COMPOUNDS. With oxygen gold forms a mon oxide, or aurous oxide, and a trioxide, or auric oxide. The former is obtained by decomposing aurous chloride with cold dilute potassium hy droxide; the latter by heating a solution of gold trichloride with an excess of magnesia and well washing the precipitate with nitric acid. Aurie oxide, which is the more common of the two, combines with bases, forming salts called au rates. Perhaps the most important of the com pounds of gold with acids is auric chloride, which is readily obtained by dissolving metallic gold in aqua regia and evaporating the solution to crystallization. The resulting orange-red crys tals may be further purified by recrystallization. It is a very deliquescent salt, and is chiefly em ployed for toning silver prints in photography.
Fulminating Gold, which was originally de scribed in a work published under the name of Basil Valentine, is a green or brown powder that readily explodes when dry; it may be obtained by the action of ammonia on gold hydroxide, or by precipitating gold chloride with ammonia or its carbonate.
Gold Purple, or Purple of Cassius, which was originally prepared by Andreas Cassius, and de scribed in 1685, is a flocculent purple precipitate obtained by treating a solution of stannous and stannic chlorides with gold chloride. The re sulting product is believed to be a mixture of tin oxide and finely divided gold. The color of ruby glass is due to small proportions of this pigment.
Mosaic Gold is a fine flaky yellow variety of tin bisulphide; it is prepared by heating a mixture of seven parts of sulphur, six parts of ammonium chloride, and eighteen parts of a powdered amalgam consisting of two parts of tin to one of mercury. When the odor of hydrogen sulphide is no longer perceptible, the heat is raised to low redness, and the mercurous chloride, ammonium chloride, and mercuric sulphide are volatilized. The mosaic gold thus obtained is used as an imitation bronze in the arts.