Chromium is not altered by exposure to atmospheric air, but when heated is susceptible of three degrees of oxidation. The protoxide, or green oxide, which may be obtained by exposing the metal to heat, or by heating chromic acids in elose vessels, is infusible, indecomposible by heat, reducible by voltaic electricity, and not acted upon by oxygen or the air. The deutoxide, or brown oxide may be procured by exposing the protonitrate, or the protoxide dissolved in nitric acid, to heat till fumes of nitrous gas cease to ascend; a brown powder, insoluble in acids, and scarcely soluble in alkalies, remains. The peroxide, or chromic acid is found in nature in red-lead ore; it has a deep scarlet colour, and consists of one part chromium, and two parts oxygen.

The oxides of this metal furnish the most beautiful pigments used in oil and water-colour painting, dyeing, calico-printing, enamelling, and in glass and porcelain painting.

The principal ores of chromium are chromate of lead, found in the Ural Mountains, in Siberia, and in Brazil; and chromate of iron, found in France, in Shetland, and near Baltimore in America. It has never been found in the metallic state, but only in the state of oxide, or in that of an acid. It is supposed to be the colouring principle of the emerald and the ruby.

CHAPTER XIV.

COBALT.

COBALT derives its name from kobold (an evil spirit), because, as it renders the working of metals with which it may be combined very difficult, the superstitious miners imagined that they were bewitched.

It has a grey colour with a tinge of red, a specific gravity of 7.834, a granular texture, and a fusing point of 130 degs. Wedgewood; it is sometimes composed of plates, sometimes of grains, and sometimes of very small fibres, adhering to

each other; it is attracted by the magnet, and may itself be converted into a magnet; it is so brittle that it may easily be reduced to powder. It is not very brilliant, and it undergoes but little change when in contact with the air, or with water, at ordinary temperatures; but, at a red heat in an open vessel, it becomes oxidized. The oxide furnishes all those beautiful blue colours employed in the manufacture of glass, porcelain, and pottery; it has such a power with vitrifiable substances, that 1 part in 240 will communicate a deep blue tint to glass.

Oxygen combines with cobalt in three proportions, forming the protoxide, the deutoxide, and the peroxide. The protoxide, which is dark blue, may be procured by igniting gently in a retort the oxide precipitated by potash from the nitric solution. It consists of 84 38 cobalt and 15.62 oxygen, and dissolves in acids without effervescence. Moist protoxide, when exposed to the air, gradually absorbs oxygen, and assumes an olive colour; this is the deutoxide, which is composed of 73 metal and 27 oxygen, or as Rothoff makes it, 100 metal + 36.77 oxygen. By gradual exposure of this oxide to the air, more oxygen is absorbed, and the peroxide is formed; this is black, and consists of 80 cobalt and 20 oxygen.

Zaffre is the term applied to a mixture of an impure oxide of cobalt and finely-ground silica, obtained by calcination of the ores. The oldest and the best-known sympatheticink is obtained by dissolving zaffre in nitro-muriatic acid. The solution is green, or if there be no excess of acid, blue; but it turns red when diluted with water. Landscapes drawn with it in this state, on paper, are colourless when cold, but acquire a fine green or blue colour (according to the quantity of acid), when heated or otherwise thoroughly dried.

Smalt is a finely pulverized deep blue glass, which owes its colour to the presence of oxide of cobalt. It is sometimes called azure blue; it is employed in washing to prevent linen from becoming yellow, and by paper manufacturers to give a blue tinge to writing and printing papers. Smalt is also used in porcelain, pottery, glass, encaustic tiles, frescopainting, &c.

The peculiar properties of cobalt were discovered by Brandt, who first obtained it in the metallic state in 1733.

It is met with in nature associated with arsenic, copper, iron, nickel, and sulphur: and in the metallic state, it occurs in meteoric stones or Aërolites. It is found in Cornwall, and Somersetshire; Sweden yields a considerable quantity; but Saxony produces the most.

CHAPTER XV.

COLUMBIUM,

Tantalum, or Tantalium.

COLUMBIUM was first obtained by Mr. Hatchett, in the year 1801, whilst analysing a mineral, which had been sent from America, to Sir Hans Sloane, and deposited in the British Museum. He evidently derived the term which he applied to it, from Columbus, the name of the discoverer of that part of the world in which this mineral was found.

Not long after this, Ekeburg, a Swedish chemist, discovered in a mineral from Finland, a metallic oxide, the base of which he called Tantalum, from the name of the mineral itself. Dr. Wollaston having examined this substance, found it to be identical with Hatchett's columbium.

Columbite, or tantalite (as it is frequently termed), the ore of columbium, first discovered in Connecticut, and occurring also in Finland, is an iron-black, brittle mineral, usually associated with beryl, and consists chiefly of columbic or tantalic acid and peroxide of iron and

manganese.

The following is Dr. Wollaston's method for obtaining the oxide of this metal, from tantalite :-Mix one part of tantalite with five parts of carbonate of potash and two of borax, and fuse in a platinum crucible; soften the mass with water, and digest it in hydrochloric acid. The iron and manganese are dissolved, and a white powder remains, which is the oxide of columbium.

Berzelius, by exposing this oxide, mixed with charcoal, to an intense heat, deprived it of its oxygen, although he was

unable to melt it. The metal had a dark-grey colour, was brittle, and consisted of particles adhering so firmly together, that water would not pass through them; which were hard enough to scratch glass, and which, when scraped with a knife, or rubbed against a fine grindstone, assumed a metallic lustre, and the appearance of iron.

Columbium, by trituration, may be reduced to a darkbrown powder. When heated, it takes fire, and burns feebly to a greyish oxide. It is not acted upon by either nitric, muriatic, or nitro-muriatic acid. Its specific gravity was found by Wollaston to be 561. It has been alloyed with iron and tungsten.

CHAPTER XVI.

COPPER.

THE name of this abundant, long and well known, metal is said to have been derived from the island of Cyprus, where it was first wrought by the Greeks; the alchemists called it Venus, the name of the protecting goddess of Cyprus. The ancients made their weapons and agricultural implements of brass or as, an alloy of copper and tin, before that iron was obtained in sufficient quantity to be applied to those purposes. Ample evidence may be obained from classic and Egyptian authorities, that brass was extensively used, and it is said the Egyptians had the art of tempering it; they used it for making bows, and arms of all kinds. The columns of the temple (I. Kings, vii., 13-21), the bath or sea, in the priest's vestibule, the forks used in the sacrifice, and the mirrors were all made of this alloy. Exodus xxxviii, 8, and II. Kings, xxv., 13.

Copper occurs in a state of native purity, in masses, of various forms, varying from an ounce to a ton, and usually possessing a reddish, but sometimes a brownish or blackish colour, Native copper is seldom met with in large masses; one, however, has been found in Brazil, weighing 2666 pounds; it occurs in Cornwall, France, Sweden, Hungary, Siberia, and South America.

Copper is also found in combination with sulphur and iron, forming a double sulphuret known as copper pyrites, or yellow copper ore; this constitutes the whole of the ore raised in Cornwall; it is also found in Devonshire, and Anglesea; in the north of Europe; and in many parts of Asia, Africa, and North and South America. On account of the supply of coal in South Wales, the ores, raised in Cornwall and Devonshire, are sent there to be smelted.

Oxygen is sometimes associated with this metal in nature; often as the red, or suboxide of copper, or ruby copper ore; and occasionally, as the black oxide.

It also is found combined with sulphur, forming black sulphuret of copper, or vitreous copper ore, which contains a small proportion of iron and silex, and is highly prized by the miner; and, with carbon, as malachite, and mountain green, or blue carbonate of copper, which ore is found in great beauty in the Ural Mountains of Siberia, and in Australia; it is a very valuable ornamental stone. It likewise occurs, associated with acids, phosphurus, and silica.

Copper is met with in several parts of England and Wales; the most extensive and the richest are those of Cornwall, and Devonshire. Cornwall has upwards of eighty copper mines, producing from 100 to 1500 tons annually. Anglesea, where there is a whole bill called Paris-mountain, consisting of copper ore, produces more than 20,000 tons every year.

Besides those of England, copper mines are worked in France, Saxony, Hungary, Germany, Spain, Norway, Sweden, Russia, Siberia, China, Japan, Sumatra, Chili, Cuba, Mexico, in North America, and North Africa. Copper is so very cheap in Sweden, that the houses are covered with it, as is the Coliseum in Regent's Park, London.

A contemporary writer describes the mode of reducing the yellow copper ore, as follows:-"It is first roasted in a reverberatory furnace, by which much of the sulphuret of iron is converted into oxide, while the sulphuret of copper remains unaltered. The product of this operation is then strongly heated with silicious sand; the latter combines with the oxide of iron to a fusible slag, and separates from the heavier copper compound. When the iron has, by a re