therefore neceffary to remelt it in an Iron ladle, when the Iron will immediately rife at top, in form of fcoria, when the Lead may be eafily poured off, and the fcoria will be left in the ladle. A little tallow may be added before the Lead is poured off, which will reduce fome of the Lead that was burned, and increafe the produce.

In this operation, the Iron having a ftronger attraction to fulphur than Lead, frees the Lead from it, which by this means is reduced to its metallick form. The Iron is alfo mineralized by the Lead, which is evident, by its melting the Mundick shine, which those scoria exhibit when broke; but cfpecially by falling abroad when expofed to the air, and being convertible into copperas, juft in the fame manner as the fulphurcous Marcafites are.

If Lead Ores have arfenical pyrites mixed with them, the affay is more difficult; for in this cafe, they must be calcined like Copper Ores, and all the arfenick must be evaporated. By adding powdered charcoal in proportion to one quarter the weight of the Ore, it will expedite the calcination, and prevent it from running into lumps, which it is very apt to do.

When it is calcined, it must be mixed with its own weight, or more, of black flux, and about a quarter or fifth part of Iron filings; put on them a layer of falt, and melt down, till it flows thin; then pour it out, and treat the Lead as was done in the former process, to free it from the Iron.

The use of the calcination in this laft procefs, is to discharge the arfenick, which renders the Iron cafily fufible; and if the Ore was not calcined, would fall down, in a reguline form, together with the Lead, and render it impure. Befides, it would caufe an imperfect feparation of the fcoria, and keep up a great deal of the Lead amongst them; for, as this arfenical regulus would incorporate with the Lead, the mixture would be much lighter than Lead. The Iron filings are added, to absorb the vitriolick acid that may be left in the Ore after calcination.

Lead is affayed for Silver or Gold on the cuppel, as directed before; and all the Silver it contains above twelve troy ounces in the ton, is profit.

Procefs

Proceís XVI. To affay Tin Ore, called Black-Tin.

The method of affaying Tin Ore is very eafy; for in its form and fize of Black-Tin (which is the Ore dreffed by stamping, feveral washings, and calcination, if mineralized with vitriolick, arfenical, or fulphureous pyrites) great part of the work is done to the affayer's hand; fo that little more remains, than to proceed to immediate fufion, which is prefently accomplished by a red heat, in the following process.

Take four or five ounces of Black-Tin as emptied from the facks; mix it well with about one-fifth part of its weight of powdered culm; put the mixture in a Black-Lead crucible on the wind furnace, and in twenty minutes (more or less, according to the ftrength of the fire, and the greater or lefs fufibility of the Ore) you will find the Metal precipitated as far as may be to the bottom of the crucible, the culm and scoria floating on the Tin, not in a vitrified, but loofe unconnected state. You will generally fee globules of Tin lying on the surface of this matter; you should therefore with an Iron rod stir the mixture, by which means most of those globules will fall through it into the Tin at the bottom. Clofe the furnace, and let the whole remain in fufion from three to five minutes. Keep by you an Iron or Brass mortar, and an ingot mould of about fix inches in length, fig. R, plate VI. Pour the Tin into the ingot, and empty the culm and scoria into the mortar, scraping off what remains in and about the crucible (which fhould always be of the Black-Lead kind) with a sharp iron. As foon as cold, put them into another mortar and pulverize them, first in a fmall degree, fo as to separate the scoria from the largest of the globules of Tin, fome of which will always remain therein after pouring out the ingot as before directed. Select the larger globules, and pulverize the remainder a second time; then put this ftuff fo twice powdered on a fhovel, and paffing it often through water, in the fame manner as the lighter parts are washed from Ore in vanning, you will have the smaller globules remaining on the fhovel; and these with the larger (both together generally called Pillion-Tin) being added to, and weighed with the ingot, fhew the produce in Metal of the four or five ounces affayed.

Process XVII. To affay Cobalt.

Z z z

Take

Take a bit of the Mineral supposed to be Cobalt, with its weight of borax; put both into a broken china cup, and blow on them with a blow-pipe till they are perfectly melted and vitrified. If the china-ware is tinged blue in the spot where the Ore was placed, it contains Cobalt. But as fome Ores contain Cobalt and Mundick together, in which cafe the Iron would render the Glafs black, the best way is regularly to aflay the Ore which is fuppofed to contain Cobalt, as follows:

Pour it

Calcine an ounce of the Ore in the fame manner as Copper Ore is directed to be calcined, only the calcination need not be carried fo far; for as foon as the fulphurcous flame evidently difappears, it is fufficiently calcined. Melt the calx with black flux, as directed in the fcorification of Copper Ore. out in the ingot, and melt a little of the regule with five or fix times its weight of flint glafs, and a little borax, for half an hour in a fmall crucible. If the glafs is of a fine blue colour, the regulus is pure; but if the glafs is black, it contains Iron, and must be refined with the white flux, in the fame way as is directed in refining the Copper affay. As long as the scoria are black or brown, the regule contains Iron; but as soon as the fcoria, and fides of the crucible, are tinged blue, it is fine: and if this does not happen, when the whole of the regule is confumed, the Mineral contained no Cobalt. The goodness and value of the Mineral is cftimated by the quantity of pure regulus it contained. If there is any Silver or Bifmuth mixed with the Cobalt, they will neither of them mix with the cobaltine regulus; but, on breaking the pot, will be found quite diftinct from it: and it is the fame if the matter is poured into an ingot.

This regulus is not to be made malleable, but from this procefs is evidently that which ftrikes the colour: for a further proof, take two ounces of fmalt or powder blue, mix it with its weight of argol or tartar, and it will depofit in fufion the regulus that gave the colour. May it not be fairly concluded from hence, that all the Semi-metals which ftrike any colour, will depofit a regulus which is the efficient caufe of it? But the knowledge of this valuable branch of Mineralogy is its infancy with us.

Process XVIII. To affay Bismuth.

yet

in

Bifmuth

Bifmuth is easily feparable from its Ore, and may be procured pure by melting the Ore in a crucible in a moderate fire, without any flux; but if it is very impure, an addition o. the black flux will foon fufe it: however, the fire muft not be too fierce, for if it be, the Bifmuth will be loft.

To difcover Silver in Goffans or very poor Ores.

Any Goffans or very poor Ores which are supposed to contain Silver, being calcined and mixed with three times their weight of litharge, may be affayed as directed in the process for affaying Mundick; only there will be no need of Glass, as the Ores are supposed to be ftony. Care must be taken, that the scoria are thin at the laft, either by the continuance of the fire (by which litharge will be formed from the Lead at bottom) or by the addition of litharge, as directed in the aforefaid procefs. The china-ware crucible is alfo best here.

CHA P. II.

Of Smelting of Copper Ores in the great Furnaces called Copper Works.

form a juft and general idea of the conftruction of

Tfurnaces, and of the difpofition of the feveral apertures in Ꭲ

them, with a view to increase or diminish the activity of the fire, it will be proper to lay down, as our ground-work, certain principles of natural philofophy founded on experience.

First, Every one knows that combuftible matters will not burn or confume unless they have a free communication with the air, infomuch that if they be deprived thereof, even when burning moft rapidly, they will be extinguished at once; that, confequently, combuftion is greatly promoted by the frequent acceffion of fresh air; and that a ftream of air, directed fo as to pafs with impetuofity through burning fuel, excites the fire to the greateft poffible activity.

Secondly, It is certain that the air which touches or comes near ignited bodies, is heated, rarefied, and rendered lighter

than

than the air about it, that is further diftant from the center of the heat, and confequently that this air fo heated and become lighter, is neceffarily determined thereby to afcend in order to make room for that which is lefs heated and not fo light, which by its weight and clafticity tends to occupy the place quitted by the other another confequence hereof is, that if fire be kindled in a place enclosed every where but above and below, a current of air will be formed in that place, running in a direction from the bottom to the top; fo that if any light bodies be applied to the opening below, they will be carried up towards the fire; but, on the contrary, if they be held at the opening above, they will be impelled by a force which will drive them up, and carry them away from the fire.

Laftly, It is a demonftrated truth in hydraulicks, that the velocity of a given quantity of any fluid determined to flow in any direction whatever, is fo much the greater the narrower the channel is to which that fluid is confined, and confequently that the velocity of a fluid will be increased by making it run from a wider through a narrower paffage. These principles being established, it is eafy to apply them to the construction of furnaces.

The materials fitteft for building furnaces are, bricks joined together with potter's clay mixed with fand, and moistened with water; potter's clay mixed with potfherds, moistened with water, and baked in a violent fire: alfo Stourbridge clay, and many of our talcy clays to be had in great plenty in the Cornish foft grouan ftrata, mixed and baked in the fame manner.

The only kind of furnace for fmelting Ores where bellows are not employed, is what is called a Reverberatory Furnace. The Germans call it a Wind Furnace. It is alfo diftinguished by the name of English Furnace, because the invention of it is attributed to an English phyfician. The Copper furnaces bear four names, viz. the Calciner, which is the largeft; the Operation, Roafter, and Refiner, which are all of one gauge or nearly fo both in shape and fize.

The hearth or bed of the calciner fhould be eighteen feet long and thirteen feet wide within, by two feet ten inches at a medium from its concave back to the bottom, which muft be flat. The fire place three feet four inches long, by two feet wide and two feet deep, so as to have two feet of flame to pass