oxide. The metal is obtained by the reduction of its oxide or chloride, or by the electrolysis of its chlorides, when chromium separates out in brittle glistening scales. It is tin-white in colour, having a specific gravity of 6.8. The fused metal is said to be as hard as corundum; it melts with more difficulty than platinum, and is only slowly oxidised when heated in air. It is used in the form of an alloy with iron and carbon, forming a hard, white, and brilliant steel, much esteemed for special purposes.

Its chief

Manganese. The pure metal, obtained by the reduction of its oxide, is a gray or reddish-white body, hard, and brittle; its specific gravity is about 8; it oxidises more readily than iron, and must therefore be excluded from air by keeping it under rock-oil, or in sealed vessels. use is in the formation of alloys with iron, steel, and copper. It is not used in the unalloyed state. Compounds of this metal are very widely distributed in nature; one of the most common is pyrolusite or black oxide of manganese MnO2.

Nickel. This is a brilliant-white, malleable, ductile, weldable, and very tenacious metal, with a melting point only a little below that of iron, but the presence of carbon and other impurities considerably lower its fusing point. Its specific gravity is 8.9; it is magnetic like iron, but in a less degree. It does not readily oxidise in air at ordinary temperatures, but when heated the monoxide NiO is formed. It readily unites with sulphur, forming nickel sulphide NiS, which is brass-yellow in colour; and with arsenic, forming nickel arsenide NiAs.

Nickel is found in commerce in the form of dull-gray cakes or cubes, and by melting these at a high temperature a compact, silver-white metal is obtained. The malleability of nickel allows of its being fashioned into various articles, which possess great lustre, hardness, and durability. These properties render it valuable for coating base metals by the process of electro-plating, especially as it is little liable to oxidation.

Commercial nickel was formerly very impure, due to the presence of carbon and other bodies, which make it hard and

D

brittle. Dr. Fleitmann and other metallurgists have devised simple and effective means of refining and toughening nickel, which are now largely practised. Fleitmann adds to the melted metal minute quantities of magnesium in several charges, and well stirs each time a dose is added. One ounce of magnesium is sufficient for refining 60 lbs. of impure nickel. The magnesium is supposed to reduce the occluded carbonic oxide CO forming magnesia, and to cause the carbon to separate out as graphite. Nickel unites readily with most metals forming alloys, some of which are of great commercial utility. The most important of these is German silver. Nickel occurs in nature as kupfer-nickel or copper-nickel NiAs, which is a copper-red coloured mineral, with a metallic lustre. As nickel pyrites NiS, which is brass-yellow in colour. As nickel-glance, which is a variable compound of nickel, arsenic, and sulphur. As garnierite, which is a hydrated silicate of nickel, iron, and magnesium.

Cobalt. This metal resembles nickel in appearance and properties, and is generally associated with it in nature. Cobalt is a white metal, highly malleable, ductile, and tenacious; its specific gravity is 8.9; it is magnetic like nickel; almost unalterable in air at ordinary temperatures, but oxidises when heated, and at a high temperature burns with a red flame. It is seldom used in the metallic state, but its compounds are largely employed in pigments. It unites with arsenic to form iron-gray, fusible, and brittle compounds. The principal ores are smaltine CoAs, cobalt glance Co2ASS, and cobalt bloom (Co.As04,4H2O).

ZINC GROUP. ZINC, CADMIUM, AND MAGNESIUM

§ 10. Zinc, commonly known by the name of "spelter when in the cast state, is a white metal, with a bluish shade, and bright metallic lustre. Ordinary zinc is hard and brittle, and when fractured exhibits a highly crystalline structure. When pure it is malleable at the ordinary temperature, while commercial cast-zinc is brittle; the latter, however,

becomes malleable and ductile if heated to a temperature of 100° to 150° C., but beyond that point it again becomes brittle. Its specific gravity in the cast state is 6.9, which may be increased to 7.1 by rolling or forging; it contracts but slightly on cooling from the liquid state, and is thus well adapted for castings. The castings made at a high temperature are brittle and crystalline; but when cast near the solidifying point are more malleable. Zinc melts at 412° C. and boils at 1040° C. At a red heat in air it rapidly oxidises, and burns with a greenish-white flame, forming zinc oxide ZnO; if raised to a bright-red heat in a closed vessel, it may be readily distilled. When rolled zinc is exposed to air and moisture a gray film of suboxide is formed, which preserves the metal from further oxidation. Ordinary zinc readily dissolves in dilute hydrochloric and sulphuric acids, while the pure metal is unaffected; both kinds dissolve in nitric acid and in alkalies. Zinc displaces silver, gold, platinum, bismuth, antimony, tin, mercury, and lead from their solutions. The chief impurities of the commercial metal are iron, lead, and arsenic.

Zinc and sulphur do not readily unite, but when a mixture of finely divided zinc and sulphur is projected into a red-hot crucible, some zinc sulphide ZnS is formed. It is also formed by heating zinc with cinnabar HgS.

Zinc forms compounds with phosphorus and arsenic, when these bodies are heated with it, having a metallic lustre and somewhat vitreous fracture.

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The chief ores of zinc are :-The oxide ZnO called zincite or red oxide of zinc, which is white when pure, but generally red from the presence of oxide of manganese. The sulphide ZnS, known as 'blende" and "black jack," is the principal source of the metal, and generally black or yellowishblack in colour, but sometimes it has a reddish tint from the presence of galena; when pure it is white, and contains 67.03 per cent of zinc. The carbonate ZnCO, called calamine. And the silicate (2ZnO,SiO2,OH2) called electric calamine. Zinc forms with other metals a most important class of

alloys, such as brass, German silver, etc. It is used in the form of sheets, worked into a variety of shapes; it protects iron from rusting, as in galvanised-iron; it forms the electropositive element in many batteries; and in the form of fine dust it is obtained in large quantities mixed with zinc oxide, and forms a valuable reducing agent.

Cadmium. In the process of zinc extraction it was observed that a volatile vapour, in some cases, was distilled off with the first portions of zinc; this was found to be the metal cadmium. It possesses a tin-white colour, has a fibrous structure, and takes a high polish. It is harder than tin, malleable, ductile, and readily volatile. It has a density of 8.6; it melts at about 320° C., and boils at 860° C. Its vapour is of a dark-yellow colour, with a disagreeable odour, Like tin it emits a crackling sound when bent. It is used

in alloys to produce a fusible metal, which melts below 100° C.; and an amalgam of cadmium is employed as a stopping for teeth, such amalgam being soft when first prepared, but soon becomes hard.

Magnesium.

This metal possesses a brilliant-white colour, but soon tarnishes when exposed to moist air, due to the formation of magnesium oxide. It is stated to possess great tensile strength, being nearly equal to that of aluminium bronze. Its specific gravity is 1.74. At a temperature of 450° C. it can be rolled and worked into a variety of forms. Screws and threads made of this metal are sharper and more exact than those made of aluminium. When ignited in a flame it burns with a dazzling white light, which is said to have been seen at sea from a distance of twenty-eight miles. This light is used for purposes of photography. Magnesium occurs abundantly in nature in combination with other elements forming compounds, such as magnesite MgCO3, dolomite MgCa(CO3)2, etc.

ALUMINIUM

§ 11. Aluminium.-With the exception of oxygen and

silicon, this is the most widely distributed of the elements, and contained in the largest quantity in the solid crust of the earth. It occurs in a variety of forms as oxide, but more generally in combination with other metals, such as zinc, iron, magnesium, etc., forming aluminates; as silicate in all clays, and as fluoride in cryolite (6NaF,AL,F).

Aluminium is a white metal which takes a fine polish. It has no taste or odour. It is soft, very malleable, and ductile, with an elasticity and tenacity about equal to that of silver. Its specific gravity is 2.5, which is increased by hammering; it melts at a temperature a little above that of zinc, and is not volatile when strongly heated out of contact with air. Its conductivity for heat and electricity is said to be very high; but, according to Mr. Roby, it is very low, and diminishes the conductivity of copper considerably when alloyed with it. It does not oxidise in air or combine with sulphur; it is insoluble in cold nitric acid; sulphuric acid has no action on it; but hydrochloric acid and alkalies dissolve it readily.

Aluminium is valuable for making articles where lightness is an important feature; this, combined with its lustre, unalterability in air and sulphuretted hydrogen, non-poisonous properties, and ease of working, gives it a widespread interest. It is, however, in its alloys that its greatest value appears. In some cases it imparts strength, in others it modifies the colour, while in others it promotes soundness in castings.

ALKALINE-EARTHY METALS

§ 12. The term "earth" was formerly used to denote those bodies which are insoluble or but slightly soluble in water, and unaltered by exposure to a high temperature. Some of these were found to have an alkaline reaction, and to easily neutralise acids; hence the term "alkaline earth." These oxides viz. baryta, strontia, lime, and magnesia-were found to be composed of metals in combination with oxygen.