in Germany. It is sold very cheap in this country, under the name of Dutch gold or Dutch metal. It is about five times as thick as gold leaf; that is to say, it is about one sixty-thousandth of an inch thick.

Why is tin preferable to other metals for lining copper vessels ?

Because it combines with copper at a much lower temperature than is necessary to fuse the copper alone. When vessels are tinned, they are first scraped or scoured; after which they are rubbed with sal-ammoniac. They are then heated, and sprinkled with powdered resin, which defends the clean surface of the copper from acquiring the slight film of oxide, that would prevent the adhesion of the tin to its surface. The melted tin is then poured in and spread about. An extremely small quantity adheres to the copper, which may perhaps be supposed insufficient to prevent the noxious effects of the copper as perfectly as might be wished.

Why do watchmakers prefer Dutch brass to the English?

Because of its superior ductility, which is owing to the large proportion of copper contained in it; the Dutch being a compound of two atoms of copper and one of zinc, while the English is of equal parts of copper and zinc.-Thomson.

COBALT.

Why is cobalt extremely valuable to the manufacturers of porcelain ?

Because it not only produces a beautiful colour, but endures the extreme heat of their furnaces unaltered. This colour is so intense, that a single grain of the pure oxide will give a deep tint of blue to 240 grains of glass. Smalt, or powder-blue, used by laundresses, consists of oxide of cobalt, ground impalpably with flint-glass. This is also used to give a blue tinge to writing and printing papers.

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 18. 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 con

tains.

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 672,4161. 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, showed 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.