gradation in their properties. Thus, as regards malleability and ductility, silver is intermediate between copper and gold, the latter possessing these properties in the highest degree. With respect to their tenacity, silver is again intermediate, copper being the most, and gold the least tenacious of the three.

COPPER.

Symbol, Cu. Atomic weight=63.6.

Occurrence. Copper is found in the elementary condition in various parts of the world, notably in the neighbourhood of Lake Superior, where native copper occurs in enormous masses. In combination, copper is a very abundant element, and is widely distributed, the most important of these natural compounds being the following

Modes of Formation. The methods by which copper is obtained from its ores vary with the nature of the ore. From ores containing no sulphur, such as the carbonates and oxide, the metal may be obtained by a method known as the reducing process, which consists in smelting down the ore in a blast-furnace with coal or coke, when the metal is reduced according to the equation

Cu2O+C

CO+2Cu.

In the case of mixed ores, containing sulphides, .the process (known as the English method) consists of six distinct stages

(1.) The ores, which contain on an average 30 per cent. of iron and 13 of copper (the remainder being chiefly sulphur and silica), are first calcined; usually in a reverberatory furnace, whereby a portion of the sulphur is burnt to sulphur dioxide, and the metals are partially oxidised.

(2.) The second step consists in fusing the calcined ore; when the copper oxides, formed during calcination, react upon a portion of the ferrous sulphide, with the formation of cuprous sulphide and ferrous oxide, thus

Cu2O+ FeS Cu,S+ FeO. 2CuO+2FeS = Cu2S+2FeO+S.

The oxide of iron combines with the silica already present (or which is added in the form of metal-slag obtained from the fourth process) to form a fusible silicate of iron, or slag, which contains little or no copper. This is run off, and a fused regulus remains, consisting of cuprous and ferrous sulphides, known as coarse-metal, and containing from 30 to 35 per cent. of copper. This molten regulus, which has a composition very similar to copper pyrites, is usually allowed to flow into water, whereby it is obtained in a granulated condition favourable for the next operation.

(3.) The third step consists in calcining the granulated coarsemetal; the result, as in the first calcination, being the removal of a part of the sulphur as sulphur dioxide, and the partial oxidation of the metals.

(4.) The calcined mass is next fused along with refinery-slag, which results in the production of a regulus consisting of nearly pure cuprous sulphide, the greater part of the iron having passed into the slag (known as metal-slag). This regulus, called finemetal, or white-metal, contains from 60 to 75 per cent. of copper.

(5.) The fifth operation consists in roasting the "white-metal" in a reverberatory furnace. A portion of the cuprous sulphide is here oxidised into cuprous oxide, which, as the temperature rises, reacts upon another portion of cuprous sulphide, thus

2Cu2O+Cu2S=6Cu+SO2.

At the same time any remaining ferrous sulphide is converted into oxide, thus

3CuO+ FeS 6Cu+ FeO+SO2.

The metallic copper so obtained presents a blistered appearance, and on this account is known as blister-copper.

(6.) This impure copper is lastly subjected to a refining process. For this purpose it is melted down upon the hearth of a reverberatory furnace, in an oxidising atmosphere. The impurities present in the metal, such as iron, lead, and arsenic, are the first to oxidise; and the oxides either volatilise or combine with the siliceous matter of which the furnace bed is composed, forming a slag, which is removed. The oxidation is continued until the copper itself begins to oxidise, when the oxide so formed reacts upon any remaining cuprous sulphide with the reduction of copper and the evolution of sulphur dioxide, according to the above equation. The metal at this stage is termed dry copper; and in order to reduce the copper

oxide which it still contains, the molten mass is stirred with poles of wood, and a quantity of anthracite is thrown upon the surface to complete the reducing process.

Wet Process.-Copper is extracted from the burnt pyrites, obtained in enormous quantities in the manufacture of sulphuric acid, which contains about 3 per cent. of copper. Although too poor in copper to be submitted to the smelting process, it is found that when calcined with 12 to 15 per cent. of common salt, the copper is all converted into cupric chloride. On lixiviating the calcined mass with water, the cupric chloride goes into solution, and metallic copper can be precipitated from it by means of scrap-iron or by electrolysis.

Properties. Copper is a lustrous metal, having a characteristic reddish-brown colour. The peculiar copper-red colour of the metal is best seen by causing the light to be several times reflected from the surface before reaching the eye.

Native copper is occasionally found crystallised in regular octahedra, and small crystals of the same form may be artificially obtained by the slow deposition of the metal from solutions of its salts by processes of reduction.

Copper is an extremely tough metal, and admits of being drawn into fine wire, and hammered out into thin leaf. Its ductility and malleability are greatly diminished by admixture with even minute quantities of impurities. When heated nearly to its melting-point, copper becomes sufficiently brittle to be powdered. The specific gravity of pure copper, electrolytically deposited, is 8.945, which by hammering is increased to 8.95.

Copper is only slowly acted upon by exposure to dry air at ordinary temperatures; but in the presence of atmospheric moisture and carbon dioxide it becomes coated with a greenish basic carbonate. When heated in air or oxygen, it is converted into black cupric oxide, which flakes off the surface in the form of scales. When volatilised in the electric arc, copper gives a vapour having a rich emerald-green colour.

Copper is readily attacked by nitric acid, either dilute or concentrated, with the formation of copper nitrate and oxides of nitrogen (page 246).

Dilute hydrochloric and sulphuric acids are without action upon copper when air is excluded, but slowly attack it in the presence of air, or in contact with platinum. Cold concentrated sulphuric acid does not act upon copper; but when heated, copper sulphate

and sulphur dioxide are formed, with the simultaneous production of varying quantities of cuprous and cupric sulphides, which remain as a black residue (page 416).

Finely divided copper is slowly dissolved by boiling concentrated hydrochloric acid, with evolution of hydrogen and formation of cuprous chloride

2Cu+2HCl=Cu2Cl2+H2.

In the presence of air, copper is acted upon by a solution of ammonia, the oxide dissolving in the ammonia forming a deep blue solution.

Copper is an extremely good electric conductor, being only second to silver in this respect; it is therefore extensively employed for cables, or leads, for purposes of telegraphy and electric lighting.

Copper possesses the property, in a high degree, of being deposited in a coherent form by the electrolysis of solutions of its salts. On this account it is extensively used in processes of electrotyping.

Alloys of Copper. The most extensive use of copper is in the formation of certain alloys, many of which are of great technical value. The following are among the most important :—

Oxides of Copper. Two oxides of copper are well known, namely, cuprous oxide (copper sub-oxide), CuO, and cupric oxide (copper monoxide), CuO.

Cuprous Oxide, Cu2O, occurs native as red copper ore. It is formed when finely divided copper is gently heated in a current of air, or when a mixture of cuprous chloride and sodium carbonate is gently heated in a covered crucible.

Cu,Cl2+NaCO3=2NaCl+CO2+Cu2O.

Cuprous oxide is also obtained when an alkaline solution of a copper salt is reduced by grape sugar.

Cuprous oxide is insoluble in water; it is converted into cuprous chloride by strong hydrochloric acid. Nitric acid converts it into cupric nitrate with the evolution of oxides of nitrogen. When acted upon by dilute sulphuric acid, it is partly reduced to metallic copper and partly oxidised into copper sulphate, thus

Cu2O+H2SO1 = CuSO1+Cu+H2O.

When heated with the strong acid it is entirely oxidised, thus—

Cu2O+3H2SO4=2CuSO4+SO2+3H2O.

Cuprous oxide fuses at a red heat, and when melted with glass, imparts to the latter a rich ruby-red colour.

Cupric Oxide, CuO, occurs as the rather rare mineral, tenorite. It is formed when copper is strongly heated in the air or in oxygen, or by gently igniting either the nitrate, carbonate, or hydroxide. It is a black powder, which rapidly absorbs moisture from the air. When heated, it first cakes together and finally fuses, giving up a part of its oxygen, and leaving a residue consisting of CuO,2Cu2O.

When heated in a stream of carbon monoxide, marsh gas, or hydrogen, it is reduced to the metallic state. Similarly, when mixed with organic compounds containing carbon and hydrogen, it oxidises these elements to carbon dioxide and water, itself being reduced on this property depends its use in the ultimate analysis of organic compounds.

:

Cupric Hydroxide, Cu(HO), is the pale blue precipitate produced when sodium or potassium hydroxide is added in excess to a solution of a copper salt. The compound, when washed, may be dried at 100° without parting with water; but if the liquid in which it is precipitated be boiled, the compound blackens, and is converted into a hydrate having the composition Cu(HO)22CuO. Cupric hydrate dissolves in ammonia, forming a deep blue liquid, which possesses the property of dissolving cellulose (cotton wool, filter paper, &c.).

Salts of Copper.-Copper forms two series of salts, namely, cuprous and cupric salts. The former, which are colourless, readily pass by oxidation into cupric salts, and serve therefore as powerful reducing agents, and are mostly insoluble in water. The cupric salts in the hydrated condition are either blue or green in colour; the anhydrous cupric salts are colourless or