1911 Encyclopædia Britannica/Zinc

Zinc, a metallic chemical element; its symbol is Zn, and atomic weight 65·37 (O＝16). Zinc as a component of brass (χαλκος, ὀρει-χαλκος) had currency in metallurgy long before it became known as an individual metal. Aristotle refers to brass as the “metal of the Mosynoeci,”^[1] which is produced as a bright and light-coloured χαλκος, not by addition of tin, but by fusing up with an earth. Pliny explicitly speaks of a mineral καδμεία or cadmia as serving for the conversion of copper into aurichalcum, and says further that the deposit (of zinc oxide) formed in the brass furnaces could be used instead of the mineral. The same process was used for centuries after Pliny, but its rationale was not understood. Stahl, as late as 1702, quoted the formation of brass as a case of the union of a metal with an earth into a metallic compound; but he subsequently adopted the view propounded by Kunckelin 1677, that “cadmia” is a metallic calx, and that it dyes the copper yellow by giving its metal up to it.

The word zinc (in the form zinken) was first used by Paracelsus, who regarded it as a bastard or semi-metal, but the word was subsequently used for both the metal and its ores. Moreover, zinc and bismuth were confused, and the word spiauter (the modern spelter) was indiscriminately given to both these metals. In 1597 Libavius described a “peculiar kind of tin” which was prepared in India, and of which a friend had given him a quantity. From his account it is quite clear that that metal was zinc, but he did not recognize it as the metal of calamine. It is not known to whom the discovery of isolated zinc is due; but we do know that the art of zinc-smelting was practised in England from about 1730. The first continental zinc-works were erected at Liege in 1807.

Occurrence.—Zinc does not occur free in nature, but in combination it is widely diffused. The chief ore is zinc blende, or sphalerite (see Blende), which generally contains, in addition to zinc sulphide, small amounts of the sulphides of iron, silver and cadmium. It may also be accompanied by pyrites, galena, arsenides and antimonides, quartz, calcite, dolomite, &c. It is widely distributed, and is particularly abundant in Germany (the Harz, Silesia), Austro-Hungary, Belgium, the United States and in England (Cumberland, Derbyshire, Cornwall, North Wales). Second in importance is the carbonate, calamine (q.v.) or zinc spar, which at one time was the principal ore; it almost invariably contains the carbonates of cadmium, iron, manganese, magnesium and calcium, and may be contaminated with clay, oxides of iron, galena and calcite; “white calamine” owes its colour to much clay; “red calamine” to admixed iron and manganese oxides. Calamine chiefly occurs in Spain, Silesia and in the United States. Of less importance is the silicate, Zn₂SiO₄·H₂0, named electric calamine or hemimorphite; this occurs in quantity in Altenburg near Aix-la-Chapelle, Sardinia, Spain and the United States (New Jersey, Pennsylvania, Missouri, Wisconsin). Other zinc minerals are willemite (q.v.), Zn₂SiO₄, hydrozincite or zinc bloom, ZnCO₃·2Zn(OH)₂, zincite (q.v.) or red zinc ore, ZnO, and franklinite, 3(Fe,Zn)O·(Fe, Mn)₂O₃.

Production.—Until about 1833 the supply of zinc was almost entirely obtained from Germany, but in this year Russia began to contribute about 2000 tons annually to the 6000 to 7000 derived from Germany. Belgium entered in 1837 with an output of about 2000 tons; England in 1855 with 3000; and the United States in 1873 with 6000 tons. The productions of Germany, Belgium and the United States have enormously and fairly regularly increased; the rise has been most rapid in the United States, England, France, Spain and Austria have been fairly constant producers Germany produced 155,799 tons in 1900, and 198,208 in 1905, Belgium, 120,000 in 1900 and 143,165 in 1905; the United States, 111,000 in 1900 and 183,014 in 1905. The world's supply was 445,438 tons in 1900, and 654,367 in 1905.

Metallurgy

The principles underlying the extraction of zinc may be summarized as (1) the ore is first converted into zinc oxide, (2) the oxide is distilled with carbon and the distillate of metallic zinc condensed. Oxide of zinc, like most heavy metallic oxides, is easily reduced to the metallic state by heating it to redness with charcoal; pure red zinc ore may be treated directly; and the same might be done with pure calamine of any kind, because the carbon dioxide of the zinc carbonate goes off below redness and the silica of zinc silicate only retards, but does not prevent, the reducing action of the charcoal. Zinc blende, however, being zinc sulphide, is not directly reducible by charcoal, but it is easy to convert it into oxide by roasting: the sulphur goes off as sulphur dioxide whilst the zinc remains in the (infusible) form of oxide, ZnO. In practice, however, we never have to deal with pure zinc minerals, but with complex mixtures, which must first of all be subjected to mechanical operations, to remove at least part of the gangue, and if possible also of the heavy metallic impurities (see Ore-Dressing).

As ores of zinc are usually shipped before smelting from widely separated places—Sweden, Spain, Algiers, Italy, Greece, Australia and the Rocky Mountains region of North America—it is important that they be separated from their mixtures at the mines. The difficulty in separating zinc blende from iron pyrites is well known, and probably the most elaborate ore-dressing works ever built have been designed with this end in view. The Wetherill system of magnetic concentration has been remarkably successful in separating the minerals contained in the well-known deposit in Sussex county, N.J. Here, very clean non-magnetic concentrate of willemite, which is an anhydrous zinc silicate and a very high-grade zinc ore, is separated from an intimate mixture of willemite, zincite and franklinites, with calcite and some manganese silicates. The magnetic concentrates contain enough zinc to be well adapted to the manufacture of zinc oxide. Magnetic concentration is also applied in the removal of an excess of iron from partially roasted blende. Neither mechanical non-magnetic concentration can effect much in the way of separation when, as in many complex ores, carbonates of iron, calcium and magnesium replace the isomorphous zinc carbonate, when some iron sulphide containing less sulphur than pyrites replaces zinc sulphide, and when gold and silver are contained in the zinc ore itself. Hence only in exceptional circumstances is it possible to utilize a large class of widely distributed ores, carrying from 10 to 35 per cent. of zinc, in which the zinc alone, estimated at 2d. a pound, is worth from about £2 to £7 per ton of ore. The ores of the Joplin district, in the Ozark uplift in the Mississippi Valley, are remarkable in that they are specially adapted to mechanical concentration. The material as mined will probably not average over 10 per cent. of zinc, but the dressed zinc ore as sold ranges from 45 to 62 per cent. of zinc. This region now furnishes the bulk of the ore required by the smelters of Illinois, Missouri and Kansas.

The ore, even if it is not blende, must be roasted or calcined in order to remove all volatile components as completely as possible, because these, if allowed to remain, would carry away a large proportion of the zinc vapour during the distillation. If the zinc is present as blende, this operation offers considerable difficulties, because in the roasting process the zinc sulphide passes in the first instance into sulphate, which demands a high temperature for its conversion into oxide. Another point to be considered in this connexion is that the masses of sulphur dioxide evolved, being destructive of vegetable life, are an intolerable nuisance to the neighbourhood in which the operations take place. For the desulphurization of zinc blende where it is not intended to collect and save the sulphur there are many mechanical kilns, generally classified as straight-line, horse-shoe, turret and shaft kilns; all of these may be made to do good work on moderately clean ores which do not melt at the temperature of desulphurization. But the problem of saving the sulphur is yearly becoming more important. In roasting a ton of rich blende containing 60 per cent. of zinc enough sulphur is liberated to produce one ton of strong sulphuric acid, and unless this is collected not only are poisonous gases discharged, but the waste is considerable. When sulphuric or sulphurous acid is to be collected, it is important to keep the fuel gas from admixture with the sulphur gases, and kilns for this purpose require some modification. If hot air is introduced into the kiln, the additional heat developed by the oxidation of the zinc and the sulphur is sufficient to keep up a part of the reaction but for the complete expulsion of the sulphur an externally-fired muffle through which the ore is passed is found to be essential.

Distillation of the Oxide with Charcoal.—The distillation process in former limes, especially in England, used to be carried out “per descensum.” The bottom of a crucible is perforated by a pipe which projects into the crucible to about two-thirds of its height. The mixture of ore and charcoal is put into the crucible around the pipe, the crucible closed by a luted-on lid, and placed in a furnace constructed so as to permit of the lower end of the pipe projecting into the ash-pit. The zinc vapour produced descends through the pipe and condenses into liquid zinc, which is collected in a ladle held under the outlet end of the pipe. For manufacturing purposes a furnace similar to that used for the making of glass was employed to heat a circular row of crucibles standing on a shelf along the wall of the furnace. This system, however, has long been abandoned.

The modern processes may be primarily divided into two groups according to the nature of the vessel in which the operation is effected. These distilling vessels are called retorts if they are supported only at the ends, and the furnace using them is termed a Belgian furnace. If they are supported at intervals along a flat side, they are called muffles, and the furnace is known as a Silesian furnace. Various combinations and modifications of these two types of furnace have given rise to distinctive names, and as each system has its advantages and disadvantages local conditions determine which is the better.

In the Belgian process the reduction and distillation are carried out in cylindrical or elliptical retorts of fire-clay, from 3 ft. 3 in. to 4 ft 9 in long and 6 to 10 in internal diameter. Some forty-six or more retorts, arranged in parallel horizontal rows, are heated in one furnace. The furnaces are square and open in front, to allow the outlet ends of the retorts to project: they are grouped together by fours, and their several chimneys are within the same enclosure. Each retort is provided with two adapters, namely, a conical pipe of fire-clay, about 15 in. long, which fits into the retort end, and a conical tube of sheet iron, which fits over the end of the fire-clay pipe, and which at its outlet end is only about an inch wide. To start a new furnace, the front side is closed provisionally by a brick wall, a fire lighted inside, and the temperature raised very gradually to a white heat. After four days' heating the provisional front wall is removed piecemeal, and the retorts, after having been heated to redness, are inserted in corresponding sets. The charge of the retorts consists of a mixture of 1100 l℔ of roasted calamine and 550 ℔ of dry powdered coal per furnace. A newly started furnace, however, is used for a time with smaller charges. Supposing the last of these preliminary distillations to have been completed, the residues left in the retorts are removed, and the retorts, as they lie in the hot furnace, are charged by means of semi-cylindrical shovels, and their adapters put on. The charging operation being completed, the temperature is raised, and as a consequence an evolution of carbon monoxide soon begins, and becomes visible by the gas bursting out into the characteristic blue flame. After a time the flame becomes dazzling white, showing that zinc vapour is beginning to escape. The iron adapters are now slipped on, and left on for two hours, when, as a matter of experience, a considerable amount of zinc has gone out of the retort, the greater part into the fire-clay adapter, the rest into the iron cone. The former contains a mixture of semi-solid and molten metal, which is raked out into iron ladles and cast into plates of 66 to 77 ℔ weight, to be sold as “spelter.” The contents of the iron recipient consist of a powdery mixture of oxide and metal, which is added to the next charge, except what is put aside to he sold as “zinc dust.” This dust may amount to 10 per cent. of the total production. As soon as the adapters have been cleareed of their contents, they are replaced, and again left to themselves for two hours, to be once more emptied and replaced, &c. The complete exhaustion of the charge of a furnace takes about eleven hours.

In the Silesian process the distillation is conducted in specially constructed muffles of a prismatic shape arched above, which are arranged in two parallel rows within a low-vaulted furnace, similar to the pots in a glass furnace. As a rule every furnace accommodates ten muffles. Through an orifice in the outlet pipe (which is closed during the distillation by a loose plug) a hot iron rod can be introduced from time to time to clear away any solid zinc that may threaten to obstruct it. As soon as the outlet pipe has become sufficiently hot the zinc flows through it and collects in conveniently placed receptacles. About six or eight hours after starting the distillation is in full swing, and in twenty-four hours it is completed. A fresh charge is then put in at once, the muffles being cleared only after three successive distillations. The distillate consists of a conglomerate of drops (“drop zinc”). It is fused up in iron basins lined with clay, and cast out into the customary form of cakes.

The chief improvements in the plant of these processes are concerned with the manufacture of the retorts or muffles, and especially with the introduction of gas-firing. Even a machine of simple type, like the ordinary drain-pipe machine, in which the retorts are made by forcing the plastic clay mixture through a die, may result in greater economy and uniformity than is possible when retorts are made by hand When hydraulic pressure to the amount of 2000 to 3000 ℔ per square inch is applied, the saving is unquestioned, since less time is required to dry the pressed retort, its life in the furnaces is longer, its absorption of zinc is less, and the loss of zinc by passage through its walls in the form of vapour is reduced.

Three modes of gas-firing are to be noticed, each of which is adapted to special local conditions. (a) The gas is made from the fuel in a detached fireplace and conducted while hot into the combustion chamber of the furnace, and the air for complete combustion is heated by the products of combustion on their way to the chimney. (b) Both the producer gas and the air are heated before they enter the combustion chamber, as in the Siemens system of regenerative firing. (c) Natural gas is piped to the furnace, where it meets air heated by the chimney gases. The primary advantages of gas-firing are that less fuel is required, that there is better control of the heat in the furnace, and that larger and more accessible furnaces can be built. In Silesia the introduction of gas-firing has led to the use of furnaces containing eighty muffles. In the United States, Belgian furnaces of type (a) are built to contain 864 retorts; of type (b), to contain 300 to 400 retorts; and of type (c), preferably about 600 retorts. The use of gas-fired furnaces greatly simplifies manual labour. On a direct-fired furnace at least one man, the brigadier, must be an expert in all the operations involved; but with a gas furnace a division of labour is possible. One man who understands the use of gaseous fuel can regulate the heat of a thousand or more retorts. The men who charge and empty the retorts, those who draw and cast the metal, and those who keep the furnace in repair, need not know anything about the making or using of gas, and the men who make the gas need not know anything about a zinc furnace. Again, in direct-fired furnaces there are commonly seven or eight rows of retorts, one above another, so that to serve the upper rows the workman must stand upon a table, where he is exposed to the full heat of the furnace and requires a helper to wait upon him. With gas-firing the retorts can be arranged in four horizontal rows, all within reach of a man on the furnace-room floor. Furthermore, with the large furnaces which gas-firing makes possible mechanical appliances may be substituted for manual labour in many operations, such as removing and replacing broken retorts, mixing and conveying the charge drawing and casting the metal, charging and emptying the retorts, and removing the residues and products.

Refining.—The specific effects of different impurities on the physical properties of zinc have only been imperfectly studied fortunately, however, the small amounts of any of them that are likely to be found in commercial zinc are not for most purposes very deleterious. It is generally recognized that the purest ores produce the purest metal. Grades of commercial zinc are usually based on selected ores, and brands, when they mean anything usually mean that the metal is made from certain ores. Chemical control of the metal purchased is not nearly as common as it should be, and the refining of zinc is at best an imperfect operation. To obtain the metal chemically pure a specially prepared pure oxide or salt of zinc is distilled. A redistilled zinc, from an ordinarily pure commercial zinc, is often called chemically pure but redistillation is seldom practised except for the recovery of zinc from galvanizer's dross and from the skimmings and bottoms of the melting furnaces of zinc rolling mills. The only other method of refining is by oxidizing and settling. A bath, even of very impure zinc, is allowed to stand at about the temperature of the melting-point of the metal for forty-eight or more hours, whereupon the more easily oxidizable impurities can be largely removed in the dross at the top, the heavier metals such as lead and iron settling towards the bottom. This method is rarely practised except by the rollers of zinc. A certain amount of refined zinc can be dipped from the furnace; a further amount, nearly free from iron, can be liquated out of the ingots cast from the bottom of the bath in a subsequent slow remelting, and it is sometimes possible to eliminate a zinciferous lead which collects in the sump of the furnace. Owing to the fact that at temperatures between its melting and boiling point zinc has a strong affinity for iron, it is often contaminated by the scraper while being drawn from the condenser, as is shown by the fact that the scraper wears away rapidly. As each retort in a furnace is in all essentials a separate crucible, and as the metal from only a few of them goes into a single ingot, there can be no uniformity either in the ingots made from the same furnace during a day's run or in those made from several furnaces treating the same ore. Some brassfounders break from a single ingot the quantity of zinc required to produce the amount of brass they wish to compound in one crucible, but when perfect uniformity is desired the importance of remelting the zinc on a large scale cannot be too strongly emphasized.

Electrolytic Separation of Zinc.—The deposition of pure zinc is beset with many difficulties. Zinc being more electro-positive even than nickel, all the heavy metals must be removed before its deposition is attempted. Moreover, unless the conditions are closely watched, it is liable to be thrown down in a spongy form. M. Kiliani found that the sponge was produced chiefly when a weak solution, or a low current-density, was used, and that hydrogen was usually evolved simultaneously; sound deposits resulted from the use of a current-density of 200 amperes, or more, per sq. ft. and strong solutions. The cause of the spongy deposit is variously explained, some (Siemens and Halske) ascribing it to the existence of a compound of zinc and hydrogen, and others, among whom are G. Nahnsen, F. Mylius and A. Fromm, F. Foerster and W. Borchers, trace it to the presence of oxide, produced, for example either by the use of a solution containing a trace of basic salt of zinc (to prevent which the bath should be kept just—almost imperceptibly—acid), or by the presence of a more electro-negative metal, which, being co-deposited, sets up local action at the expense of the zinc. Many processes have been patented, the ore being acted upon by acid, and the resulting solution treated, by either chemical or electrolytic means, for the successive removal of the other heavy metals. The pure solution of zinc is then electrolysed. E. A. Ashcroft patented a process of dealing with complex ores of the well-known Broken Hill type, containing sulphides of silver, lead and zinc but the system was abandoned after a long trial on a practical scale. A full account of the process (Trans. Inst. Min. and Met., 1898, vol. vi. p. 282) has been published by the inventor, describing the practical trial at the Cockle Creek Works. The ore was crushed roasted, and leached with sulphuric acid (with or without ferric sulphate); the solution was purified and then electrolysed for zinc with lead anodes and with a current-density of 5 amperes per sq. ft. at 2.75 volts when diaphragms were used, or 2.5 volts when they were dispensed with, or with 10 amperes per sq. ft. at 3 or 2.5 volts respectively, the electrolyte containing 1.2 ℔ of zinc in the form of sulphate and 1/2 to 3/4 oz. of sulphuric acid, per gallon. The current efficiency was about 83 per cent. Canvas diaphragms were used to prevent the acid formed by electrolysis at the anode from mixing with the cathode liquor and so hindering deposition. C. Hoepfner has patented several processes, in one of which (No. 13,336 of 1894) a rapidly rotating cathode is used in a chloride solution, a porous partiton separating the tank into anode and cathode compartments, and the chlorine generated by electrolysis at the anode being recovered. Hoepfner’s processes have been employed both in England and in Germany. Nahnsen’s process, with an electrolyte containing alkali-metal sulphate and zinc sulphate, has been used in Germany, and a process invented by Dieffenbach has also been tried in that country, Siemens and Halske have proposed the addition of oxidizing agents such as free halogens, to prevent the formation of zinc hydride, to which they attribute the formation of zinc-sponge. Borchers and others deposit zinc from the fused chloride. In Borchers process the chloride is heated partly by external firing, partly by the heat generated owing to the use of a current-density of 90 to 100 amperes per sq. ft.

Properties

Zinc is a bluish white metal showing a high lustre when freshly fractured. It fuses at 415° C. and under ordinary atmospheric pressure boils at 1040° C. Its vapour density shows that it is monatomic. The molten metal on cooling deposits crystals belonging to the hexagonal system, and freezes into a compact crystalline solid, which may be brittle or ductile according to circumstances. If zinc be cast into a mould at a red heat, the ingot produced is laminar and brittle; if cast at just the fusing-point it is granular and sufficiently ductile to be rolled into sheet at the ordinary temperature. According to some authorities, pure zinc always yields ductile ingots. Commercial “spelter” always breaks under the hammer; but at 100° to 150° C. it is susceptible of being rolled out into even a very thin sheet. Such a sheet, if once produced, remains flexible when cold. At about 200° C the metal becomes so brittle that it can be pounded in a mortar. The specific gravity of zinc cannot be expected to be perfectly constant; according to Karsten, that of pure ingot is 6.915, and rises to 7.191 after rolling. The coefficient of linear expansion is 0.002,905 for 100° from 0° upwards (Fizeau). The specific heat is 0.09555 (Regnault). Compact zinc is bluish white; it does not tarnish much in the air. It is fairly soft, and clogs the file. If zinc be heated to near its boiling-point, it catches fire and burns with a brilliant light into its powdery white oxide, which forms a reek in the air (lana philosophica, “philosopher’s wool”). Boiling water attacks it appreciably, but slightly, with evolution of hydrogen and formation of the hydroxide, Zn(OH)₂. A rod of perfectly pure zinc, when immersed in dilute sulphuric acid, is so very slowly attacked that there is no visible evolution of gas; but, if a piece of platinum, copper or other more electro-positive metal be brought into contact with the zinc, it dissolves readily, with evolution of hydrogen and formation of the sulphate. The ordinary impure metal dissolves at once, the more readily the less pure it is. Cold dilute nitric acid dissolves zinc as nitrate, with evolution of nitrous oxide. At higher temperatures, or with stronger acid, nitric oxide, NO, is produced besides or instead of nitrous. Zinc is also soluble in soda and potash solutions, but not in ammonia.

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. It is also a constituent of many valuable alloys; brass, Muntz-metal, pinchbeck, tombac, are examples. In technological chemistry it finds application as a reducing agent, e.g. in the production of aniline from nitrobenzene, but the use of iron is generally preferable in view of the cheapness of this metal.

Compounds

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—magnesium, manganese, iron, nickel, cobalt and cadmium.

Zinc oxide, ZnO, is manufactured for paint by two processes—directly from the ore mixed with coal by volatilization on a grate, as in the Wetherill oxide process, 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 also manufactured by the latter process from the metallic zinc liquated out of galvanizer's dross. 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 produced from the metal by combustion; that produced from the carbonate, if once made yellow at a red heat, retains a yellow shade permanently. By heating the nitrate it is obtained as hemimorphous pyramids belonging to the hexagonal system; and by heating the chloride in a current of steam as hexagonal prisms. It is insoluble in water; it dissolves readily in all aqueous acids, with formation of salts. It also dissolves in aqueous caustic alkalis, including ammonia, forming “zincates” [e.g. Zn(OK)₂]. 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 discoloured in sulphuretted hydrogen. It is used also in medicine.

Zinc hydroxide, Zn(OH)₂, is prepared as a gelatinous precipitate by adding a solution of any zinc salt to caustic potash. The alkali must be free from carbonate and an excess of it must be avoided, otherwise the hydrate redissolves. It is a white powder, and is insoluble in water. To acids and to alkalis it behaves like the oxide, but dissolves more readily.

Zinc chloride, ZnCl₂, is produced by heating the metal in dry chlorine gas, when it distils over as a white translucent mass, fusing at 250° and boiling at about 400°. Its vapour-density at 900° C. corresponds to ZnCl₂. It is extremely hygroscopic and is used in synthetical organic chemistry as a condensing agent. It dissolves in a fraction of its weight of even cold water, forming a syrupy solution. A solution of zinc chloride is easily produced from the metal and hydrochloric acid; it cannot be evaporated to dryness without considerable decomposition of the hydrated salt into oxychloride and hydrochloric acid, but it may be crystallized as ZnCl₂·H₂O. A concentrated solution of zinc chloride converts starch, cellulose and a great many other organic bodies into soluble compounds; hence the application of the fused salt as a caustic in surgery and the impossibility of filtering a strong ZnCl₂ solution through paper (see Cellulose). At a boiling heat, zinc chloride dissolves in any proportion of water, and highly concentrated solutions, of course, boil at high temperatures; hence they afford a convenient medium for the maintenance of high temperatures.

Zinc chloride solution readily dissolves the oxide with the formation of oxychlorides, some of which are used as pigments, cements 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 bromide, ZnBr₂, and Zinc iodide, ZnI₂, are deliquescent solids formed by the direct union of their elements. With ammonia and alkaline bromides and iodides double salts are formed.

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, ZnSO₄+7H₂O, or white vitriol, is prepared by dissolving the metal in dilute sulphuric acid. If care be taken to keep the zinc in excess, the solution will be free from all foreign metals except iron and perhaps manganese. Both are easily removed by passing chlorine through the cold solution, to produce ferric and manganic salt, and then digesting the liquid with a washed precipitate of basic carbonate, produced from a small portion of the solution by means of sodium carbonate. The iron and manganese are precipitated as hydroxides, and are filtered off. The filtrate is acidified with a little sulphuric acid and evaporated to crystallization. The salt crystallizes out on cooling with 7 molecules of water, forming colourless orthorhombic prisms, usually small and needle-shaped. They are permanent in the air. According to Poggiale, 100 parts of water dissolve respectively of (7H₂O) salt, 115.2 parts at 0°, and 653.6 parts at 100°. At 100° C. the crystals lose 6 of their molecules of water; the remaining molecule goes off at 250°, a temperature which lies close to that at which the salt begins to decompose. 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 100 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+6H₂O, isomorphous with one another and the magnesium salts.

Zinc carbonate, ZnCO₃, occurs in nature as the mineral calamine (q.v.), but has never been prepared artificially, basic carbonates, ZnCO₃.xZn(OH)₂, where x is variable, being obtained by precipitating a solution of the sulphate or chloride with sodium carbonate. To obtain a product free of Cl or SO₄, there must be an excess of alkali and the zinc salt must be poured into the hot solution of the carbonate. The precipitate, even after exhaustive washing with hot water, still contains a trace of alkali; but from the oxide, prepared from it by ignition, the alkali can be washed away. The basic carbonate is used as a pigment.

Of zinc phosphates we notice the minerals hopeite, Zn₃(PO₄)₂⋅4H₂O, and tarbuttite, Zn₃(PO₄)₂⋅Zn(OH)₂, both found in Rhodesia.

Analysis.—From neutral solutions of its salts zinc is precipitated by sulphuretted hydrogen as sulphide, ZnS—a white precipitate, soluble, but by no means readily, in dilute mineral acids, but insoluble in acetic acid. In the case of acetate the precipitation is quite complete; from a sulphate or chloride solution the greater part of the metal goes into the precipitate; in the presence of a sufficiency of free HCl the metal remains dissolved; sulphide of ammonium precipitates the metal completely, even in the presence of ammonium salts and free ammonia. The precipitate, when heated, passes into oxide, which is yellow in the heat and white after cooling; and, if it be moistened with cobalt nitrate solution and re-heated, it exhibits a green colour after cooling.

Zinc may be quantitatively estimated by precipitating as basic carbonate, which is dried and ignited to zinc oxide. It may also be precipitated as zinc ammonium phosphate, NH₄ZnPO₄, which is weighed on a filter tared at 100°. Volumetric methods have also been devised.

Pharmacology and Therapeutics of Zinc Compounds

Zinc chloride is a powerful caustic, and is prepared with plaster of Paris in the form of sticks for destroying warts, &c. Its use for this purpose at the present day is, however, very rare, the knife or galvanocautery being preferred in most cases. The salt is a corrosive irritant poison when taken internally. The treatment is to wash out the stomach or give such an emetic as apomorphine, and, when the stomach has been emptied, to administer demulcents such as white of egg or mucilage. Numerous other salts of zinc, used in medicine, are of value as containing this metal. Certain others are referred to in relation with the important radicle contained in the salt. Those treated here are the sulphate, oxide, carbonate, oleate and acetate. All these salts are mild astringents when applied externally, as they coagulate the albumen of the tissues and of any discharge which may be present. In virtue of this property they are also mild haemostatics, tending to coagulate the albumens of the blood and thereby to arrest hemorrhage. Lotio Rubra, the familiar “Red Lotion,” a solution of zinc sulphate, is widely used in many catarrhal inflammations, as of the ear, urethra, conjunctiva, &c. There are also innumerable ointments.

These salts have been extensively employed internally, and indeed they are still largely employed in the treatment of the more severe and difficult cases of nervous disease. The sulphate is an excellent emetic in cases of poisoning, acting rapidly and without much nausea or depression. For these reasons it may also be given with advantage to children suffering from acute bronchitis or acute laryngitis.

Bibliography.—For the history of zinc see Bernard Neumann, Die Metalle (1904); A. Rossing, Geschichte der Metalle (1901). For the chemistry see H. Roscoe and C. Schorlemmer, Treatise on Inorganic Chemistry, vol. 2 (1897); H. Moissan, Traité de chimie minerale; O. Dammer, Handbuch der anorganischen Chemie. For the metallurgy see Walter Renton Ingalls, The Metallurgy of Zinc and Cadmium, Production and Properties of Zinc; A. Lodin, Metallurgie du zinc (1905); C. Schnabel, Handbook of Metallurgy, English translation by H. Louis (1907). See also The Mineral Industry (annual).

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1. From the name of this tribe the German word Messing, brass, is undoubtedly derived (see K. B. Hoffmann, Zeit. f Berg. und Huttenwesen, vol. 41).