IRIDIUM. Mr. Tennant, on examining the black powder left after dissolving pla tina, which, from its appearance, had been supposed to consist chiefly of plumbago, found it contained two distinct metals, never before noticed, which he has named iriclaurn and osmium. The former of these was observed soon after by Descostils, and by Vauquelin.
To analyse the black powder, Mr. Ten nant put it into a silver crucible, with a large proportion of pure dry soda, and kept it in a red heat for some time. The alkali being then dissolved in water, it had acquired a deep orange or brownish yellow colour, but much of the powder remained undissolved. This digested in muriatic acid gave a dark blue solution, which afterwards became of a dusky olive green, and finally, by continuing the heat, of a deep red. The residuum being treat ed as before with alkali, and so on alter nately, the whole appeared capable of solution. As some silex continued to be taken up by the alkali, till the whole of the metal was dissolved, it seems to have been chemically combined with it. The alkaline solution contains oxide of os mium, with a small proportion of iridium, which separates spontaneously in dark. coloured thin flakes, by keeping it some weeks.
The acid solution contains likewise both the metals, but chiefly iridium. By slow evaporation it affords an imperfectly crystallized mass ; which being dried on blotting-paper, and dissolved in water, gives by evaporation distinct octaedral crystals. These crystals, dissolved in wa ter, produce a deep red solution, inclin ing to orange. Infusion of galls occasions no precipitate, but instantly renders the solution almost colourless. Muri ate of tin, carbonate of soda, and prussiate of pot ash, produce nearly the same effect. Am monia precipitates the oxide, but, possi bly from being in excess, retains a part in solution, acquiring a purple colour. The fixed alkalies precipitate the greater part of the oxide, but retain a part in solu tion, this becoming yellow. All the me
tals that Mr, Tennant tried, except gold and platina, produced a dark or black, precipitate from the muriatic solution, and left it colourless.
The iridium may be obtained pure, by exposing the octaedral crystals to heat, which expels the oyxgen and muriatic acid. It was white, and could not be melt ed by any heat Mr. Tennant could em ploy. It did not combine with sulphur, or with arsenic. Lead unites with it ea sily, but is separated by cupellation, leav ing the iridium on the cupel as a coarse black powder. Copper forms with it a very malleable alloy, which, after cup ella tion, with the addition of lead, leaves a small proportion of the iridium, but much i less than in the preceding instance. Silver forms with it a perfectly malleable cont pound, the surface of which is tarnished merely by cupellation : yet the iridium appears to be diffused through it in fine powder only. Gold remains malleable, and little altered in colour, though alloy ed with a considerable proportion ; nor is it separable either by cupellation or quartation. lithe gold or silver be dissolv ed, the iridium is left as a black-powder.
The French chemists observed, that this new metal gave a red colour to the triple salt of platina and sal ammoniac, was not altered by muriate of tin, and was precipitated of a dark brown by caustic alkali. Vauquelin added, that it was pre cipitated by galls, and by prussiate of pot ash : but Mr. Tennant ascribes this to some impurity.
Mr. Tennant gave it the name of iri dium, from the striking variety of colours it affords while dissolving in muriatic acid.
Dr. Wollaston hasobserved, that among the grains of crude platina, there are some scarcely distinguishable from the rest but by their insolubility in nitro-muriatic acid. They are harder, however, when tried by the file ; not in the least malleable ; and of the specific gravity of 19.5. These ap peared to him to be an ore, consisting en tirely of the two new metals.