It must be observed, that, when Gold and Silver are mixed together in equal parts, they cannot be parted by the means of _aqua fortis_. To enable the _aqua fortis_ to act duly on the Silver, this metal must be, at least, in a triple proportion to the Gold. If it be in a less proportion, you must either employ _aqua regis_ to make the separation, or, if you prefer the use of _aqua fortis_, melt the metalline mass, and add as much Silver as is necessary to make up the proportion above-mentioned: and hence this Process is called _Quartation_. 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.