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dually tarnished, and at length is covered with a dark-green crust of hydrate of a carbonate of copper. It is not acted upon by water, unless air have access to it, and hence water may be kept in a copper vessel, except that at the surface a green crust is formed. Copper oxidates slowly in the air at common temperatures, but when it is raised to a red heat, it combines with oxygen quickly. When a plate of the metal is heated red-hot, it is covered in a few minutes with a crust, which separates in scales as the plate is allowed to cool; and any quantity of this oxide may be obtained by alternately heating the plate of metal and plunging it in cold water. When copper is heated below redness, its surface assumes finely variegated shades of orange, blue, and yellow, and in this manner thin plates of it are sometimes tinged for toys and rude ornaments. Nitric acid acts powerfully upon it, but it is scarcely at all affected by being boiled in hydrochloric acid.

Its specific gravity, when fused, is about 8.667, and when forged, 8.95. Its combining weight is generally reckoned 31.71, although many chemists, perhaps with better reason, reckon it double, 63.42. It combines with oxygen, chlorine, sulphur, and other bodies, and it forms, with the other metals, alloys, some of which are of great importance in the arts, such as brass and bronze.

Copper exists most largely in the natural state in combination with sulphur, but it is found also in combination with oxygen and chlorine, and forms various salts, constituting distinct mineral species. The most abundant salts of copper are the sulphates, produced by the decomposition of its sulphurets. The relations of copper with sulphur itself have been pointed out, and we are thus led to associate its production with that mighty volcanic agency, to which the production of sulphur may not unreasonably be ascribed. Be this as it may, copper is the production of an early period in the history of the globe, it being never found, it is believed, in more recent rocks than the earlier members of the carboniferous era.

XXXIV. ZINC.

The early nations of Europe were acquainted with a mineral, which, melted with copper, formed brass; but it is not known whether they were acquainted with the metal zinc in its separate state. It is first mentioned by Paracelsus in the 16th century. But there is reason to believe that it was, from early times, known to the natives of the East, and that they possessed the art of rendering it malleable. It is found in several minerals, but the ores which chiefly yield it are the native carbonates, classed under the common name calamine, termed by miners blende, or zinc-blende. The extraction of the metal from the first of these classes of minerals, is effected by heating the ore along with carbonaceous substances; and so likewise it is from the sulphuret, after the process of roasting, as it is termed, or exposing the ore to a low red heat. The clay pots, or crucibles, in which the smelting process is performed, are closed at the top, while through the bottom there passes an iron tube, the upper aperture of which is within the crucible, while the lower terminates just above a vessel of water. Through this tube the vapour of the zinc, with the other gaseous products, passes, and the zinc is condensed in the water. It is then melted, and, being cast into ingots, forms the zinc of commerce.

Zinc is a brilliant metal, of a bluish-white colour. Its structure is lamellated, and it is seen to be composed of thin crystalline plates adhering together. It is a somewhat hard metal, resisting the action of the file. When pure, it may be hammered into leaves at a low temperature, but the impure zinc of commerce is not malleable at a low temperature, nor until heated somewhat above 210°, when it be

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comes, to a certain extent, malleable and ductile; but at 400° again, it becomes so brittle, that it may be pulverized. This property of becoming malleable and ductile at a certain temperature is taken advantage of in the arts to form it into plates, and draw it into wires; and by a process of annealing and working which it then undergoes, it can be made to retain the malleability which it had acquired. Pure zinc, according to Mr Daniel, fuses at a temperature of 773° F. When fused in open vessels, it combines with the oxygen of the air, and is converted into white oxide of zinc, popularly termed flowers of zinc. When the metal is raised to a very strong heat, it takes fire, burning with a brilliant white flame, like phosphorus, and throwing off a vast quantity of light white flakes of the oxide. When heated to whiteness in close vessels, it volatilizes without change. Its density, when cast, is 6.862, and it increases by forging to 7.21. Its combining weight is reckoned 32.31.

Zinc is somewhat abundantly diffused in the mineral kingdom, and largely in combination with sulphur. Its ores are usually found associated with those of lead. It possesses various characters in common with bismuth, and other members of the sulphur group, but indicates an approach to iron, and the members more immediately connected with silicium and aluminum.

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XXXV. CADMIUM.

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Cadmium is a metal found associated in small proportion with zinc, and partakes of its properties. It was discovered in the year 1817 by Professor Stromeyer of Göttingen. He detected it in an oxide of zinc, prepared for medical use; and it has since been found in several of the ores of zinc. It is a brilliant metal, of a bluish-white colour, resembling zinc. It is very malleable and ductile, so that it may be hammered and drawn into wires. It is flexible, and yields to the knife; and, like zinc, tin, and lead, it stains paper or the skin. It fuses before it arrives at a red heat, and, in close vessels, volatilizes, condensing again in drops, and crystallizing as it cools. It is little changed by exposure to the atmosphere. When heated in the open air, it takes fire, like zinc, emitting fumes of an orange-coloured oxide. Its density after fusion is 8.064, and after being hammered, 8.694. Its combining weight is calculated to be 55.83.

XXXVI. TIN.

Tin, although far less abundant than copper, was, like it, known to the earliest nations. It occurs in two mineral species, the peroxide, called tin-stone or tin-ore, and the cupreous sulphuret, called tin pyrites, or bell-metal ore. It is from the first of these ores that nearly all the tin of commerce is derived. It occurs in veins, or disseminated in granite and the primary rocks. It is found in Cornwall, Saxony, Bohemia, Hungary, in the peninsula of Malacca, and in some parts of the Eastern Archipelago, and especially in the island of Banca. In Cornwall, this ore occurs either in veins, or as rounded grains amongst beds of sand and rolled materials deposited by the action of water; the former yielding the block tin of commerce, which is the least pure, the latter, the grain tin, which is the most free from mixture. The smelting of tin is, like that of copper, a tedious process of art. The ore is first ground and washed, by which the heavier tin is separated from several of the lighter ores with which it is mixed. It is then exposed to the heat of a reverberatory furnace, when the sulphur and arsenic are expelled from the substances with which they were combined, by which means these substances are rendered so much lighter, that they may be separated by washing. The ore is then fused with lime and coal, by which process the lime combines with the earthy matters, and the coal reduces the oxide of tin to the metallic state.

Tin possesses considerable lustre, and a colour approaching to that of silver, but on exposure to the air, its surface becomes tarnished. It has an unpleasant taste, and, like copper, emits a peculiar odour when rubbed. It is soft and flexible, and, when bent backwards and forwards, produces

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