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acid being troublefome and imperfedf, (owingto the conftant regeneration of pruffian blue):the fame excellent cliemift difcovered a Hillbetter way, and which is the one now followed.When pruffiat of potalli or pruffian blue isdigefted with any oxyd of mercury, the prufficacid unites with this metal, and a folublepruffiat of mercury is formed which cannot bedecompofed fingly by acids or alkalies, and maybe brought to cryftallize by evaporation. When■this pruffiat is digefted with metallic iron thejirft aftion of the iron is to deprive the mercuryof its oxygen, and precipitate it vifibly in themetallic itate; and the pruffic acid is fet atliberty. This however would immediatelyunite with the iron if it were not. prevented bythe prefence of a ftronger acid, for which thefulphuric (or indeed any other) may be added.Then on applying heat, the difengaged prufficacid rifes in diftillation with the firft water thatcomes over, and may be collected. The exaftprocefs of Scheele is the following: to twoounces of pruffian blue mixed with one ounceof red mercurial precipitate, 6 oz. of waterwere added, and the whole was boiled for forneminutes with conftant agitation, when the bluecolour entirely difappeared, and the mafs be-came of a yellowifli grey. It was then filtered,and the mafs on the filter edulcorated with alittle hot water which was added to the clearliquor. This, which had a ltrong mercurialtafte and was pure pruffiat of mercury, wasthen poured upon i f oz. of clean iron filings,to which were added 3 drams of ftrong fulphu-ric acid. The whole was well agitated, and ina few minutes it was turned quite black by thereduced quickfilveri and at the fame time loftits mercurial tafte and acquired the peculiarpeach-flower fmell of free pruffic acid. Theliquor, poured off from the mercury that hadfubfided, was then put into a retort, and afourth part of it was diftilled over into a well- luted receiver, and this liquor was now aqueouspruffic acid, with no other admixture than thatof a little fulphuric acid which had rifen alongwith it. This was got rid of by mixing it•with a little chalk, and a fecond diftillation.Probably barytic water would be preferable, asevery diftillation volatilizes and deftroys a por-tion of the pruffic acid.

There appears at firft fight a contradiction inthe order of affinities between pruffic acid, iron,and mercury refpeCtively, when it is found onthe one hand that oxyd of mercury decompofesjpruffian blue, and takes this acid front the iron

and potafli with which it is united; and on theother hand when iron filings again compleatlydecontpofe pruffiat of mercury. Berthollet explains this apparent contradiction by al-ledging that the oxyd of mercury has aftronger affinity for this acid than the oxyd ofiron, even combined with potafh, and henceany mercurial oxyd or fait will decompofepruffian blue and give a pure pruffiated mercury;but on the other hand iron has a greater affinityfor oxygen than mercury has, and hence metalliciron decompofes the oxyd of mercury exiftingin the pruffiat, and the mercury, now broughtto the metallic ftate, feparates from the folution.

We fir all now deferibe the properties of thisacid and its compofition, and afterwards proceedto its combinations, in which it will be neceffaryin fome degree to return to thofe pruffiats ofiron and potafli which have been already par-tially deferibed.

The Prussic Acid , (the preparation ofwhich in a liquid form has been already deferi-bed) has a fweetifli and acrid tafte, and a fmellrefembling that of peach-flowers or bitteralmonds; nor is this latter circumftance a merecoincidence, fince this acid is actually foundin many parts of vegetables. It is ftronglypoifonous. It does not redden vegetable blues,and hence, as well as from the abfence offournefs of tafte, it is very doubtful how far itought to be entitled to the name of an acid,though from its property of neutralizing alkaliesand forming cryftallizable falts with them, andwith metallic oxyds, it is more convenientlyclaffed among thefe bodies. It does not appearto contain any oxygen. It precipitates fulphu-rets, and curdles foap. It is extremely volatilewhen Amply united with water, or with analkaline bafe fingly, and is expelled from thelatter by every known acid, and even by light,and by a moderate heat. It cannot be cryftal-lized.

It has no a&ion upon metals, but unites withtheir oxyds, forming combinations for the moltpart infoluble, and not eafily decompofable. byother acids. It has the greateft tendency to formtriple falts when in contadl with an alkali anda metallic bafe, and it adheres to the alkaliesand earths with much more force when alfounited to a metallic oxyd (particularly of iron)than when only in fingle combination. It istotally decompofed by oxymuriatic acid. Thedouble and triple falts formed by this acid withthe different bafes will be prefently deferibed,but we iliall firft mention the experiments of