The menstruum best adapted for electro-plating is a solution of silver in cyanide of potassium. During the process of deposition, the same quantity of metal that is deposited from the liquid on the objects connected with the negative pole of the battery is restored to it, by dissolving an equal quantity from the silver plate connected with the positive pole, and in this manner the solution is maintained at the same strength. Any thickness of silver may be deposited by continuing the process, but about one ounce and a half to a square foot of surface is considered a full quantity. Those parts on which no silver is required to be deposited are covered with varnish or wax, which protects them from the voltaic action. Where the operation of electro-plating is carried on extensively, decomposing troughs, holding nearly 300 gallons, are employed, and the silver plates in a single trough expose a surface of thirty square feet to the dissolving action of the menstruum under the influence of the voltaic battery. By the aid of electro-plating the most elaborate designs of the artist in metal can be covered with silver on every part; and a group, finely engraved in copper, may be made to resemble in every particular a work cut out of solid silver. The metal is usually deposited in a granulated state, resembling "frosted" silver, and the parts required to be bright are subsequently burnished; but by the addition of a few drops of the sulphuret of carbon to the solution, the silver may be precipitated perfectly bright. GAS LIGHTING. The invention of Gas Lighting had its origin in the earliest times of history; not, indeed, as we now see it, burning at the end of a pipe supplied with gas made artificially, and stored in gas-holders, but blazing from fissures in the ground, supplied from natural sources in the bowels of the earth. The Greek fire-altars are supposed to have been supplied with gas, either issuing from bituminous beds, or made artificially by the priests, by pouring oil on heated stones placed in cavities beneath. Fountains of naphtha, and fires issuing from the earth at Ecbatana, in Media, are mentioned by Plutarch in his life of Alexander, and many other ancient historians record the knowledge of similar instances of natural gas lighting. In later times, the inflammable gas issuing into the galleries of coal mines, and either exploding when mixed with atmospheric air, or blazing as it issued from fissures in the coal, afforded instances of the natural production of gas, the ignition of which too frequently proved fatal to those in the mines.