518 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



by the file ; and very brittle, breaking from a moderate blow, of a rough sur- 

 face, and dull bell-metal colour. It bore a good polish, and did not tarnish in 

 the air. 



2. Platina and copper, of each 1 oz., and 4 oz. of tin, melted perfectly to- 

 gether, and without loss. This compound filed freely and easily, bore to be cut 

 with a knife, but broke readily on the anvil, of an irregular surface, and dull 

 whitish colour. Polished, it looked like polished iron. The fracture soon tar- 

 nished to a yellow; the polished part grew dull, but retained its colour. 



3. A mixture of platina and copper, of each 1 part, and 8 of tin, proved 

 softer than the foregoing ; and bore to be flattened a little under the hammer. 

 It broke of a very irregular surface, composed of a great number of bright white 

 plates. The fracture soon tarnished ; the polished part retained its colour. 



Remark. It is observable, that in the first of these experiments, platina was 

 perfectly taken up by less than half its weight of a mixture of copper and tin ; 

 though it could scarcely be made to melt with less than its own weight of either 

 of them separately, in a fire equally, or rather more, intense. 



The specific gravity of these mixtures turned out, on exjjeriment, a little less 

 than by calculation ; though the copper and tin, melted together without platina, 

 formed a compound specifically heavier than even the copper by itself. 



The several mixtures with zinc, bismuth, regulus of antimony and brass, were 

 likewise weighed hydrostatically, and found all somewhat lighter than they ought 

 to have been by calculation. As few hydrostatical experiments seem to have been 

 made on zinc and bismuth, it may be proper to mention, that the gravity of pure 

 zinc turned out 7-050, and that of bismuth 9.733. 



Hitherto we have considered the miscibility of platina with metallic bodies, 

 and the alterations which different proportions of it produce in their appearance 

 and qualities : employing the necessary precautions for preventing the scorifica- 

 tion and dissipation, which most of the metals suffer in the fire ; and which some 

 remarkably promote in those which by themselves are more difficultly, or not at 

 all, destructible. We shall now examine the relation of platina, in this respect, 

 to those metallic substances, which are the most destructive. 



1. Cupellation and Scorification of Lead with Platina. 

 1. A mixture of platina and lead was cupelled, under a muffie, in an assay- 

 furnace. For some time the process went on well ; the lead gradually changing 

 into scoriae, which were thrown off to the sides, and absorbed by the cupel, or 

 dissipated in fume. In proportion as the lead worked off, the matter required a 

 stronger fire to keep it fluid ; and at length, collecting into a dull flat lump, 

 could no longer be made to flow in the greatest degree of heat which the fur- 

 nace was capable of giving. The lump broke verj' easily, appeared of a dull grey 



