TUN

TUN

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be taken up, and the yellow oxyd of tungftenwill remain behind, hill however combined,perhaps, with a little alkali. A third method,,and upon the whole the belt, of preparing thisoxyd, is by calcining the eryftallized tun flat ofammonia in an open crucible of .platina, by■which the alkali will be entirely driven off anddeftroyed, and perfectly pure oxyd will remainbehind: xoo parts of this fait afford, accordingto Vauquelin, 78 of yellow oxyd.

The proportion of oxygen in the yellow oxydhas not yet been afcertained with much pre-cifion. The Elhuyars obtained from 100 parts•of oxyd 60 parts of metal, according to whichthe oxyd contains 40 per cent, of oxygen ; huton the other hand, from 100 parts of metal.they obtained by calcination only 124 parts ofoxyd, which would make the oxyd contain only19.4 per cent, of oxygen. This latter propor-tion appears the moil worthy of credit, bothfrom the cir.cumflances of the experiment, andbecaufe it does not very much differ from theproportions obtained by combining the refultsof two other experiments, the one by Vauquelinand the other by myfelf, according to whichtoo parts of tunftat of ammonia afford 78 ofyellow oxyd, and 240 parts of the fame faitafford r6i of metal, which would give for thecompofition of the yellow oxyd 86 parts ofmetal to 14 of oxygen.

The yellow oxyd is infipid and inodorous:its fp. gr. is 6.12. When expofed to theblowpipe in a platina fpoon, it acquires a deepgreen colour; it gives a deep blue when fufedwith microcofmic fait; when heated on char-coal it becomes of a dark bluifh black. By aftrong heat in a clofe crucible its colour ischanged to a black iff grey, according to Vau-quelin ; but according to the Elhuyars it formsa fpungy mafs of a bluiff black colour, andcovered fuperficially with minute cryftals; itsfp. gr. is increafed to 9.55, and by calcinationin a muffle it again acquires a yellow colour,and its weight is increafed by 2 per cent. (Itis to be obferved however, that the oxyd em-ployed by the laft mentioned chemifts was pre-pared according to the fecond of the methodsabove defcribed, and therefore might poffiblycontain a little potaffl). The yellow oxyd whendigefted with a little dilute muriat of tin, ac-ouires a deep blue colour, and if in this ftateit is removed into dilute nitric acid and warm-ed, nitrous gas will be given out, and the yellowcolour will be again reftored, thus clearly ffow-

ing that the blue powder is the fub-oxyd ofthis metal.

The yellow oxyd even if diftilled repeatedlyto drynefs with nitric acid, undergoes no ap-parent alteration. It is entirely infoluble inwater, but if rubbed in a mortar with this fluidit forms a yellow emulflon, which pafl'es turbidthrough the pores of filtering paper, and re-quires very long reft before it becomes clear.This yellow fluid has no tafte, nor does it atall redden litmus or other blue vegetable fub-ftances ; it may therefore be queftioned how farit has a title to its ufual appellation, tunfticacid, (the tunftic acid of Scheele being a verydifferent fubftance, as we fir-all ffow prefentlyj,as, however it unites with the alkalies, and dis-lodges carbonic acid from fome of its combina-tions, it may without impropriety be confideredas at leaft affting the part of an acid.

The yellow oxyd is entirely infoluble in thefulphuric, nitric, or muriatic acids; by digeftionwith acetous acid it is converted into the blueoxyd, but is not diffolved. Cauftic potaflr ei-ther in the dry or moift way combines with theyellow oxyd, forming a fait with excefs ofalkali, readily foluble in water, from which byevaporation may be obtained a glittering powder,but without any appearance or regular cryftal-lization. Tunjlat of Potajh thus obtained has afomewhat cauftic metallic tafte, is deliquefcentin a moift air, and is eafily foluble in water.When fuperfaturated with nitric acid a whiteprecipitate falls down, which, when waffledwith cold water and dried, prefents. the follow-ing properties 5 . It is acid to the tafte, co-lours litmus red, and is foluble in about 20parts of boiling water. The folution whenmixed with lime water, and boiled, depofits aheavy white powder which appears to he tun-ftat of lime. When faturated with an alkaliit yields by evaporation a cryftalline fait. Whenboiled with filings of zinc, tin, or iron, the li-quor becomes of a deep blue, and depofits aprecipitate of the fame colour. Nitric or mu-riatic acid when boiled on the above mentionedwhite precipitate, change its colour to yellow,as fulphuric acid does to blue. From the aboveproperties it was confidered by Scheele as thepure acid of tungften, but later- experimentshave proved that this was a miftake, it being atriple fait compofed of oxyd of tungften, potaflr,and nitric acid. The compofition of this fait isfatisfadtorily (hewn by an experiment of theElhuyars.

* Schcele’s