VOL. L.] PHILOSOPHICAL TRANSACTIONS. lOK 



about the edges, than mixtures of fine gold with a larger quantity of platina. 

 By repeated fusion, and frequent nealing, it became a little softer and tougher, 

 so as to be drawn into pretty fine wire ; but the colour was still exceedingly dull, 

 more resembling that of bad copper than of gold. The specific gravity of this 

 compound was 1 7.91 5, a little less than the medium of the 3 ingredients un- 

 mixed, and a little greater than the mean gravity resulting from the platina by 

 itself, and the copper and gold mixed ; for copper, in the standard proportion, 

 appears to diminish the gravity of gold more than it ought to do according to 

 calculation. 



From the foregoing experiments it appears, that platina is miscible with gold, 

 in certain proportions, without injuring either its colours or ductility, or occa- 

 sioning any considerable alteration in the gravity: experiments related in former 

 papers have showed, that it stands aquafortis, and the other trials by which the 

 purity of gold is estimated. It is to be hoped, that the abuses manifestly prac- 

 ticable by this mineral have hitherto been but rarely made use of. To guard 

 against them is the object of this paper; to detect them, of the next. 

 Paper vi. — Experiments for Distinguishing and Purifying Gold mixed tvitk, 



Platina. 

 1. By Amalgamation with Mercury. — In an experiment related in the 4th 

 paper, an amalgam of 1 part of platina and 2 of gold, with a suitable quantity- 

 of mercury, liaving been triturated with water for a considerable time, and occa- 

 sionally washed over, the platina was gradually thrown out, and the gold retained 

 by the quicksilver. Repetitions of this experiment have showed, that though 

 the separation succeeds in some cases, it does not perfectly in all: that if there 

 is any particle of the platina imperfectly dissolved in the gold (which will gene- 

 rally be the case, unless the quantity of gold is 3 or 4 times greater than that of 

 the platina), this part will be retained, after long trituration, undissolved by the 

 mercury, uncomminuted by the pestle, and too ponderous to be washed off 

 in its gross form. A variety of mixtures of platina and gold were treated in 

 the manner above described; and the gold, recovered from the amalgams, sub- 

 mitted to further examinations. Where the proportion of platina was large, the 

 microscope almost always discovered still some granules of it on the fracture of 

 the ingot; where the proportion was small, the recovered gold was frequently, 

 but not constantly, found to be pure. 



From these experiments it appears, that mercury has a greater alHnity with 

 gold than platina, and that platina is capable of being totally separated by elu- 

 triation ; but that the process is too vague and undetermined to be applicable in 

 the way of assay, as we have no mark of the precise time for discontinuing it, 

 and as we can never be certain, without making another assay, whether the 

 whole of the platina is separated or not. As a preparatory examination, where,. 



