perfectly well done by this process. The fulphur and arsenick, are continually flying off through the Glafs, which is likely to detain any of the nobler Metals, which the arfenick might otherwife volatilize; at the fame time, the Iron which was mineralized by them, burns to fceria, and rifing a-top of the metallick part mixes with the Glafs, and is vitrified with it; the Mundick at bottom grows more and more metallick, and, as I apprehend, the Lead, if not entirely, is at least greatly mineralized by the fulphur and arfenick. The Iron and Lead, in this Mineral state, are mixed; but the Iron parting from these matters eafier, as well as attracting them ftronger, than the Lead, difcharges them up through the Glafs, and is gradually turned into scoria, till the whole of it is feparated from the Lead, leaving with it the nobler Metals it contained.

The only hazard of miffing in this procefs, is from the vessels being corroded by the Glafs of Lead, which is very penetrating, when brought to that thinnefs by the litharge which is neceffary; but this may be effectually prevented by the ufe of a porcelain or china-ware crucible, which as it is a new invention, and may be of great ufe to the curious in Metallurgy, without remarking on what others have done, I fhall here give it to the publick in few words.

I

Process IV. Whoever hath been converfant in Mineral chymistry, must know, that vessels which will hold Glass of Lead, prove a great defideratum. Now the micofe clay, which is one part of china ware, is known to be abfolutely unvitriable; for though mixed with an equal part of vitriable stone, it stands the greatest heat that art knows, without being vitrified.* believe all the grouan clays would answer to make the veffels in queftion; and, I know that the porcelain clay at St. Stephen's will. The compofition I would recommend, is two parts of the washed clay, and one part of the gravel it contains, ground to a very fine powder, mixed and made into a pafte. Let a potter form them into the shape of coffee dishes of a moderate thickness, and of different fizes, according to the purposes they are defigned for. They must be burned in a crucible, or with crucibles, or porcelain, if you are in the neighbourhood of a factory of either kind. The fire must be full as strong as is neceffary to burn china ware or crucibles; but if one hath not the advantage of a neighbouring pottery, the highest heat that

* See book i. chap. iii. upon Steatitæ, or Soap-rock.

can be given in a finart wind furnace, is fufficient. When burned, they are a true unglazed porcelain as it is poffible; the St. Stephen's clay without mixture, may make the strongest veffels; it might be tried: but I know common porcelains anfwer extremely well.

As thefe veffels will by no means bear an open fire, they must be guarded: the best way of doing which, is to place them in crucibles made round, and about two-thirds of an inch, or an inch wider. Lay in the bottom of the large crucible the thicknefs of half an inch of flint fand; if this cannot be had, Quartz, or (as it is improperly called in Cornwall) Spar, may be powdered and fifted through a hair fieve: fill up the vacancy between the two crucibles with the fand or powder, and let the outfide crucible have a cover made to it exactly like that of a

teapot, and the apparatus is finished. See plate VI, fig. 3. This apparatus must be fixed on a conical bafe made of two parts pipe clay, and one part fand; the fhape of it is to be seen plate VI, fig. 4, a little excavated at top, to let in the crucible that it may ftand fteady.

I have thought proper to give this procefs on Pyrites, as there has been much contention about the matter; people will now have it in their power to know whether or not they are of any value.

Any Ore that is fuppofed to contain Silver, or Gold, mixed with a proper quantity of litharge, with revived Lead at bottom, and a mixture of Glass, if the Ore has no vitrifiable ftone in it, may be tried the fame way. The want of vitrifiable ftone or earth, may be known by the scoria, which will be tough and metallick, not glassy.

Litharge is easily revived, by mixing it with a proper quantity of black flux, and a little charcoal duft, and melting the whole in a strong fire, till the furface melts fmooth and equal, without bubbling.

Procefs V. Cuppellation; and the feparation of Silver and Gold by Aqua Fortis.

The veffels used in this procefs, are called Cuppels, and are formed ordinarily of bones burned white and powdered, or of the ashes of vegetables from which the falts have been thoroughly

separated

feparated by water. But for the formation of cuppels, I refer to Cramer's Art of Affaying, or Maquer's Chymistry, where the manner of doing it is very rightly directed. Some of these cuppels are made in moulds, and others in Iron rings. The former are inverted fruftums of a cone, much about the fize of fig. 5, plate VI, which is a fection of a cuppel. The others are formed in Iron rings, larger or lefs at pleasure. The method of forming them, is to fill the ring with the bone or other ashes, or a mixture of both. The afhes are brought much to the fame temper of moisture with water, as fand is for cafting Metals: the fand is then beat down as clofe as poffible, and a hollow place is formed in the cuppel, for holding the Metal. These cuppels are made either round or oblong. The kind of fig. 6, plate VI, may be ufed, four or five inches wide; which will work off four or five pounds of lead.

As the cuppelling furnace will hold several tests, when one wants to cuppel, it is right to put three or four dry cuppels into the reverberatory with their bottoms upward. Light a fire in the wind furnace, and raife it gradually till the cuppels are red hot; then fet one or more of them with their hollows upward, and with a small Iron ladle put the Lead to be tried into one of the cuppels: the Lead is ufually beat flat, and cut into pieces, which will melt immediately and contract a scum, and if the fire is fufficiently ftrong, in fome little time the fcum will feparate, and discover the melted furface of the Metal, as bright as Quickfilver. If the procefs goes on well and right, there will be little particles or drops refembling oil, continually rifing on the furface of the Metal, which will be thrown off to the fides, and abforbed by the cuppel. The fire is to be conftantly and uniformly kept up, fo as to keep the affay in this way of working, till the Lead is all converted into litharge, and the Silver or Gold fets on the cuppel. Expertnefs in this process is only attained by practice. Cramer's defcription of it, is very exact; but as the furnace here directed, is different from his, it is neceffary to obferve, that if the fire wants to be fuddenly quickened, fresh lumps of coal, or fmall pieces of dry wood, are to be thrown into the wind furnace, by juft opening one of the bricks that cover it. When the affay is too hot, a covering brick or two may be taken off, or even the ftopper in the reverberatory left open, till the heat is funk to a proper temper. The marks of too great or too fmall a degree of heat, are accurately described by Cramer.

Weigh the grain left on the cuppel, and fee what proportion it bears to the Mundick affayed; from whence it is easy to calculate the quantity of noble Metals, in any given quantity of the Mundick. Lead reduced from litharge is ufed in this operation, as it contains no Silver, at least fo inconfiderable a quantity, as is not worth attending to.

Process VI. To discover, whether the product of the affay contains Gold, and the quantity it contains.

Pour on the grain, four or five times its weight of proof aqua fortis; place the phial on warm fand, and if the Silver entirely diffolves without any black fediment, it contains no Gold; but if there is any black fediment, this is Gold. Pour the folution of Silver from it, and pour water on it, fhaking the whole; let this water settle, and then decant it off into the folution of Silver; repeat this till the water has no bitter taste. Wash out the black powder into a small tea dish; and when it is fettled, pour off the water from it, and dry this powder of Gold by placing the dish on hot fand. Weigh the powder, and make the calculation. If the Gold is in fo fmall a quantity, that you have no scales or weights nice enough to weigh it, the Lead must be enriched by the operation of fcorification, being repeated with the fame Lead, on three or four more parcels of fresh Mundick.

If the grain or bead of Metal contains much Gold, fay as much, or more, or even one-third of Gold, the aqua fortis will not diffolve it; in which case, three or four times its weight of Silver (which contains no Gold) may be melted with it, or fo much as will render it diffolvable in the aqua fortis. The Silver may be precipitated from the solution, by evaporating the water from it in a fuitable china-ware veffel fet in hot fand, till the quantity is properly reduced; that is, till the water used in wafhing the Gold is moftly evaporated from it; when by putting clean bits of Copper into it, the attraction between the aqua fortis and the Copper, being ftronger than with the Silver, this latter will be precipitated in the form of a white shining powder, to be feparated from the bits of Copper. If clean bright pieces of Iron are put into the folution, the Copper will be precipitated; and alkali falt will precipitate the Iron. Wash this precipitate till the water is no longer faline; evaporate the whole, and what is left will be a true good nitre, formed by

the

the spirit of nitre, and the pot-ash; the vegetable alkali being the bafis of nitre.

Process VII. Proof Aqua Fortis.

Take any quantity of good aqua fortis, which will diffolve Silver; drop into it a few drops of a faturated folution of Silver : if there appears to be any precipitate or cloud of a white colour, as there will if the aqua fortis has spirit of falt in it, which I believe is always the cafe; if this precipitate falls foon to the bottom, it is proof the aqua fortis contains much spirit of falt, and one may be bolder in dropping in the solution of Silver; but if it is thin and light, it is neceffary to proceed with more caution. Let this milkiness settle; and to a small

quantity of the aqua fortis in a phial, add a drop of the solution of Silver; and if there ftill appears a milkinefs, more of the folution may dropped in, always aiming to add no more of the Silver folution, than is neceffary to separate the whole of the fpirit of falt from the aqua fortis, which may be known by adding a drop of the solution to a little of the aqua fortis in a phial; for if the aqua fortis is proof, it will continue quite clear without the least milk inefs.

There is an eafier way of preparing proof aqua fortis, which is by putting a bit of Silver into it, and fhaking it feveral times in a few hours; and if, the next morning, it is fettled quite clear, and any of the Silver is left, it is proof. The only question is, whether it doth not contain Silver; to determine this, drop a few drops of it into filtered brine, and if there arifes no cloudinefs in the mixture, the aqua fortis contains no Silver.

Spirit of falt will not diffolve Silver; but being diffolved in aqua fortis, there is a stronger attraction between the spirit of falt and the diffolved Silver, than between it and the aqua fortis, as it dislodges the spiritus nitri, and unites with the Silver into a falt that is not diffolvable in water, and so finks to the bottom in a white curd called Luna Cornea, which may be reduced into Silver with pot-afh, by being melted with it; and if the pot-afh is not in too great a quantity, it will be converted into a fea falt, with a vegetable alkali bafis; by which it appears, that the fea falt was decompofed, or feparated from its mineral alkaline bafis, in the operation of precipitating the Silver. What is called the Mineral Alkali, or Bafis of Sea Salt, is of the