VOL. L.] PHILOSOPHICAL TRANSACTIONS. 105 



Where the quantity of the mixture to be assayed was very small, the precipita- 

 tion was usually performed in a filter, that the gold, which separates in very 

 minute moleculas, some of which might possibly remain unobserved in the bot- 

 tom of a glass, might be detained on the paper. The colourless sorts of filter- 

 ing paper are preferable for this use to the coloured ; as these last may be impreg- 

 nated with astringent matter, which would extricate some of the ferrugineous 

 part of the vitriol. The vitriol was dissolved in about 6 times its quantity ot 

 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 through before they 

 had been duly mixed, poured back to the rest. The gold remaining in the filter 

 was washed with fiesh 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 

 aether imbibing, and green vitriol precipitating, gold alone. Care should be 

 had that the piece of the mixture, 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, while the gold continued 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, 



VOL. XI. P 



