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

by Pierre Joseph Macquer

This Precipitate of Silver, procured by means of the marine acid, is very easily fused, and when fused changes to a substance in some measure transparent and flexible, which hath occasioned it to be called by the name of Luna Cornea. If it be proposed to decompound this luna cornea, that is, to separate the marine acid from the Silver with which it is united, the luna cornea must be melted along with fatty and absorbent matters, with which the acid will unite, and leave the metal exceeding pure.

It must be observed, that if, instead of the marine acid, sea-salt in substance be added to a solution of Silver in the nitrous acid, a Precipitate is also produced, which by fusion appears to be a true luna cornea. The reason is, that the sea-salt is decomposed by the nitrous acid, which seizes its basis, as having a greater affinity therewith than its own acid hath; and this acid being consequently disengaged and set at liberty unites with the Silver, which, as has been shewn, has a greater affinity with it than with the nitrous acid. This is an instance of decomposition effected by means of one of those double affinities mentioned by us in our seventh proposition concerning Affinities.

From what hath been already said it is clear, that all these combinations of Silver with acids may be decompounded by absorbent earths and by fixed alkalis; it being a general law with regard to all metallic substances. We shall not therefore repeat this observation when we come to treat of the other metals; unless some particular occasion require it.

With regard to Silver I must take notice that, when separated by these means from the acids in which it was dissolved, it requires nothing but simple fusion to restore it to its usual form; because it does not, any more than Gold, lose its Phlogiston by those solutions and precipitations.

Silver unites with sulphur in fusion. If this metal be only made red-hot in a crucible, and sulphur be then added, it immediately flows; the sulphur acting as a flux to it. Silver thus united with sulphur forms a mass that may be cut, is half malleable, and hath nearly the colour and consistence of Lead. If this sulphurated Silver be kept a long time in fusion, and in a great degree of heat, the sulphur flies off and leaves the Silver pure. But if the sulphur be evaporated by a violent heat, it carries off with it part of the Silver.

Silver unites and mixes perfectly with Gold in fusion. The two metals thus mixed form a compound with properties partaking of both.

Metallurgists have hitherto sought in vain for a perfectly good and easy method of separating these two metals by the dry way only: (this term is used to signify all operations performed by fusion): but they are conveniently enough parted by the moist way, that is, by acid solvents. This method is founded on the above-mentioned properties of Gold and Silver with respect to acids. It hath been shewn that aqua regis only will dissolve Gold; that Silver, on the contrary, is not soluble by aqua regis, and that its proper solvent is the acid of nitre; consequently, when Gold and Silver are mixed together, if the compound mass be put into aqua fortis, this acid will take up all the Silver, without dissolving a particle of the Gold, which will therefore remain pure; and by this means the desired separation is effected. This method, which is commonly made use of by Goldsmiths, and in Mints, is called the Parting Assay.

It is plain, that if aqua regis were employed instead of aqua fortis, the separation would be equally effected; and that the only difference between this process and the former would consist in this, that now the Gold would be dissolved, and the Silver remain pure. But the operation by aqua fortis is preferable; because aqua regis does take up a little Silver, whereas aqua fortis hath not the least effect on Gold.

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.