The chrome yellow is next placed in suitable receptacles, and dried in the open air or in specially constructed drying rooms, after which it is packed in boxes, kegs, &c., and is then ready for use. The liquid nitrate of potassium, or saltpetre lye, removed from the receptacles in which the chrome yellow is precipitated, and the first water used for washing the chrome yellow, as above described, are placed in large open flat receptacles or excavations, so as to be exposed to the action of the air and sun; or the liquids may be operated on by a small graduation work, so that a great portion of water evaporates. The residue is then heated in suitable vessels or troughs by a slow heat until a salt crust is formed, which, when cooled off and left to dry, is nitrate of potassium or saltpetre in a pure state. From 100 lb. of galena having 80 per cent. metallic lead, some 28 to 30 lb. of pure and dry saltpetre are produced by the above described process. The sulphur produced by the dissolving of the galena by nitric acid is melted in a small stove or furnace in the usual manner, and then refined, so as to produce bars of sulphur called “brimstone.” About 10 lb. of such sulphur are produced from 100 lb. of such galena treated in the manner described. The chrome yellow thus produced is said to be chemically pure, and of great covering power, equal to the best chrome yellow in the market. The process is very simple, and the crude lead ore is transformed into chrome yellow in from three to four days. _Characters._--Pure chromate of lead has an orange-yellow colour, and in whatever manner it be made it always has this tint. Commercially, chromes are made of a great variety of tints, from a pale-lemon chrome to a deep scarlet, through all the intermediate shades of yellows and oranges; in fact, most colour makers produce not less than eight, and some more shades of chromes, whence it is obvious that they cannot be chemically pure, but must be mixed with some other bodies. In commerce, chromes are distinguished as “pure” and “common”: the distinction between them is that the “pure” chromes are made from a lead base, and consist of chromate of lead mixed to a larger or smaller amount with sulphate of lead, the paler shades containing most of the latter; while the “common” chromes are mixed with china clay, barytes, gypsum, or similar bodies. To distinguish the two kinds of chromes, treat with boiling hydrochloric acid. If pure, the chrome will completely dissolve, the solution usually having a green colour. On cooling, crystals of lead chloride separate out. The liquor will give a white precipitate with barium chloride.