 parts of tin, 7 parts of sulphur, 3 parts of mercury, and 3 parts of sal ammoniac. It may be formed by heating together in a retort, a mixture of equal parts of sulphur and oxide of tin. It is used principally for rubbing the cushions of electrical machines, and for bronzing wood. In fire-works, it is sometimes employed under the name of _gold-powder_. It was supposed to be a combination of sulphur with the oxide of tin. Dr. J. Davy (_Phil. Trans._ 1812, p. 199) and Berzelius, (_Nich. Jour._ xxxv, 165), have proved, however, that it is nothing more than metallic tin and sulphur; the proportions of which, according to the former, are 100 of tin + 56.25 of sulphur. Mosaic gold is of a yellow colour, resembling that of gold. It is insoluble in water, and is not acted upon by muriatic or nitric acid. The nitromuriatic, however, decomposes it. A solution of caustic potassa dissolves it, forming a green solution, which is decomposed by acids, letting fall a hydrosulphuret of tin. It deflagrates with nitre. When it is used in fire-works, it is pulverized, and sifted. It is more generally employed as a pigment to impart a golden colour to small statues of plaster-paris. When mixed with melted glass, it is said to imitate lapis lazuli. _Sec. XLVI. Of Iron and Steel._ Both iron and steel are used abundantly in fire-works. It would be unnecessary to detail the preparations, in which they are employed, which may be seen by a reference to the different kinds of fire, and to their respective formulæ. Cast iron is more employed in artificial fire than forged iron or steel, at least in the preparation of some, as gerbes, white fountains, and Chinese fire.