Page:Proceedings of the Royal Society of London Vol 1.djvu/351

Rh thought it deserved particular examination, although the quantity which he could obtain “ms too small for accurate analysis.

The appearance of this mineral is whiter than Peruvian platina ; the grains are rougher and more angular, being evidently fragments of larger masses, very little worn at their surfaces. When examined by solution and precipitation, the greatest part of the grains appeared to be platiua nearly pure, as they are free from iron, which forms a considerable part of the Peruvian ore; and apparently free from the several metals, which have within these few years been discovered in that mineral; but they contain, on the contrary, a small quantity of gold, which is not contained in the grains of Peruvian platina.

The author discovered also, among the grains of native platina, a few fragments of native palladium, which he describes as resembling, in the whiteness of their colour, the grains of platina, but differing from them in presenting an appearance of ﬁbres diverging from one extremity. These grains are readily detected by their solubility, and by the red colour of the solution : that they consisted of palladium, was proved by precipitation with prussiate of mercury, or green sul- phate of iron, as well as by their fusibility by assistance of sulphur. It is remarked, however, that these grains are not absolutely pure, but contain a very small quantity of platina, which, by its redness when precipitated, seems to be contaminated by iridium.

The mineral of which this account is given was raised in a very rich copper-mine called Huel-Unity, in the parish of Gwennap, having been found at the depth of ﬁfty fathoms, at the junction of two small lodes or veins. This ore is mixed with some native copper, very rich gray copper, and black copper ore.

It crystallizes in the form of a hexahedral prism, terminated in general by a plane, but sometimes by a taper six-sided pyramid. The colour is generally a shade of yellow, but sometimes wine- yellow, like the Brazilian topaz, and sometimes as dark as brown sugar-candy. The hardness varies, and is sometimes sufﬁcient to scratch ﬂint«glass. The speciﬁc gravity at 50° temperature is 6'41.

Being exposed to heat upon a gold spoon, it melts into abrownish- yellow mass, and remains unaltered in a state of ignition. But if heated upon charcoal, it is rapidly decomposed. arsenical vapours being extricated, while the lead is reduced to its metallic state.

The mode of analysis adopted by the author consisted in reducing the ore to a ﬁne powder, and decomposing it by a solution of pure potash, with due precaution to avoid the solution of lead by the alkali along with the arsenic acid. The arscniate of potash was de- composed by nitrate of lead, which gave an arseniate of lead, con- sisting of known proportions. from Which the quantity of arsenic acid in the one was found to be 26'4 per cent.