Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

This effect, which is pretty singular, probably arises from hence, that when the Gold exceeds or even equals the Silver in quantity, the parts of both being intimately united, the former are capable of coating over the latter, and covering them so as to defend them from the action of the aqua fortis; which is not the case when there is thrice as much Silver as Gold.

There is one thing more to be taken notice of with regard to this process; which is, that perfectly pure aqua fortis is rarely to be met with, for two reasons; first, it is difficult in making it wholly to prevent the rising of the medium employed to disengage the nitrous acid; that is, a little of the vitriolic acid will mix with the vapours of the aqua fortis: secondly, unless the salt-petre be very well purified it will always hold some small portion of sea-salt, the acid of which, we know, is very readily set loose by the vitriolic acid, and consequently rises together with the vapours of the aqua fortis. It is easy to see that aqua fortis mixed either with the one or the other is not proper for the Parting Process; because, as has just been said, the vitriolic and the marine acid equally precipitate Silver dissolved in the nitrous acid; by which means, when they are united with that acid, they weaken its action upon the Silver, and hinder the dissolution. Add, that aqua fortis adulterated with a mixture of spirit of salt becomes an aqua regis, and consequently is rendered capable of dissolving Gold, in proportion as its action upon Silver is diminished.

In order to remedy this inconvenience, and free aqua fortis from the vitriolic or marine acid with which it is tainted, Silver must be dissolved therein: by degrees as the metal dissolves, those heterogeneous acids lay hold of it, and precipitate with it in the form of a white powder, as we observed before. This precipitate being wholly fallen, the liquor grows clear; after which, if it be found capable of dissolving more silver, without turning milky, it may be depended on as a perfectly pure aqua fortis. Then filtre it, dissolve more Silver in it, as long as it will take up any, and you will have a solution of Silver in a very pure aqua fortis. By means of this solution may other aqua fortis be purified: for pour a few drops thereof into a very impure aqua fortis, and immediately the vitriolic or marine acid, with which that aqua fortis is contaminated, will join the Silver and fall therewith to the bottom. When the solution of Silver, prepared as above, does not in the least affect the transparency of the aqua fortis, it is then very pure, and fit for the purposes of Quartation.

This operation of purifying aqua fortis by a solution of Silver is called the Precipitation of Aqua Fortis, and aqua fortis thus purified is called Precipitated Aqua Fortis.

When Silver is dissolved in aqua fortis it may be separated therefrom, as hath been shewn, by absorbent earths and fixed alkalis.

We shall see by and by that there are other means of effecting this: but whatever way it be separated from its solvent it recovers its metalline form, as Gold does, by being simply fused without any additament.

SECTION III.

Of Copper.

Of all the imperfect metals Copper comes the nearest to Gold and Silver. Its natural colour is a deep-red yellow. It resists a very violent degree of fire for a considerable time; but losing its phlogiston at last, it changes its metalline form for that of a calx, or a pure reddish earth. This calx is hardly, if at all, reducible to glass, without the addition of something to promote its fusion; all that the fiercest heat can do being only to render it soft. Copper, even while it retains its metalline form, and is very pure, requires a considerable degree of fire to melt it, and does not begin to flow till long after it is red-hot. When in fusion, it communicates a greenish colour to the flame of the coals.

This metal is inferior to Silver in point of gravity; nor is its ductility so great, though it be pretty considerable: but, on the other hand, it exceeds that metal in hardness. It unites readily with Gold and Silver; nor does it greatly lessen their beauty when added to them in a small quantity: nay, it even procures them some advantages; such as making them harder, and less subject to lose their ductility, of which those metals are often liable to be deprived, by the mixture of the smallest heterogeneous particle. This may probably arise from hence, that the ductility of Copper has the peculiarity of resisting most of those causes which rob the perfect metals of theirs.