From the top a wide pipe conducts away fume which rises into it to one of the leaden condensing chambers already described.

It is to the advantage of the refiner to save as much as possible of the nitric acid evolved during the nitric acid process. At Johnson and Matthey's works this is effected by carrying the stoneware pipe attached to the mouth of the platinum vessel into a capacious stoneware jar, shaped like a chemist's Wolffe'a bottle, and from this by long stoneware pipes placed vertically and rising to the roof of the chamber, and then bent downwards, to 2 other similar bottles. In these bottles the greater part of the acid condenses and is collected. So much of the nitrous acid as is not condensed with the nitric acid is conducted by a pipe proceeding from the last bottle to the back part of the ashpit of a coke fire, which is always in use and well kept up. Nuisance is thus avoided. (Dr. Ballard's Report.)

Rossler improves on the sulphuric acid process by allowing the sulphuric acid solution of silver to crystallize, with addition of a little water. The yellow crystals of silver sulphate are then put into water, and metallic iron is gradually added, by which all the silver is precipitated, but the copper remains in solution. The impurities introduced with the iron are removed in the slag on fusion, and the silver is purer than when the copper is deposited with it.

Coinage gold, not containing more than 1/10 silver, is generally refined by Millers chlorine process, which consists in melting the gold in a clay crucible glazed inside with borax, and passing chlorine gas through the liquid metal from a clay tube; the silver is attacked immediately by the chlorine, and rises to the surface as chloride, while the chlorides of other metals present (e.g. antimony, arsenic, bismuth, zinc) are volatilized, leaving the gold pure, except about 3 to 9 parts of silver per 1000. On the top of the fused mass in the crucible is put a layer of melted borax, which prevents volatilization of the silver chloride. Absolutely pure gold may be prepared by Rossler's process just described, or by the following method: - The alloy is dissolved in aqua - regia, and concentrated to drive off the nitric acid; after diluting with water, and filtering, the metallic gold is precipitated by a reducing agent, ferrous sulphate (FeS04) being commonly employed. Another good reducing agent is oxalic acid; on adding a little oxalic acid and an excess of potassium carbonate to a gold solution, a clear solution is produced, which decomposes on further addition of oxalic acid, and precipitates the gold when quickly heated to boiling.

The character of the gold Varies with the mode of precipitation, thus: - (1) The gold is obtained in very fine powder when the gold solution is poured into the ferrous sulphate solution; (2) it is scaly and more lustrous when the ferrous sulphate is poured into the gold solution; (3) it forms a fine yellow spongy mass when precipitated by oxalic acid. It is rendered compact and coherent by fusion with borax and nitre.

The physical properties of gold render it one of the most useful metals. Whilst resisting the action of oxygen and of water at all temperatures, its divisibility is such that 1 gr. can be made to coat 2 miles of fine silver wire; and its extreme ductility and malleability are shown by the fact that 1 gr. can be hammered out to cover an area of 56 3/4 sq. in., or so that 280,000 leaves are required to make a thickness of 1 in.; and its wire can be drawn so fine that 1 gr. will make 700 ft. It is insoluble in all simple acids but selenic, and it suffers no change when fused with potash chlorate; but it is attacked by nitrates and alkalies, and dissolves readily in any acid liquid evolving chlorine (e.g. aqua - regia). Its sp. gr. is 19.265, and its fusing - point is placed by different authorities at 1898° to 2517° F. (1037° to 1381° C).

The reader desiring more detailed information on "gold - mining " is referred to A. G. Lock's 'Gold: Its Occurrence and Extraction/