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who performed this experiment by fattening abottle half full of mercury to a mill wheel, and■thus giving it a very long continued and violentihaking. The fame has been done by fatteningit to the wheel of a carriage during a longjourney. Dr. Prieftley c found this black powderto be formed in a variety of circumftances by along agitation of mercury in phials with water,alcohol, &c. This powder heated gradually on aniron plate became of an uniform orange colour,and when digefted in muriatic acid, it gave ex-actly the fame appearances as the red oxyd, theprecipitate per i'e. This black mercurial powderis ftated by Fourcroy d to be compofed of g 6parts of mercury and 4 of oxygen, though byothers it is confidered as mercury Amply in aftate of extreme divifion. Mercury is broughtto the fame ftate by being rubbed for fometime with lard, gum arabic, honey, manna, ve-getable extract, or any other clammy cohefivematter, which foon becomes black, or rather ofa dark flate blue, by the divided metal. In thisway mercury is conttantly prepared for medi-cinal purpofes.

Another and very important oxyd of mercuryis made by keeping it nearly at its boiling point,and at the fame time with fufficient accefs ofair. This is generally performed in a very flatbottomed matrafs with a tall neck, the mouthof which is unftopped, and which is fet on afand-heat, and kept there for many days. Inthis procefs as a certain renewal of the atmo-fpheric air is required on the one hand, and onthe other as the efcape of the vapour of theboiling mercury fhould be prevented, it is ratherdifficult to contrive the beft form of veflel forpreparing it. If the neck be very narrow thereis fcarcely enough change of the air in contactwith the mercury, fo that on the whole it feentspreferable to ufe a retort with the bottom flat-tened, and die neck only flightly bent, that thecondenfed globules of mercurial vapour mayfall back into die veflel . e When the mercuryhas boiled for a few hours, a dull film forms onits furface, giving it the appearance of tarnifhedlead, and in about 48 hours after, fmall cryttalsof a dark brown or flea-coloured oxyd appearfloating on the furface, which gradually increafelift they form a friable cruft over the whole ofthe metal. It is economical at this time toempty the retort, and feparate the oxyd, which,when carefully collected, (liould be heated on anOpen pan in a heat juft below vifible rednefs, to

diflipate the globules of uncalcined mercurywhich may be entangled within the cryftals.

This brown.oxyd is called Calcined Mercury or Precipitate per fe, and is a perfe£tly pureoxyd of this metal of a dull ruby colour, andoften regularly cryftallized either in octahedronsor in lengthened four-fided pyramids. It con-fifts, according to Lavoifier f , of about 92 percent, of mercury and 8 of oxygen. If this oxydis kept fome time gently heated with contact ofair it becomes brown, and with a ftronger heat(ftill below rednefs) it partly fublimes of a finered. But when the heat is raifed to rednefs theoxyd gradually difappears, oxygen gas of greatpurity is given out, and the metal returns tothe reguline ftate, and may be colledled in dropsof running mercury. This experiment whichfrom its fimplicity is one of the moft unexcep-tionable in fupport of the prefent theory of theoxydation of metals, was employed very earlyby Lavoifier, as will be mentioned under thearticle Oxygen .

When this oxyd is rubbed with runningmercury 8 both fubftances lofe their peculiarform, and the refult is a black powder refem-bling in every refpedl the black oxyd procuredfrom this metal by trituration, the red oxydparting with a portion of its oxygen to the frefttmercury, and each being thereby converted tothe black oxyd.

Another oxyd of this metal, very clofelyrefembling the precipitate per fe, is prepared bydiftilling nitric acid off this metal, which formsthat beautiful red powder called Red Precipitate,or Red Nitrous oxyd of mercury. It is howevernot absolutely pure like the laft, but alwayscontains a very minute portion of nitric and ofmuriatic acid, and hence when reduced by dif-tillation in a red heat, fome azotic gas is givenout, together with a little vapour of corrofivefublimate, owing to the muriatic acid fromwhich the nitric acid is never abfolutely free.

The oxygen gas from each oxyd alfo holdsin folution a fmall portion of mercury in theftate of vapour, which is gradually depofited,but is fufficient, when recent, to caufe a faliva-tion in the perfons that frequently inhale it.The exact preparation of the red precipitatewill be prefently noticed.

Water either hot or cold has no afcertainableefFedl upon mercury, but if common mercury,which generally contains a little alloy of othermetals, be boiled with water, the latter becomes

' Ob Air, vol.iii. p. 463. a J our , 4.5 Mines, No. 70. * Biggin's Minutes. 1 Elements of Chemiftry,

g Fourcroy Syftemc> tom. v.

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