The white particles that appear and settle to the bottom, on dissolving silver in an _aqua fortis_ adulterated with a mixture of some foreign Acid, are no other than that very Silver, which is no sooner dissolved by the Nitrous Acid than it deserts that solvent to unite with the Vitriolic or Marine Acid, wherewith it has a greater affinity, and falls to the bottom with them. And this happens as long as there remains in the _aqua fortis_ a single atom of either of those two Acids. When therefore your _aqua fortis_ hath dissolved as much Silver as it is capable of taking up, and when all the white particles formed during the dissolution are settled to the bottom, you may be assured that the portion which remains clear and limpid is a solution of Silver in an exceeding pure _aqua fortis_. But if the solution of Silver thus depurated be mixed with an _aqua fortis_ adulterated with the Vitriolic or Marine Acid, a like precipitation will immediately ensue, for the reasons above given, till the very last particle of the heterogeneous Acid contained in the _aqua fortis_ be precipitated. _Aqua fortis_ purified by this method contains no extraneous substance whatever, except a small portion of Silver; so that it is very fit for the parting process. But if it be intended for other chymical purposes, it must be rectified in a glass retort with a moderate heat, in order to separate it from the small portion of Silver it contains, which will remain at the bottom of the retort. The third condition necessary to the success of this operation is, that your _aqua fortis_ be neither too aqueous, nor too highly concentrated. If too weak, it will not act upon the Silver: and the consequence will be the same if it be too strong. Both these inconveniencies are easily remedied: for in the former case part of the superfluous phlegm may be drawn off by distillation; or a sufficient quantity of much stronger _aqua fortis_ may be mixed with that which is too weak: and, in the latter case, very pure rain water, or a weaker _aqua fortis_, may be mixed with that which is too strong. You may satisfy yourself whether or no your _aqua fortis_ hath the requisite degree of strength, by dissolving therein a thin plate consisting of one part Gold and two or three parts Silver; which plate must be rolled up in form of a paper coffin. If, when all the Silver contained in the plate is dissolved, the Gold remains in the form of the coffin, it is a sign that your solvent has a due degree of strength. If, on the contrary, the Gold be reduced to a powder, it is a proof that your _aqua fortis_ is too strong, and ought to be weakened. The Gold remaining after the dissolution of the Silver must be melted in a crucible with Nitre and Borax, as hath already been said under the process for parting Gold and Silver by means of _aqua regis_. As to the Silver which remains dissolved in the _aqua fortis_, there are several ways to recover it.