5l6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



is always changed in these kinds of operations. The amalgam, which looked 

 very bright, left, on evaporation, a spongy mass, of a high colour, which being 

 melted, and poured into an ingot, proved very soft, extremely malleable, and in 

 all respects resembled the pure gold made use of, without the least appearance 

 of platina. 



Remark. — It is greatly to be wished that this method of purifying gold from 

 platina may prove sufficiently accurate to exactly determine the quantity of each 

 in the mixture. The experiments hitherto made do not sufficiently clear up this 

 point ; a great number are still necessary before it can be fully ascertained. 

 . 2. ff^ith Bismuth. — Equal parts of platina and bismuth, injected into a mix- 

 ture of black flux and common salt, previously brought into fusion, and urged 

 with a quick fire, strongly excited by bellows, melted perfectly in a few minutes, 

 and suffered very little loss. Without these precautions, the bismuth could 

 scarcely be made to take up above 4- its weight; great part of which, on an abate- 

 ment of the heat, subsided. 



Mixtures of platina with different proportions of bismuth proved all, like the 

 bismuth itself, extremely brittle : one was not remarkably more so than another. 

 To the file, they were scarcely harder than pure bismuth. They broke of an 

 irregular surface, composed chiefly of striae, with some plates. When newly 

 broken, they looked bright and sparkling ; except the compositions with a large 

 proportion of platina, which were of a dull greyish colour, without any bright- 

 ness. They all tarnished slowly in the air, to a dark yellowish, purplish or bluish 

 colour. Several acquired in part a fine deep blue, which has suffered no change 

 in above a 12-month ; some parts of the masses still remaining white as at first, 

 and others inclining to purple. 



3. IVith Zinc. — On J oz. of platina, covered with borax, and heated in a blast 

 furnace to a strong white heat, was injected an equal quantity of zinc. A violent 

 deflagration arose, and the platina was almost instantly dissolved : the matter, 

 immediately poured out, was found to have lost near half an ounce 



On several times repeating this experiment with different proportions of the 2 

 metals, both in a quick fire, and in 1 more gradually raised in a wind furnace, 

 the zinc was constantly found a powerful menstruum for platina, but suffered 

 great loss from the heat requisite for rendering the mixture sufficiently fluid. When 

 so much of the zinc had been dissipated, that the remainder amounted to no 

 more than -l of the platina, the compound still continued fluid enough to run 

 freely into a long mould. 



Compositions of platina and zinc differed little in appearance from zinc itself; 

 except that where the quantity of platina was large, they were of a closer texture, 

 and a duller hue, with rather more of a bluish cast. They did not tarnish, or 

 change their colour, on being exposed for several months to the air, in a dry 



