102 PHILOSOPHICAL TRANSACTIONS. [aNNO 1757. 



the quantities of platina and gold to be separated are large, it is however of good 

 use, as greatest part of the platina may by this means be washed over with little 

 trouble, and the gold brought into a less compass, so as to be commodiously 

 submitted to a perfect purification by the means hereafter pointed out. This 

 process has a similar effect on platina and gold to that of stamping and washing 

 on metallic ores ; which could not be reduced into pure metal in the furnace to 

 advantage, without the previous separation of great part of the earthy and stony 

 matter by water. 



1. By Precipitation with Alkalies. — Gold is precipitated totally by fixed alka- 

 line salts, but platina only in part. When solutions of the two metals are 

 mixed together, so much of the platina remains suspended, after saturation with 

 the alkali, as to be readily distinguishable by the yellow colour, which it com- 

 municates to the liquor. It has been objected, that though the platina was dis- 

 coverable, when thus mingled superficially with the gold, it may nevertheless, 

 when combined more intimately by fusion, elude this method of trial. 



1. Mixtures of gold with small proportions of platina were therefore kept in 

 fusion, by a very strong fire, for several hours, and afterwards dissolved in aqua- 

 regis. The solutions being diluted with water, and a pure fixed alkaline salt 

 gradually added, so long as any efterv^escence or precipitation ensued, the liquors 

 remained manifestly coloured, though apparently paler than when the two metals 

 had been dissolved by themselves. 2. A more convincing proof, that part of 

 the platina remains suspended, after the precipitation of the gold, was obtained, 

 by putting into the filtered liquors some plates of pure tin, which presently con- 

 tracted an olive hue, and threw down a large quantity of a brownish precipitate, 

 as from the common solutions of the crude mineral. It was observable, that 

 the tin plates were often sensibly acted on, even while the liquor was overcharged 

 with alkali. 3. It has been further suggested, and with great appearance of 

 probability, that as a part of platina is precipitated as well as gold by alkaline 

 salts, if only this part be mixed with gold, it will be thrown down by them 

 again on dissolving the compound. To determine this point, a precipitate of 

 platina made by fixed alkali was melted with thrice its weight of fine gold, and 

 kept in strong fusion for above an hour: they united more easily than gold does 

 with so large a proportion of the crude mineral, and formed a smooth neat bead, 

 which hammered well into a pretty thin plate before it cracked, and appeared in- 

 ternally uniform and equal. This compound being dissolved in aqua regia, and 

 a fixed alkaline salt added by degrees till the acid was more than saturated, tlie 

 liquor became indeed pale; but tin plates put into it quickly discovered that it 

 held a very considerable quantity of platina. It appears therefore a constant 

 property of this mineral to remain partially dissolved in the neutralized liquor; 

 and that minute proportions of it, mixed with gold, are by this means distin- 



