against folid Bodies, renders them flippery: whence, Workmen rub their Preffes, and other Tools, with Black-Lead inftead of Soap; partly to facilitate Motion, and partly to cover and keep off Ruft, by fuch a Lay of a fhining Black Colour. It is likewife commonly used for Writing Pencils. It hardly fuffers any Alteration in the ftrongeft open Fire*; except that, being thus divided into very fmall Particles, it lofes its Colour entirely, and becomes of a Confiftence fomewhat fofter. We fhall, when there is Occafion, in our fecond Part, at large mention this Mineral, and all the Particulars thereto belonging.

443. T

CHA P. V.

Of docimaftical Operations.

HOSE Actions which change the Objects of the Art, according to the Rules it prefcribes, and by Means of the Inftruments it employs, are called by Chemifts Operations.

444. All the Changes juft (§ 443.) mentioned are effected by Solution and Conjunction; fo that whatever the Chemift does may, in general, be eafily referred to these two Claffes, nor can a third Species of Mutation be imagined, notwithstanding all fome can fay.

445. As the Art of Affaying confifts in a wellmade Separation of Minerals, efpecially of Metals, and in a Divifion from each other (§ 1.) of the feveral conftituent Parts of them, that the Quantity and Quality of each in particular may be known; it is plain, that thofe Operations, which belong to the

*Black-Lead is of great Ufe for making Crucibles which bear the frongest Fire; the Powder of it mixt with Windfor-Loam makes a good Lute for coating Veffels over with; and it is of great Ufe among the Brafs-Founders, who rub over the Infides of their Moulds with it.

general

general Clafs of Solution (§ 444) appertain ftrictly, and primarily to this Place; and that the others, which are performed by the Affayer, are only fecondary or auxiliary Operations.

446. As therefore Conjunctions are to be confidered ($445.) here as only fecondary; one may eafily apprehend, that the docimaftical Operations cannot be divided in general into Solutions and Conjunctions, as chemical Operations are. Nay, any Operation performed by a Chemift or an Affayer, commonly has the Effect of both the Solution and the Conjunction; fo that no Solution can ever be performed without a new Conjunction, nor any Conjunction without a Solution. For Inftance, while heterogeneous Bodies are separated from Gold and Silver by Scorification, do not the Particles of Gold and Silver melt together into a greater Bulk? Likewife, do not the Bodies feparated, either of their own Accord, or with an Addition of fome fcorifying Body, melt together into Glafs, and thus join again in another Manner? It will then be very wifely done, always to be attentive to either of thefe Effects in any Operation whatfoever, and to derive the Differences and Denominations of the Operations from the principal of the faid Effects, and from the Diverfity of the Apparatus of the Inftruments (Chap. II. and III.), by which the Changes to be effected are made in the Objects (Chap. I. and IV.) of the Art.

447. But there is hardly any chemical Operation, which is not fometimes neceffary to be performed in the Art of Affaying. There are many, on the contrary, which are peculiar to Affaying alone. Therefore, we shall here give a general View of those which belong properly to it, or of thofe which, tho' taken from Chemistry at large, are nevertheless very frequently used by Affayers; and thall then add fhort Defcriptions of each of them in particular, derived from their primary Differences, their Effects, and the Methods ufed in performing them; that any Novice in this Art may thus be eafily led to the Practice, when

N 4

when we fhall attempt to propofe, in a more exact and clear Manner, fpecial Obfervations upon every individual Operation.

448. Every primary docimaftical Operation may, on Account of its Effect, be called Solution (§ 444); fince, in every Operation, the Menftrua, among which I think the Air and Fire have a right to be claffed, effect a true Solution, by interpofing themfelves between the Particles of the Objects to be changed.

Eliquation.

Now a doci

maftical Solution is either

or at least by much the greater Part of the Subject in Hand.

We may call these
Partial, as they all dif

Solve but one Part of

or Precipitation by Fufion. the Subject in Hand.

Quartation.

Moift Precipitation.

449. When the Cohesion of a folid Body is fo diminished by the Strength of the Fire, as that it may become fluid; then it is faid to be in Fufion, or melted; and the Action itself, whereby this Effect is produced, is called Fufion or Melting, which is the most frequent among the Operations, that are proper to the Art of Affaying.

450. All the fixt Bodies hitherto known in the Nature of things may be brought into ($449.) Fufion; with this Difference however, that fome melt of themselves in the Fire, and fome not, unless you add tothem a diffolving Body, that is, a Flux (§ 159-166.) There are alfo many among the volatile Bodies, that

may

may be put in Fufion; but then they evaporate at the fame Time.

451. When a Body melted ($449.) and grown. cold becomes brittle, fixt, not diffoluble by Water, and melts again when expofed to the Fire; then it is called Glafs; and the Operation which changes it in this Manner, is called Vitrification.

452. This Vitrification (§ 451.) is made use of by Metallurgifts, to the End, that any Metal imprifoned in a folid Body may, on account of its Weight, defcend and feparate itself therefrom: and when the Metal itself affumes the Appearance of Glafs, this Operation is technically called Scorification, in German Werfchlackung; and the Glafs thus produced is called Scoria, in German Schlacke, in English Drofs.

453. All fixed Bodies experience the fame (§ 452.) Alteration, not totally excepting even Gold and Silver. There are alfo among the volatile Bodies, fome that can be fixed, and which affume the Form of Glafs again, by adding fixing Bodies to them.

Scholion. They also call Scoria that faline Mafs, which is produced by melting Metals and Ores, together with faline (§ 159–166) and reducing Fluxes: Nor ought, however, the Word Scoria to be then understood of all this Mafs, but only of the vitrified Particles which adhere between the fmall Maffes of the Salts, and which may be feparated from them by a Diffolution in Water, and by Filtration.

454. It is oftentimes proper, to make the Scorification (§ 352.) in a Veffel, that may abforb the Scoria itself, and retain the metallick Part: In this Cafe, it is called Coppelling. The Tefts and Afh-Veffels mentioned (§ 168, 184, 185,) are designed for this Operation.

455. It is then felf-evident, that a great Attenuation of the Scoria is requifite, that it may be able to pass through the Veffel. Nor is there any fitter Body for this Purpofe than Lead, which, by its felf undergoing a like Attenuation in the Fire, for this Rea

fon

fon difpofes to the fame Attenuation the other Bodies to be reduced into fo fubtile a Scoria.

456. There are Earths, which turn into Metal, by being intimately joined with an inflammable Prin ciple: As Metals deftroyed, and changed into Scoria or Ashes, are, by their Union with the fame Matter, again restored to their metallick Form. This Ope

ration is called Reduction.

457. All Metals and Semi-Metals may be reduced bythis Method (§ 456), except Zink; which, being burnt to Afhes, admits of no Reduction. However, the Mixture of Gold and Silver was never radically diffolved by any Experiment hitherto known; wherefore, both thefe Metals need not be reduced by the Addition (§ 456.) of Phlogiston.

Coroll. Therefore, we must upon no Account refer to this Clafs, that Reduction by which the foreign Bodies, which do but conceal the metallick Form, are taken away. For Inftance, Sulphur, Arfenick, Salts, &c.

458. The Obfervations to be made concerning Amalgamation, are already clearly hinted (§ 65--68), by what was faid, when we treated of Mercury confidered as a Menftruum.

459. When you are to join to a folid fixt Body, by Vapour, another that is volatile, the Vapours must be confined, left they fhould be too cafily diffipated in the Air, and the Body itself, which is to be penetrated, must be fo placed, that the Vapours, very much agitated by the Fire, may as it were continually lick it *. The Manner of producing them has been already (§ 153.) explained, when we fpoke of the menftrual Acids, which are to be applied in this Manner to Bodies, efpecially to Metals. Therefore, as thefe Things are very ftrictly joined to our Subject, they mult of Course be repeated. But as they most commonly, in this Operation, lay first one Stratum of the diffolving Menftruum, reduced into Pow

*Pafs lightly over it.

der,