in a porcelain tube by means of a current of hydrogen gas at 700° C.; the iron is obtained in the form of a dark powder, which, when somewhat heated, fires spontaneously in contact with air, forming ferric oxide Fe2O. When the reduction is effected at a much higher temperature, a spongy mass of a silvery gray colour is obtained. 2°. By strongly heating the purest variety of iron wire with a little pure oxide of iron, covering the mixture with powdered glass free from lead, and exposing the whole to a high temperature in a covered clay crucible. The small portion of carbon present in the wire is absorbed in reducing the oxide, while the other impurities pass into the slag. 3°. By electrolytic decomposition of a solution of pure ferrous chloride or sulphate a mass of silvery white, soft, malleable iron is obtained, which, after annealing, has a specific gravity of 7·81.

Iron may be exposed to dry air for an indefinite period without alteration, but in the presence of moisture a layer of rust Fe2O is formed. The oxidation is accelerated by the presence of carbonic acid, which is always present in the air, a carbonate of iron being formed. This rapidly absorbs

a further portion of water and oxygen from the air, and in this way the rusting is slowly conveyed to the centre of the mass of iron. The layer of oxide and carbonate is electronegative with regard to iron, so that a galvanic action is set up, causing decomposition of the water. This electrical condition still further augments the liability of iron to rust.

When iron is strongly heated in contact with air or oxygen its surface is rapidly coated with a scale of black oxide Fe3O4, which peels off when struck with a hammer.

Iron and Sulphur.-Compounds of iron and sulphur occur in nature as pyrites. These elements readily unite when heated together, forming ferrous sulphide FeS. Sulphur, even in small quantities, has a very injurious effect on wrought-iron, making it red-short, although the metal may be readily worked in the cold. With cast-iron a small quantity of sulphur is sometimes an advantage, making it stronger, more fusible, and more liquid when poured. Sul

phur in pig-iron tends to the production of the white variety. The surface and fractured portions often exhibit black patches, which are characteristic of sulphur in iron. Ferrous sulphide, heated with carbon, is but little affected, but it is decomposed at a high temperature by oxidising substances. When this sulphide is heated with ferrous sulphate in suitable proportions, the whole of the sulphur is removed as sulphurous acid SO2, leaving ferric oxide FeO.

Iron and Phosphorus.-These bodies readily unite when phosphorus is dropped into red-hot iron, forming a phosphide of iron Fe,,P. When oxide of iron is reduced. in the presence of an earthy phosphate, phosphorus is separated, and unites with the iron. 3 per cent of phosphorus in wrought-iron makes it harder and somewhat diminishes its tenacity. 5 per cent makes it cold-short but not red-short. 1 per cent makes it very brittle. The effect of phosphorus on iron is, to impart a coarsely crystalline structure, diminish its strength, increase its fusibility, and make it cold-short. The presence of phosphorus in cast-iron diminishes its strength, but on account of its imparting fluidity to the metal, its presence is beneficial in making fine castings.

Iron and Arsenic. The effect of arsenic on iron is much the same as that of sulphur, a very small amount will make the metal red-short. Several compounds of iron and arsenic are known, varying from gray to white in colour. Arsenic is not a very frequent impurity in iron.

Iron and Silicon.-A compound of silicon and iron, highly crystalline, and of a silver-white colour, known as silicon iron, is now an article of commerce, and is used for producing soundness in steel castings. It is obtained by reducing silica with carbon in the presence of iron. If iron be heated alone with silica no action takes place. The effect of silicon on cast-iron is to set the combined carbon free, so that, as a rule, the grayer the pig, the higher the amount of silicon present. Silicon makes iron hard, more easily fusible, and brittle.

Iron and Carbon.-Carbon unites with iron in various proportions up to about 41 per cent, forming the different varieties of steel and cast-iron. When manganese is present a larger proportion of carbon may be taken up. The difference between malleable-iron, steel, and cast-iron is chiefly dependent on the relative amounts of carbon in combination with the iron. The more the carbon, the harder and more fusible the metal becomes, and this effect is considerably increased by the presence of other bodies, such as phosphorus, sulphur, etc. Combination takes place when iron is heated in contact with gaseous fuel, such as carbonic oxide, cyanogen, and hydrocarbons, or by a prolonged exposure to a high temperature in contact with solid carbon, such an operation being termed cementation. When the carbon present in iron reaches 15 per cent, the iron is sensibly harder; this may be considered the greatest amount of carbon which can be present in malleable iron without diminishing its softness and malleability. Steel may be considered as iron containing from 15 to 1.8 per cent carbon. When the proportion of carbon is low the metal is termed "mild steel," and in like manner those with the higher proportions of carbon are termed "hard steels." Carbon exists in pig-iron in two states-free and combined. When the carbon is chiefly in the combined form, the iron is "white." On the other hand, when the carbon is free, being diffused through the iron in crystalline scales, the iron is called "gray"; but neither variety is entirely free from graphite or combined carbon respectively. In some varieties the carbon is partly combined and partly free, which gives to the fractured surface of the metal a speckled appearance, consisting of gray spots, enclosed by reticulating lines of white; it is then termed "mottled iron."

Chromium is a comparatively rare metal, which only occurs in nature in combination with other elements, the chief ore being chrome-iron-stone FeO, Cr,O,. Chromium, or its oxides, forms the colouring matter of several minerals; the green colour of the emerald, for example, is due to chromium

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

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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 Co,ASS, and cobalt bloom (Co,AsO4, 4H,O).

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,