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.