496 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



Remarks. — 1. It appears from the foregoing observations, that this mineral 

 has not come to us in its native form ; being probably taken out of the mines in 

 large masses, which have been broken, and treated with mercury, to extract the 

 gold, of which possibly it at first contained a considerable quantity. The quan- 

 tity left by the workmen is extremely small ; some pounds of the mixture having 

 yielded only a few grains. A moderate fire renders more of these golden particles 

 discoverable, than can be seen at first ; the mercury evaporating, by which se- 

 veral of them were concealed. 2. Some part of the brownish powder is probably 

 adventitious, as well as the mercury ; being worn off from the stampers and 

 mills employed for comminuting the mineral, and triturating it with the mer- 

 curv. 3. The roughness and dark colour of the cavities of the grains of platina, 

 seem to proceed from a substance similar to the black dust adhering in them. It 

 is probably owing also to this heterogeneous magnetic matter, that some of them 

 are attracted by the loadstone. 



Exper. 1. — Some of the purer grains of platina, by gentle strokes of a flat 

 hammer, on a smooth anvil, bore to be considerably flattened, without breaking 

 or cracking about the edges : some quickly cracked, and discovered internally a 

 close granulated texture. All are reducible, by rude strokes in an iron mortar, 

 though with difficulty, into powder. They seemed to be rather more brittle 

 when ignited, than when cold. 



Exper. 3. — The specific gravity of platina, with its heterogeneous admixtures, 

 as brought to us, was found to be to that of water, as 16.995 to 1.000. The 

 quantity weighed for this purpose was no less than 2000 Troy grains. 



The larger grains of platina, separated as much as possible from the other 

 matters by the sieve, and cleansed by heating, boiling in aqua fortis, mixing 

 them with sal ammoniac, and forcing off" the salt by fire, and afterwards washing 

 them; weighed in air 642, in water 606.75 : whence their gravity turns out 

 18.213. The microscope still discovered a considerable portion of blackish mat- 

 ter in their cavities. These trials were several times repeated on different parcels 

 of platina : the result was nearly the same in all. 



Remark. — The gravity of this mineral, great as it appears to be from the fore- 

 going experiments, would probably turn out still greater on a further purifica- 

 tion of the platina, since it is manifestly mixed with some of the lighter hetero- 

 geneous matters. 



Exper. A. — A quantity of platina, containing its usual admixture of magnetic 

 dust, was kept for some time of a moderate red heat in an iron ladle. The 

 bright particles became somewhat duller coloured ; the magnetic ones were no 



metallic substances. See Wollaston in Phil. Trans, for 1804 and 1805, as before quoted at p. 103 

 of this vol. of these Abridgments. 



