_Aqua fortis_ is the true solvent of Silver, and is utterly incapable of dissolving the least atom of Gold. If therefore a mass consisting of Gold and Silver be exposed to the action of _aqua fortis_, that Acid will dissolve the Silver contained in the compound, without touching the Gold, and the two metals will be separated from each other. This method of parting them is just the reverse of that described before under the head of Gold, which is effected by the means of _aqua regis_. To the success of this separation, by means of _aqua fortis_, several conditions are essentially necessary. The first is, that the Gold and Silver be in due proportion to each other; that is, there must be at least twice as much Silver as Gold in the metalline mass, otherwise the _aqua fortis_ will not be able to dissolve it, for the reason formerly given. If therefore the mass contain too little Silver, it must either be melted down again, and a proper quantity of Silver added; or else, if the Gold be in a sufficient proportion to the Silver, they may be parted by means of _aqua regis_. Secondly, it is necessary that the _aqua fortis_ employed in this operation be absolutely pure, and free from any taint of the Vitriolic or Marine Acid: for, if it be adulterated with the Vitriolic Acid, the Silver will precipitate as fast as it dissolves, and so the precipitated Silver will again mix with the Gold. If the _aqua fortis_ contain any of the Marine Acid, the Silver will be precipitated in that case also; and this inconvenience will be attended with another, namely, that the menstruum, being partly an _aqua regis_, will dissolve some of the Gold. You must therefore be very sure that your _aqua fortis_ is pure, before you set about the operation. In order to discover its quality, you must try it by dissolving, in a small portion thereof, as much Silver as it will take up: if the _aqua fortis_ grow opaque and milky as it dissolves the Silver, it is a sign it contains some foreign Acid, from which it must be purified. In order to effect this, let the portion of _aqua fortis_ used for the above trial stand to settle: the white milky part will gradually fall to the bottom of the vessel. When it is all fallen, gently decant the clear liquor, and pour a few drops of this decanted solution of Silver into the _aqua fortis_ which you want to purify. It will instantly become milky. Let the white particles precipitate as before, and then add a few more drops of your solution of Silver. If the _aqua fortis_ still become milky, let it precipitate again, and repeat this till you find that a drop of your solution of Silver, let fall into this _aqua fortis_, does not make it in the least turbid. Then filter it through brown paper, and you will have an _aqua fortis_ perfectly fit for the Parting Assay.