§ 857. Carbonate of Zinc, in the native form of calamine, contains, as is well known, 64·8 per cent. of oxide of zinc; but the carbonate obtained in the course of an analysis by precipitating the neutral hot solution of a soluble salt of zinc by carbonate of potash or soda, is carbonate of zinc plus a variable quantity of hydrated oxide of zinc. Unless the precipitation takes place at a boiling temperature, the carbonic anhydride retains a portion of the oxide of zinc in solution. By ignition of the carbonate, oxide of zinc results.
§ 858. Oxide of Zinc (ZnO = 81; specific gravity, 5·612; Zn, 80·24, O, 19·76) is a white powder when cool, yellow when hot. If mixed with sufficient powdered sulphur, and ignited in a stream of hydrogen, the sulphide is produced; if ignited in the pure state in a rapid stream of hydrogen gas, metallic zinc is obtained; but, if it is only a feeble current, the oxide of zinc becomes crystalline, a portion only being reduced.
§ 859. Sulphide of Zinc (ZnS = 97; specific gravity, 4·1; Zn, 67·01, S, 32·99).—The sulphide obtained by treating a neutral solution of a soluble salt of zinc by hydric sulphide is hydrated sulphide, insoluble in water, caustic alkalies, and alkaline sulphides, but dissolving completely in nitric or in hydrochloric acid. When dry, it is a white powder, and if ignited contains some oxide of zinc. The anhydrous sulphide is produced by mixing the precipitated sulphide with sulphur, and igniting in a crucible in a stream of hydrogen gas.
Pharmaceutical Preparations.—The officinal compounds of zinc used in medicine are the acetate, carbonate, chloride, oxide, sulphate, sulphocarbolate, and valerianate.
Sulphate of Zinc (ZnSO47H2O 161 + 126; specific gravity, crystals, 1·931).—This salt is officinal in all the pharmacopœias, is used in calico-printing, and is commonly known as white vitriol. By varying the temperature at which the crystals are allowed to be formed, it may be obtained with 6, 5, 2, or 1 atoms of water. The commercial sulphate is in crystals exactly similar to those of Epsom salts; it is slightly efflorescent, and gives the reactions of zinc and sulphuric acid.
§ 860. Chloride of Zinc is obtained by dissolving zinc in hydrochloric acid, or by direct union of zinc and chlorine. Chloride of zinc is the only constituent in the well-known “Burnett’s disinfectant fluid.” A solution of chloride of zinc may be heated until it becomes water-free; when this takes place it still remains fluid, and makes a convenient bath, for warmth may be applied to it above 370° without its emitting fumes to inconvenience; at a red heat it distils. A concentrated solution of zinco-ammonic chloride (2H4NClZnCl2) is used for the purpose of removing the film of oxide from various metals preparatory to soldering.
§ 861. Zinc in the Arts.—The use of zinc as a metal in sheeting cisterns, articles for domestic use, alloys, &c., is well known; oxide of zinc enters largely into the composition of india-rubber. Sulphide of zinc has been employed as a substitute for white lead, and may possibly supersede it. Zinc white is further employed as a pigment, and, mixed with albumen, is an agent in calico-printing; it is also used in the decoloration of glass, in the polishing of optical glasses, and in the manufacture of artificial meerschaum pipes.[948]
[948] Artificial meerschaum pipes are composed of zinc white, magnesia usta, and caseine ammonium.