only in England, but in France, Holland, Sweden, &c. though from the cheapness of our tin, and the excellency of some sorts of our iron, the greatest share of the tin plate trade must ever center with ourselves. Our coal is another circumstance which tends to give Great Britain an advantage over some other countries, in such manufactures as require a great consumption of fuel. Wood was scarce in Saxony about a century ago, and it is now still more scarce in France. They are beginning, it is said, in that country, to use coal and coak, or charred pit-coal, called by them charbon de terre épuré, and they have granted a patent to an individual for the preparation of it*. Another individual has begun to distil tar from pit-coal, and he gets about five pounds weight of tar from an hundred of coal (which is pretty nearly what I suggested, in 1781, as possible to be obtained from the same quantity, Vol. II. p. 352). The French + expect great advantage from this mode of depurating coal: but we have nothing to apprehend on that score; for the patriotic zeal of the Earl of Dundonald has put us in possession of every advantage which can be expected from a discovery, which he has had the honour of bringing to perfection.

The plating of copper is performed in the following manner: Upon small ingots of copper they bind plates of silver with iron wire, generally allowing one ounce of silver to twelve ounces of copper. The surface of the plate of silver is not quite so large as that of the copper ingot; upon the edges of the copper, which are not covered by the silver, they put a little borax; and exposing the whole to a strong heat, the borax melts, and in melting contri. butes to melt that part of the silver to which it is contiguous, and to attach it in that melted state to the copper. The ingot, with it's silver plate, is then rolled under steel rollers, moved by a water wheel, till it is of a certain thickness; it is afterwards further rolled by hand rollers, to a greater or less extent, according to the use for which it is intended; the thinnest is applied to the lining of drinking horns. One ounce of silver is often rolled out into a surface of about three square feet, and its thickness is about the three thousandth part of an inch; and hence we need not wonder at the silver being soon worn off from the sharp angles of plated copper, when it is rolled to so great an extent. Plated copper has, of late years, become very fashionable for the mouldings of coaches, and for the buckles, rings, &c. of horse harness. It might be used very advantageously in kitchen utensils, by those who dislike the use of tinned copper, and cannot afford to be at the expence of silver saucepans, &c. The silver, instead of being rolled on the copper to so great a thinness as it is in most works, might be left in kitchen furniture considerably thicker, so that an ounce of sil. ver might be spread over one square foot; the silver coating would in this case still be very thin, yet it would last a long time. Fire does not consume silver, and the waste in thickness, which a piece of plate sustains from being in constant use for a century, is not much; as may be collected from comparing the present weight of any piece of college plate, which has been daily used, with the weight it had an hundred years ago.

I do not know whether any attempt has ever been made to plate copper with tin instead of silver; I am aware of some difficulty, which might attend the operation; but yet it might, I think, be performed; and if it could, we might then have copper vessels covered with a coat of tin of any required thickness, which is the great desideratum in the present mode of tinning: but it ought to be remarked, that the thicker the coat of tin the more liable it would be to be melted off the copper by strong fires.

The art of plating copper has not been long practised in England; nor do I know whether it was practised at an early period in any other country; for the Roman method of silvering copper was different, I think, from that now in use. Thomas Bolsover of Sheffield, in the year 1742, was the first person in England who plated copper; it was applied by him to the purposes only of making buttons and snuff-boxes: soon after it was used for various other works: a person of the name of Hoyland, at Sheffield, was the first who made a plated candlestick.

What is commonly called French plate, is not to be confounded with the plated copper of which we have been speaking; for though

both these substances consist of copper covered with a thin coat of real silver, yet they are not made in the same way. In making French plate, copper, or more commonly brass, is heated to a certain degree, and silver leaf is applied upon the heated metal, to which it adheres by being rubbed with a proper burnisher. It is evident, that the durability of the plating must depend on the number of leaves which are applied on the same quantity of surface. For ornaments which are not much used, ten leaves may be suffici ent; but an hundred will not last long, without betraying the me. tal they are designed to cover, if they be exposed to much hand. ling, or frequently washed. After the same manner may gold leaf be fixed, either on iron or copper. Gold is applied on silver, by coating a silver rod with gold leaf; and the rod being afterwards drawn into wire, the gold adheres to it; the smallest proportion of gold, allowed by act of parliament, is 100 grains to 5760 grains of silver; and the best double-gilt wire is said to have about twenty grains more of gold to the same quantity of silver*. It has been calculated, that when common gilt wire is flatted, one grain of gold is stretched on the flatted wire to the length of above 401 feet, to a surface of above 100 square inches, and to the thinness of the 492090th part of an inch: and M. de Reaumur says, that a grain of gold may be extended to 2900 feet, and cover a surface of more than 1400 square inches; and that the thick. ness of the gold, in the thinnest parts of some gilt wire, did not exceed the fourteen millionth part of an incht. The gold, when thus applied, is thinner than when silver is gilt in the following manner, which is yet reckoned one of the cheapest ways, and is used in making various toys. Gold is dissolved in aqua regia; and linen rags being dipped into the solution, they take up some particles of gold; the rags being burned to ashes, and the ashes being rubbed on the silver, the gold adheres to it, and is rendered visible by being well burnished.

CHAP. V.

GILDING IN OR MOULU; USE OF QUICKSILVER IN EXTRACTING GOLD AND SILVER FROM EARTHS; EXРЕRIMENTS OF BOERHAAVE ON QUICKSILVER; SILVERING LOOKING-GLASSES, AND THE TIME WHEN THAT ART WAS DISCOVERED.

THERE is another method of applying gold on copper or silver, which is much practised; it is called gilding in Or Moulu. Quick. silver dissolves gold with great facility: if you spread a gold leaf (not what is called Dutch leaf, which is made of brass) on the palm of your hand, and pour a little quicksilver upon it, you will see the quicksilver absorbing the gold, just as water absorbs into its substance a piece of salt or sugar. Persons who have taken mercurial preparations internally, seldom fail to observe the readiness with which the mercury transudes through their pores, attaching itself to the gold of their watches, rings, sleeve-buttons, or ear-rings, and rendering them of a white colour. A piece of gold, of the thickness even of a guinea, being rubbed with quicksilver, is soon penetrated by it, and thereby made so fragile, that it may be broken between the fingers with ease: and if more quicksilver be added, the mixture will become a kind of paste, of different degrees of consistence according to the quantity of quicksilver which is used. A piece of this paste is spread, by ways well known to the artists, upon the surface of the copper which is to be gilded in or moulu, and the metal is then exposed to a proper degree of heat: quicksilver may be evaporated in a far less degree of heat, than what is required to melt either gold or copper; when therefore the mixture of gold and quicksilver is exposed to the action of fire, the quicksilver is driven off in vapour; and the gold, not being susceptible of evaporation, remains attached to the surface of the copper, and undergoing the operations of burnishing, &c. too minute to be described, becomes gilt. This method of gilding copper, by means of quicksilver and gold, was known to the Romans*. Quicksilver will not unite with iron, yet by an easy operation, iron may be gilded in the same way that copper or silver may. The iron is first to be made bright, and then immers. ed in a solution of blue vitriol, its surface will thereby become covered with a thin coat of copper, and it will then admit the gild. ing as if its whole substance was copper.

It is this property which quicksilver has of uniting itself with gold, and it does the same with silver, which has rendered it of such great use to the Spaniards in America. They reduce the earths or stones, containing gold or silver in their metallic states, into a very fine powder; they mix this powder with quicksilver; and the quicksilver, having the quality of uniting itself with every particle of these precious metals, but being incapable of con. tracting any union with any particle of earth, extracts these metals from the largest portions of earth. The quicksilver, which has absorbed either gold, or silver, or a mixture of both, is separated from the substance it has absorbed by evaporation; the quicksilver flies off in vapour, and the substance remains in the vessel used in the operation. We have no mines of mercury in England; Sir John Pettus, indeed, says, that a little cinnabar is now and then met with in our copper mines; and Mr. Pennant observes, that quicksilver has been found in its native state on the mountains of Scotland; and I have been shewn a piece of clay, said to have been dug near Berwick, in which there were some mercurial globules: but there are no works at present, where mercury is procured in any part of Great Britain; nor are there many mines of mercury in any part of the world. In the Philoso. phical Transactions for 1665, we have an account of the quicksil. ver mines of Idria, a town situated in the country anciently called Forum Julii, now Padria de Friouli, subject to the regency, and included in the circles of the lower Austria, in Germany. These mines have been constantly wrought for above 280 years, and are thought, one year with another, to yield above one hundred tons of quicksilver. In Hungary also, there are mines which yield quick. silver, but not so copiously now as formerly. Alonso Barba men.

* Æs inaurari argento vivo, aut certe hydrargyro, legititum erat. Plin. Hist. Nat. XXXIII. Pliny understood by argentum vivum, native quicksilver, which is found in a fluid state in many mines; and by hydrargyrum he uuderstood quicksilver separated from its ore by fire; they are the same substance.