4. By metallic Solutions.
All the metals, which precipitate gold from aqua-regia, have been already shewn to precipitate platina also. As gold is thrown down by some metallic solutions, as well as by the metals in substance, particularly those of mercury and iron, it remains to apply these liquors as precipitants for platina.
1. A saturated solution of mercury in aqua-fortis, which readily and totally threw down gold in its metallic form, being added to a solution of platina, the liquor became immediately turbid, and, on standing for a little time, nearly the whole of the platina fell to the bottom. A solution of mercury in the marine acid, or of corrosive sublimate, likewise precipitated platina, but less perfectly, and with this difference, that the former precipitate was of a greyish brown colour, the latter of a sparkling red.
2. Solutions of iron in the vitriolic acid, or of common green vitriol in water, which totally throw down gold, happily made no change in solutions of platina. Compositions of platina and gold being dissolved in aqua-regis, the solutions diluted with about twice their quantity of water, and a filtered solution of the vitriol gradually added; the mixtures instantly grew turbid, and, on standing, deposited the gold in form of a purplish grey calx, the whole of the platina remaining dissolved. It appeared, on numerous repetitions of this experiment, that no part of the platina was precipitated along with the gold, nor any of the gold kept suspended with the platina. Where the quantity of the mixt to be assayed was very small, the precipitation was usually performed in a filter, that the gold, which separates in very minute moleculæ, some of which might possibly remain unobserved in the bottom of a glass, might be detained on the paper. The colourless sorts of filtering-paper are preferable for this use to the coloured; as these last may be impregnated with astringent matter, which would extricate some of the ferrugineous part of the vitriol. The vitriol was dissolved in about six times its quantity of water, and a few drops of oil of vitriol added, to prevent the separation of any of its iron in the filter. This solution was put into the filter first, the solution of gold and platina immediately poured into it, the whole stirred together with a clean glass rod, and such part of the liquor, as had run thro' before they had been duly mixed, poured back to the rest. The gold remaining in the filter was washed with fresh parcels of water, the paper cautiously rolled up, and burnt in a crucible, as mentioned in a former experiment.
3. Solutions of the vitriol, recommended by Kunckel and others for precipitating gold of an uncommonly high colour, made no change in the solutions either of gold or platina. The bluish green did indeed precipitate the gold; not as blue vitriols, but by virtue of the ferrugineous matter, of which these kinds largely participate. White vitriol was likewise made trial of, but without producing any sensible effect in either solution.
4. The experiments with green vitriol were repeated on the solutions of platina and gold made in spirit of salt. The event was the same as with those made in aqua-regis; the gold being constantly precipitated, and the platina remaining dissolved.
Remarks.
It may be proper to observe, that by the processes here pointed out, the gold is purified from other metallic admixtures at the same time that it is separated from platina; the inflammable spirits reviving, essential oils and æther imbibing, and green vitriol precipitating, gold alone. Care should be had, that the piece of the mixt, taken for examination, be totally dissolved before any trials are made with the solution; the menstruum not acting with equal facility on the two metals, but dissolving the gold more readily than the platina. Where the acid has been dilute, and only a gentle heat applied, great part of the gold has appeared to be taken up before the platina was considerably acted on. Where the filter, with the gold in it, is burnt in the crucible, borax is the most commodious flux: but as this salt gives a sensible paleness to gold, a little nitre may be injected, after the metal has come into fusion, to restore its colour. If the nitre was added at first, whilst the gold continues subtilely divided, some particles of the metal would be dissipated during the deflagration, which that salt produces with the coaly remains of the paper.
As the foregoing experiments exhibit platina and gold dissolved in a mineral fluid, which by simple mechanic agitation rejects the one and retains the other, and which discovers this different appetite of union so much the more remarkably, as the two metals have been the more intimately combined:—as they further exhibit platina dissolved in liquors incapable of holding gold suspended,—gold dissolved in liquors incapable of holding platina suspended,—gold totally precipitated by substances, which precipitate no particle of platina,—and gold, when mixed per minima with platina, perfectly recovered from it by these means, without increase as well as without diminution:—it follows, that platina is not, as some believe, gold naturally debased by the admixture of some other metallic body, but a metal of a peculiar kind, essentially different from all the others. Before the discriminating characters of platina were discovered, such a notion was highly plausible, and direct experiment seemed to confirm it: a portion of the platina might be separated in the process; a quantity of gold mixed with the remainder, in order to collect the gold supposed to be contained in it; the mixture submitted to operations, which gold alone was supposed capable of withstanding; and the augmentation, which the noble metal still retained, held to be true gold gained from the platina.
The methods of trial above related will, it is presumed, be sufficient to undeceive those, who may have been imposed upon by such appearances, and betrayed into the practice of unintended frauds: to convince them, that all they have gained from platina, after the most laborious attempts to divest it of its imaginary coat, is no other than platina still: and, which is of more extensive utility, to distinguish all the abuses, that may be made with this metal, and restore the gold, so debased, to its original purity and value.