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whole folutlon acquires a dark colour, and afterfome time the ofmium is depoiited (probably inthe metallic ftate) in the form of black films.A fimilar eft'eft, but much more fpeedily, isproduced by ether. The watery folution of thisoxyd is decompofed by all the metals exceptgold and platina, a black or dark grey precipitatebeing thrown down confifting of the mixedoxyds of ofmium and of the precipitant. Alittle mercury fhaken with the above folutionfpeedily deprives it of all odour, and combineswith the ofmium into an amalgam; this amal-gam being expofed to heat in clofe velfels, themercury is volatilized and a dark grey or bluepowder remains behind, which is the pure metal. When heated with accefs of air it evapor-ates with the ufual odour; but if the air is care-fully excluded it feems neither volatile nor fufi-ble even at a full white heat and in contactwith charcoal. But when heated with copperor gold it melts with them forming uniformmalleable alloys.

The pure metal after having been ftronglyheated is not affected by nitro-muriatic acid,but when heated in a filver crucible with cau-ftic alkali it combines readily with it, and givesa yellow folution, from which acids expel theoxyd of ofmium with its ufual properties.

We have already mentioned that the blackpowder obtained from crude platina, containsboth ofmium and iridium. The method offeparating the former has been juft now treated_of, we fltall now therefore proceed to fhow howto procure the latter. Vauquelin’s m methodis to fufe the black powder with four times itsweight of cauftic potafh, which gives a greenfaline mafs : by digeftion with water a greenfolution is obtained, and fome green powderremains undiffolved. The alkaline folution is-to be faturated with muriatic acid, by which agreen precipitate will be obtained, and this,together with the green powder, is to be digeftedin ftrong muriatic acid. The deep green mu-riatic folution thus prepared, contains theoxyds of iridium, iron, and ofmium, and whenheated to ebullition, its colour changes to abright red. It is now to be gently evaporatedto drynefs, and the refidue being treated withalcohol, the muriat of iron will be dilfolved,leaving behind a red powder entirely free fromt tis metal. This red powder being calcinedat a red heat in an open crucible, there firftap es muriatic' acid, and then a vapour whichtinges the fiatne.of the coals of a fine blue, andwhich doubtlefs is the oxyd of ofmium; a

m An. de Ch.

black powder remains behind, which, whenmixed with borax and expofed to a very highheat, is reduced into a half fufed granular metal,of a white colour, and very brittle, which ispure Iridium . The metal in this ftate is folubleonly in nitro-muriatic acid, and that in lmallquantity, and with confiderable difficulty. '1 hecolour of the folution is a very deep red, whichis deftroyed by green fulphat of iron. It'affordsno precipitate with muriat of ammonia; withpruffiat of potafh it gives a brown precipitate,which becomes green by the action of the air.When added to a folution of pure platina, itcaufes the precipitate of this latter by fal-am-moniac, to be of a deep-red colour, inftead ofthe light yellow which it affords when pure.

Mr. Tennant feparated the iridium from theother metals with which it is mixed, in thefollowing manner. He treated the black pow-der alternately with cauftic foda, and muriaticacid: the acid folution, confifting chiefly ofmuriat of iridium, was of a dark-blue colour,which afterwards became of a dufky olive-green,and finally, by continuing the heat, of a deep-red colour. By flow evaporation of the folutiononly an imperfeftly cryftallized faline mafs wasobtained, but this being dried on blotting paperand again diffolved in water, afforded on evapo-ration diftindt o£tohedral cryftals- Thefe cryf-tals are the pure muriat of iridium, and whendiffolved in water give a deep orange-red co-loured folution. With an infufion of galls noprecipitate is formed, but the colour of thefolution is inftantly and almoft entirely takenaway. Muriat of tin, carbonat of foda, andpruffiat of potafh, produce nearly the fameeffect. Ammonia precipitates the oxyd, butif in excefs, rediflolves a portion, forming apurple liquor. The pure fixed alkalies throw-down the oxyd of a brown colour, but retaina part in folution, -and thus acquire a yellowcolour. Muriat of iridium is decompofed byall the metals except gold and platina, a blackprecipitate being thrown down, which doubtlefsis metallic iridium. It may alfo (be obtainedpure by heating the oritohedral muriat, whichexpelis the oxygen and acid ; the'metal remainsof a white colour, and nearly infufible Itdoes no* combine either with iulphur or arienic;it unites with lead, but is readily ieparabffi bycupellation. With copper it forms a very mal-leable alloy, as it alfo does with filver. Goldalloyed even with a confiderable quantity ofiridium, is perfectly malleable, and nearly ofthe fame colour with pure gold : ' it is not fepa-

xlviii. p. 178.