Zinc

Applications.—Zinc is largely used for "galvanizing" iron, sheets of clean iron being immersed in a bath of the molten metal and then removed, so that a coat of zinc remains on the iron, which is thereby protected from atmospheric corrosion. For the same purpose zinc may be sprayed on to other metals, or it may be deposited electrolytically. It is also a constituent of many valuable alloys; brass, Muntz-metal, pinchbeck and tombac are examples.

Zinc forms only one oxide, ZnO, from which is derived a well characterized series of salts. It is chemically related to cadmium and mercury, the resemblance to cadmium being especially well marked ; one distinction is that zinc is less basigenic. Zinc is capable of isomorphously replacing many of the bivalent metals, e.g., magnesium, manganese, iron, nickel, cobalt and cadmium, in certain salts.

Zinc Oxide, ZnO, is manufactured for paint by two processes —directly from the ore mixed with coal by volatilization on a grate, and by oxidizing the vapour given off by a boiling bath of zinc metal. The oxide made by the latter method has generally a better colour, a finer texture, and a greater covering power. It is an infusible solid, which is intensely yellow at a red heat, but on cooling becomes white. This at least is true of the oxide pro duced from the metal by combustion ; that produced from the carbonate, if once made yellow at a red heat, retains a yellow shade permanently. Crystalline zinc oxide is obtained by heating the nitrate, or by heating the chloride in a current of steam. It is insoluble in water, but dissolves readily in all aqueous acids, with formation of salts. It also dissolves in aqueous caustic alkalis, in cluding ammonia, forming "zincates" [e.g., Zinc oxide is used in the arts as a white pigment (zinc white) ; it has not by any means the covering power of white lead, but offers the advantages of being non-poisonous and of not becoming dis coloured in sulphuretted hydrogen. It is used also in ointments, as a polish for glass, and in dental cements.

Zinc Hydroxide, is prepared as a gelatinous precipi tate by adding a solution of any zinc salt to pure aqueous caustic potash. It is a white powder, is insoluble in water, but soluble in excess of alkali and in acids.

Zinc Peroxide, obtained from zinc sulphate and barium perox ide, or by electrolysis of neutral zinc chloride solutions in the presence of hydrogen peroxide, is a valuable antiseptic, being odourless and non-irritant ; it is much used for skin troubles under various proprietary names (e.g., dermogen), and as produced com mercially contains about so% of together with hydroxide and moisture.

Zinc chloride,

is produced by heating the metal in dry chlorine gas, or by heating a mixture of zinc sulphate and sodium chloride. It condenses as a white translucent mass, boil

ing at about 700°. Its vapour density at 900° C corresponds to It is extremely hygroscopic and is used in synthetical organic chemistry as a condensing agent. It dissolves in a frac tion of its weight of even cold water and in any proportion of boil ing water, forming a syrupy solution. A solution of zinc chloride is easily produced from the metal and hydrochloric acid; it can not be evaporated to dryness without considerable decomposition of the hydrated salt into oxychloride and hydrochloric acid, but it may be crystallized as ZnC12.H20. A concentrated solution of zinc chloride converts starch, cellulose and a great many other organic substances into soluble compounds ; hence the application of the fused salt as a caustic in surgery and the impossibility of filtering a strong solution through paper. (See CELLULOSE.) The solution is also used as a flux in soldering.

Zinc chloride solution readily dissolves the oxide with the forma tion of oxychlorides, some of which are used as pigments, ce ments and for filling teeth in dentistry. A solution of the oxide in the chloride has the property of dissolving silk, and hence is employed for removing this fibre from wool.

Zinc sulphide,

ZnS, occurs in nature as blende (q.v.), and is artificially obtained as a white precipitate by passing sulphuretted hydrogen into a neutral solution of a zinc salt. It dissolves in mineral acids, but is insoluble in acetic acid.

Zinc sulphate,

or white vitriol, is prepared by dissolving the metal in dilute sulphuric acid, concentrating, and cooling the solution. The hydrated salt crystallizes out on cooling, forming colourless orthorhombic prisms, usually small and needle shaped. They are permanent in the air. According to Poggiale, oo parts of water dissolve respectively of salt, 115.2 parts at o°, and 653.6 parts at ma°. At 39° C the crystals lose one, and at ioo° six of their molecules of water; the remaining molecule goes off at 250°. The anhydrous salt, when exposed to a red heat, breaks up into oxide, sulphur dioxide and oxygen. An impure form of the salt is prepared by roasting blende at a low temperature. In the arts it is employed in the preparation of varnishes, and as a mordant for the production of colours on calico. A green pigment known as Rinmann's green is prepared by mixing Ioo parts of zinc vitriol with 2.5 parts of cobalt nitrate and heating the mixture to redness, to produce a compound of the two oxides. Zinc sulphate, like magnesium sulphate, unites with the sulphates of the potassium metals and of ammonium into crystalline double salts, ZnSO4.R2SO4+6H20, isomorphous with one another and with the magnesium salts.