operations, the gold is found to be deprived of much of its usual ductility; this however is easily restored to it, by fusing it with nitre and borax. The first part of this process is founded on a property of sulphur, by which it is incapable of uniting with gold, and is strongly disposed to unite with all other metallic substances, excepting platina and zinc ; and also upon the property of sulphur, that it has less affinity with regulus of antimony than with any metallic substance with which it can unite. Hence, when gold, alloyed with silver, copper, iron, lead, &c. is fused together with sulphuret of antimony, these latter metals unite with the sulphur of the antimony, while the reguline part, disengaged from them by its sulphur, unites with the gold.

The sulphur of the antimony, though it unites with the baser metals, does not destroy them, but forms with them a scoria, from which they may be separated by treatment as an ore.

Parting.

When the quantity of silver united to the goldis considerable, they may be separated by other processes. Nitric acid, muriatic acid, and sulphur, which cannot dissolve gold, attack silver very easily; and, therefore, these three agents furnish methods of separating silver from gold, which operation is called parting.

Parting by nitric acid is the most convenient, and, therefore, most used; and is even almost the only one employed by goldsmiths and coiners. Wherefore it is called simply, parting. That made with muriatic acid is only made by cementation, and is known by the name of concentrated parting.

Lastly, parting by sulphur is made by fusion, and is, therefore, called dry parting.

Parting gold from silver by nitric acid or aqua fortis. Although parting by nitric acid be easy, it cannot succeed, or be very exact, unless we attend to some essential circumstances. The gold and silver must be in a proper proportion; for if the gold be in too great a quantity, the silver would be covered and guarded by it from the action of the acid; therefore, when assayers do not know the proportion of gold to silver in the mass, they rub the mass upon a touch-stone (which is usually composed of black basalt, though black pottery will do very well,) so as to leave a mark upon it; theythen make similar marks with the proof-needles, (which are needles composed of gold and silver alloyed together in graduated proportions,) and by comparing the colour of the several marks, they discover the probable Scale of admixture.

If the trial shows that in any given mass the silver is not to the gold as three to one, this mass is improper for the operation of parting by aqua fortis. In this case, the quantity of silver necessary to make any alloy of that proportion-must be added. This operation is called quartation, because it reduces the gold to a fourth of the whole mass. No inconvenience arises from too great quantity of silver, except a waste of aqua fortis. The nitric acid, or aqua fortis employed, must be very pure, and especially free from mixture of sulphuric and muriatic acids. Its purity must, therefore, be ascertained; and if this be found not suffi cient, the acid must be purified by nitrate of silver.

If the purity of the nitric acid were not attended to, à quantity of silver proportionable to these two foreign acids, would be separated during the solu

tion; and this portion of silver converted by these acids to sulphate of silver, and to muriate of silver, would remain mingled with the gold.

When the metallic mass is properly alloyed, it is to be reduced to plates rolled up spirally, called cornets, or to grains. These are to be put into a matrass, and upon them a quantity of aqua fortis is to be poured, the weight of which is to that of the silver as three to two; and as the nitric acid employed for this operation is rather weak, the solution is assisted, especially at first, by the heat of a sand-bath, in which the matrass is to be placed. When, notwithstanding the heat, no further mark of solution appears, the aqua fortis charged with silver is to be decanted. Fresh nitric acid is to be poured into the matrass, stronger than the former, and in less quantity, which must be boiled in the remaining mass, and decanted as the former. Aqua fortis must even be boiled a third time on the remaining gold, that all the silver be certainly dissolved. The gold is then to be washed with boiling water. This gold is very pure, if the operation has been performed with due attention. It is called gold of parting.

The silver dissolved in the aqua fortis may be separated either by distillation-in which case all the aqua fortis is recovered very pure, and fit for another parting-or it may be precipitated by some substance which has a greater affinity than this metal with nitric acid. Copper is generally employed for this purpose in the mint.

The solution of silver is put into copper vessels. The aqua fortis dissolves the copper, and the silver precipitates. When the silver is all precipitated, the new solution is decanted, which is then a solu

tion of copper. The precipitate is to be well

It is

washed, and may be melted into an ingot. called parted silver. When this silver has been obtained from a mass which had been refined by lead, and when it has been well washed from the solution

of copper, it is very pure. Or the silver may be separated from the nitric acid by adding to it muriatic acid, with which it forms muriate of silver. Muriate of silver may be decomposed by mixing it with soda, and exposing it to a sufficient heat in a crucible, whereby the soda unites to the muriatic acid, and sets the silver free.

The refiners frequently employ this solution of copper obtained in the process of parting, for making verditer; which is prepared by adding quick lime to the solution; a precipitate takes place, which is the blue pigment known by the name of verditer.

Parting gold from silver by cementation.-This is also called parting by concentration, and is, usually employed when the quantity of gold is so great to that of the silver, as to render it a difficult task by aqua fortis. The mixed metal to be cemented is to be reduced to plates, as thin as small pieces of money. At the bottom of the crucible, or melting-pot, is to be laid a stratum of cement, composed of four parts of bricks powdered and sifted, one part of green copperas (sulphate of iron) calcined to redness, and one part of common salt, about the thickness of a finger in depth. Upon this stratum a layer of plates of the metal is to be placed, and then another stratum of cement, and so on till the crucible is filled.. It is now to be placed in a furnace, or oven (after a top has been luted on the crucible,) and exposed for twentyfour hours, till it is gradually made red hot, but by no means to be melted. The fire is now left to

go out, and the metal is permitted to cool, that it may be separated from the cement, and bolled repeatedly in large quantities of pure water. This gold is afterwards to be tried on a touch-stone; and if it is not sufficiently purified, the process must be performed a second time. By the above method, we see how powerfully silver is dissolved by muriatic acid, when it is in a state of subtile vapour which is disengaged from the common salt of the cement. Instead of common salt, nitre may be used, as the nitrous acid readily dissolves silver; but the mixture of common salt and nitre together is highly injudicious, because the joint acids are able to dissolve some of the gold with the silver. Whatever silver has been separated, will now remain in the cement; but it may be freed from this by lead, in the method described in cupellation.

Parting gold from silver in the dry way. This is also called parting by fusion, and is performed by means of sulphur, which has the property of uniting easily with silver, while it does not attack gold. This dry parting is troublesome, and even expensive, and ought not to be undertaken but when the silver far exceeds the gold, because sulphur will not separate it so easily as aqua fortis, and will therefore require a further application to cupellation and solution.

Refining Silver by Nitre.

The principle upon which this operation is founded, consists in the property of nitre to oxydate very powerfully all base metals; whereas, on the contrary, the noble metals are not at all affected by it. For as the metallic oxides and glasses do