Cobalt ores generally contain arsenic: they are so contaminated with it, that the workmen who are employed seldom live many years.

Why is cobalt especially valuable in the fine arts?

Because its oxide forms the most permanent blue colour that we are acquainted with. La Grange says that the old painters used this oxide mixed with oil in their paintings, which is the reason why the sky and drapery in some old pictures are of so durable a blue.

BISMUTH.

Why is bismuth important in the composition for printing-types?

Because it has the singular property of expanding as it cools; and from this expansive property are obtained the most perfect impressions of the moulds in which the letters are cast. The larger kind of types are generally made with lead and antimony, in the proportion of from 4 to 16 parts of the former to one of the latter.

SILVER.

Why is silver alloyed with copper for plate and coin? Because the former metal is thus rendered harder and more sonorous, while its colour is scarcely impaired.

The silver mines of Mexico and Peru far exceed in value the whole of the European and Asiatic mines: for we are told by Humboldt, that three mines, in the space of three centuries, afforded 316,023,883 pounds troy of pure silver; and he remarks that this quantity would form a solid globe of silver, 91,206 English feet in diameter. (Jameson.) Mr. Helms is of opinion that the Andes, if properly examined, would afford silver enough to overturn our present commercial system, by making silver as common as copper.

Silver has also been obtained from some of the lead mines of Great Britain. Bishop Watson, in his Chemical Essays, notes, "By the silver which was produced

from the lead mines in Cardiganshire, Sir Hugh Middleton is said to have cleared two thousand pounds a month, and that this enabled him to undertake the great work of bringing the new river from Ware to London."

Why does silver tarnish and blacken?

Because of the sulphureous vapours in the atmosphere: pure water has no effect upon silver; but if the water contain vegetable or animal matter, it often slightly blackens its surface in consequence of the presence of sulphur.

Why is the German "silver" improperly so named? Because it is nothing more than the white copper long known in China, and does not contain a particle of silver; it is only an alloy of copper, metal, and nickel. Although only now coming into known use in England, it has been no stranger to the manufactories of Birmingham for at least twenty years or more.

Why is plating so called?

Because it is performed by the application of a plate of silver to the surface of copper, which is afterwards beaten or drawn out. Amalgam of silver is sometimes employed for plating; it is applied to the surface of the copper, and the mercury being evaporated by heat, the remaining silver is burnished. A mixture of

chloride of silver, chalk, and pearlash is employed for silvering brass: the metal is rendered very clear, and the above mixture, moistened with water, rubbed upon its surface. In this way, thermometer scales and clock dials are usually silvered.

A note upon the duty on plate will show how large a portion of gold and silver is annually diverted from the purposes of coin to those of ornament and luxury. The rate of duty upon silver wrought plate in 1804, was 1s. 3d.; upon gold 16s. per ounce; it was afterwards raised to 1s. 6d. upon silver, and to 17s. upon gold. At this time the annual net duty was less than 5000l.;

in 1828, it was upwards of 105,000l.; a rise more than twenty-fold, notwithstanding the greatly diminished supply from the mines, and the consequent increasing value of the precious metals.

Why is coal gas injurious to silver and plated goods? Because of the sulphuretted hydrogen which it contains.

Why is the Birmingham and Sheffield plate superior to that formerly made?

Because the old method was by dissolving mercury in nitrous acid, dipping the copper, and depending on the affinity of the metals, by which a very slight article was produced. But at Sheffield and Birmingham, all plate is now produced by rolling ingots of copper and silver together. About the eighth of an inch in thickness of silver is united by heat to an inch of copper in ingots about the size of a brick. It is then flattened by steel rollers worked by an eightyhorse power. The greater malleability of the silver occasions it to spread equally with the copper into a sheet of any required thickness, according to the nature of the article for which it is wanted. Plated metal, the eighth of an inch thick, is thus rolled by the hand into ten times the surface, the silver spreading equally; and the plating would be perfect if the rolling had reduced it to the thinness of silver paper! This mode of plating secures to modern plate a durability not possessed by any plate silvered by immersion. Hence plated goods are now sought all over the world, and, if fairly used, are nearly as durable as silver itself. Of this material, dinner and dessert services have been manufactured at from fifty to three hundred guineas, and breakfast sets from ten to two hundred guineas, as sold on the spot.

GOLD.

Why is gold alloyed with copper for coin?

Because it is thus made harder than pure gold, and

resists wear better than any other alloy except that with silver.

The produce of the Ural gold mines amounted, in 1827, to 672,4167. Gold is also found in the Rhine; but the quantity is so scanty, that, the washer considers it a good day's work, if he succeed in collecting to the value of 5s. or 6s. From the official accounts of the yearly produce obtained from that stream in the Grand Duchy of Baden, we observe the value was, in 1821-2, £603; 1826, £808; 1827-8, £943. The last produce, small as it may appear, for it scarcely exceeded 17 pounds in weight, shewed so considerable an increase upon preceding years, that a great impulse was given to this branch of industry in Baden.

We have already noticed the malleability of gold, though not its actual limit;* for the gold-beaters find it necessary to add three grains of copper in the ounce, to harden the gold, which otherwise would pass round the irregularities of the newest skins, and not over them and in using the old skins, which are not so perfect and smooth, they even add twelve grains. The wire which is used by the lace-makers, is drawn from an ingot of silver, previously gilded. In this way, from the known diameter of the wire, or breadth when flattened, and its length, together with the quantity of gold used, it is found, by computation, that the covering of gold is only 1-12th part of the thickness of gold leaf, though it is still so perfect as to exhibit no cracks when viewed through a microscope.

Fifty thousand pounds worth of gold and silver are said to be annually employed at Birmingham in gilding and plating, and of course lost for ever as bullion. The ductility of gold is such that one ounce of it is sufficient to gild a silver wire more than 1,300 miles long.

* See MECHANICS, p. 17.

Why is mercury used to separate gold and silver from the extraneous matter found with those metals

Because, by triturating the mass with mercury, the gold and silver become amalgamated with it; and afterwards this amalgam is submitted to heat, when the mercury sublimes; the precious metals leaving in a state of purity.

The gilding of buttons is, in part, similarly effected. When the buttons, which are of copper, are made, they are dipped into dilute nitric acid, to clean them, and then burnished with a hard black stone: they are then put into a nitric solution of mercury, and stirred about with a brush till they are quite white: an amalgam of gold and mercury is then put into an earthen vessel with a small quantity of dilute nitric acid; and in this mixture the buttons are stirred till the gold attaches to their surface: they are then heated over the fire till the mercury begins to run, when they are thrown into a large cap made of coarse wool and goat's hair, and in this they are stirred about with a brush. The mercury is then volatilized, by heating over the fire in a pan. By Act of Parliament, a gross of buttons, of an inch diameter, are required to have five grains of gold on them; but many are deficient even of this small quantity.

Why is false gilding so called?

Because it is the art of applying thin leaves of silver, or of tin, to the substance to be gilded, and then rubbing them over with a yellow transparent colour, through which the metallic splendour appears: this is very old; and a method of affixing a white metal to paper, and then covering it with a gold varnish, has been known in China from the earliest ages. It appears also to have been employed at a very remote period for gilding leather, of which many specimens may be found in ancient leathern tapestry.