THE three elements composing this family do not exhibit such a close resemblance to each other as exists between barium, strontium, and calcium; for although zinc and cadmium are very closely related, mercury in many respects differs widely from these, and from all the other elements in the same group.

Cadmium and zinc are almost invariably found associated together in nature, they are both fairly permanent in the air, and both readily take fire and burn when strongly heated, forming the oxides. Both are acted upon by dilute hydrochloric and sulphuric acids, with evolution of hydrogen, and most of their salts are isomorphous.

Mercury is peculiar in being liquid at ordinary temperatures. Zinc and cadmium melt at 430° and 320° respectively, while mercury melts at -38.8°. It is quite unacted upon by oxygen at ordinary temperatures, and combines with extreme slowness when heated. Its oxide, also, is readily decomposed by heat into its elements.

Dilute hydrochloric and sulphuric acids are entirely without action upon it, and it forms no hydroxide.

The hydroxide of zinc, Zn(HO), differs from the corresponding cadmium compound, in being soluble in alkaline hydroxides.

These three elements resemble each other, and differ from those of family A of this group, in that they can be volatilised, mercury at a temperature about 357°, cadmium and zinc at temperatures approaching 1000°.

These three elements are also alike, in that their vapours consist of mono-atomic molecules.

ZINC.

Symbol, Zn. Atomic weight=65.4.

Occurrence.-Zinc is stated to have been found in Australia in the uncombined condition; with this exception, it is always met with in combination, chiefly as carbonate in calamine or zinc-spar, ZnCO, and as sulphide in zinc-blende, or black-jack, ZnS. Other ores are red zinc ore, ZnO ; and franklinite, (ZnFe)O, Fe2O3.

Gahnite, or zinc-spinnelle, has the composition ZnO,AlO3. Modes of Formation.-The ores chiefly employed for the preparation of zinc are the carbonate and sulphide, although in New Jersey the red oxide and franklinite are used. The process consists of two operations, namely, first, the conversion of the ore into oxide of zinc, by calcination; and, second, the reduction of the oxide by means of coal at a high temperature. The calcination of the natural carbonate is readily accomplished, this compound merely giving up its carbon dioxide at a high temperature—

ZnCO3=ZnO+CO2.

In the case of zinc-blende, the operation consists in the oxidation of both the sulphur and the zinc by atmospheric oxygen, thus—

ZnS+30=ZnO+SO

Considerable care has to be exercised in order to prevent the formation of zinc sulphate, which, in the subsequent operation, would be reconverted into sulphide, and so lost. The finely crushed calcined ore is mixed with coke or coal and heated to bright redness in earthenware retorts, when the oxide is reduced, with the formation of carbon monoxide, and the metal distils and is collected in iron receivers. Zinc ores frequently contain small quantities of cadmium, and as this metal is more readily volatilised than zinc, it passes over in the first portions of the distilled product.

The two processes now almost exclusively in use for the reduction of zinc, known as the Silesian and the Belgian process,* differ only in metallurgical details, &c.

* The old method, known as the English process, or distillation per descensum, is entirely obsolete. For details of this and all other metallurgical processes, the student is referred to treatises on metallurgy, such as Percy.

Commercial zinc usually contains carbon, iron, and lead, and occasionally arsenic and cadmium. It may be obtained in a higher degree of purity by careful distillation, but pure zinc is best obtained by first preparing the pure carbonate by precipitation, and then calcining and finally reducing with charcoal obtained from

sugar.

Properties.--Zinc is a bluish-white, highly crystalline, and brittle metal. At a temperature of 300° it can be readily powdered in a mortar, while between 100° and 150° it admits of being drawn into wire or rolled into thin sheet. The presence of a small quantity of lead greatly enhances this property, but is detrimental when the zinc is required for making brass. Zinc which has been either rolled or drawn no longer becomes brittle when cold, but retains its malleability.

Zinc melts at a temperature about 430,° and when heated in air much beyond this point the metal takes fire and burns with a bluishwhite flame, the brilliancy of which becomes dazzling if a stream of oxygen be projected upon the burning mass. The product of its combustion is zinc oxide, ZnO, which forms a soft, white, flocculent substance resembling wool, and formerly known as philosopher's wool.

The boiling-point of zinc is about 930°.

Zinc is permanent in dry air at ordinary temperatures, but when exposed to moist air it tarnishes superficially; it is also unattacked by water at the boiling temperature. It is soluble in a hot solution of sodium or potassium hydroxide, with evolution of hydrogen (p. 175).

Pure zinc is scarcely acted upon by pure sulphuric or hydrochloric acid, either dilute or strong. The presence of small quantities of impurities, however, determines the solution of the metal with the rapid evolution of hydrogen, hence ordinary commercial zinc is readily attacked by these acids, and also decomposes water at the boiling-point, with the evolution of hydrogen.*

* The difference between the behaviour of acids towards pure and commercial zinc was formerly explained on the ground that the impurities present formed with the zinc a voltaic couple, whereby local electric currents were set up, while in the case of pure zinc no such action took place. The recent observations of Pullinger (Chem. Soc., 57) and Weeren (Berichte, 24) show that this is not a complete explanation. Weeren concludes that the insolubility of pure zinc in dilute acids is due to the formation of a film of condensed hydrogen upon the surface of the metal, which stops all further action. The addition of oxidising agents, such as hydrogen peroxide, or dilute sulphuric acid which has

Zinc is extensively used in the process of galvanising iron, which consists in coating iron with a film of zinc, not by electrical deposition, as would be implied by the name, but by dipping the iron into a bath of molten zinc. The layer of zinc preserves the iron from rusting. Galvanised iron is better able to withstand the action of air and moisture than tinned iron, hence it is extensively used for wire netting, corrugated roofing, water tanks, and other purposes where the metal is exposed to the oxidising influence of air and water.

Alloys of Zinc.-Zinc forms a number of useful alloys, the most important of which are the various forms of brass (see Copper). With certain metals, such as tin, copper, and antimony, zinc will mix in all proportions; while with others, such as lead and bismuth, it is only possible to obtain solid alloys of definite composition. When, therefore, lead and zinc are melted together, although in the molten condition the mixture is homogeneous, on cooling the metals separate into two layers, the lighter zinc rising to the surface. The separation of the metals, however, is not perfect, for the zinc will have dissolved a certain quantity of the lead (1.2 per cent.), and the lower layer of lead is found to have dissolved a small proportion of zinc (1.6 per cent.), just as water and ether, when shaken together, separate into two layers, the uppermost being an ethereal solution of water, and the lower an aqueous solution of ether.

This property is made use of in the extraction of silver from lead (see p. 561).

The so-called German silver, or nickel silver, is a nearly white alloy of copper, nickel, and zinc.

Bronze coinage consists of 95 parts of copper, 4 of tin, and 1 of zinc, the small proportion of zinc giving to the alloy an increased hardness and durability.

Zinc Oxide, ZnO, the only oxide of zinc, occurs native as red zinc ore, the colour being due to the presence of manganese. It is

been electrolysed, and therefore contains presulphuric acid, tends to destroy this film by oxidising the hydrogen, and therefore promotes the solution of the zinc. He also finds, that by mechanically removing this layer of hydrogen, either by constantly brushing the metallic surface or placing the materials under reduced pressure, the solution of the zine by the acid is promoted. It is also found that the character of the surface of the metal, whether smooth or rough, affects the result; zinc that is unacted upon when its surface is perfectly smooth is more readily attacked by the dilute acid when its surface is rough.

formed as a soft white substance when zinc is burnt in the air. It is manufactured under the name of zinc white by the combustion of zinc, the fumes being led into condensing-chambers, where the oxide collects.

Zinc oxide is a pure white substance, which when heated becomes yellow, but again becomes white on cooling. When strongly heated in oxygen, it may be obtained in the form of hexagonal crystals; such crystals are occasionally found in the cooler parts of zinc furnaces. The oxide does not fuse in the oxyhydrogen flame, but, like lime, under these circumstances it becomes intensely incandescent; for some time after being so heated it appears phosphorescent in the dark. It is insoluble in water, and does not combine directly with water to form the hydroxide. It dissolves in acids, giving rise to the different zinc salts. Zinc oxide is largely used in the place of "white lead" as a pigment; although it does not equal white lead in covering power, or body, it possesses the advantage of not being blackened by exposure to atmospheric sulphuretted hydrogen.

Zinc Hydroxide, Zn(HO)2, is formed as a white flocculent precipitate, when either sodium or potassium hydroxide, or a solution of ammonia, is added to a solution of zinc sulphate. The compound is soluble in an excess of either alkali, and is deposited from a strong solution in regular octahedra of the hydrated hydroxide, Zn(HO), H2O. Both of these compounds on heating readily lose water, and are converted into the oxide.

Zine Chloride, ZnCl2, is formed by the direct combination of zinc with chlorine, or by the action of hydrochloric acid upon the metal. It is also obtained in the anhydrous state by distilling a mixture of mercuric chloride and zinc, or a mixture of anhydrous zinc sulphate and calcium chloride.

It is usually prepared on a large scale by dissolving zinc in hydrochloric acid, and after precipitating any manganese and iron, the liquid is boiled down in enamelled iron vessels, until on cooling it solidifies; it is usually cast into sticks.

Zinc chloride is a soft, white, easily fusible solid, which volatilises and distils without decomposition. It is extremely deliquescent, and readily soluble in water and in alcohol, its solution being powerfully caustic. From a strong aqueous solution deliquescent crystals are deposited, having the composition ZnCl2, HO.

When the aqueous solution is evaporated, partial decomposition takes place, hydrochloric acid being evolved and basic compounds