P R U

PRU

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was again foluble in acids, and was thereforenot pruffiated iron, fo that the lixivium hadbeen converted into Ample carbonated potafh.This was owing to the carbonic acid confinedwith it having united with the potafh, and dis-placed the pruflic acid which was volatilized,'lhe Pruflic acid therefore is fhewn to have lefsaffinity for alkalies than even the carbonic, ai dto affume the gafeous form when expelledfrom its bafe. Some of the lixivium fanguiniswas then firft mixed with a little vitriolatediron, which turned it yellow, and afterwardsexpofed to carbonic acid as in the former cafe,but now the pruflic acid was not expelled, fothat it ftill was able to form a perfect Pruffianblue by a further addition of vitriolated iron,and fubfequent wafhing with muriatic acid.Another portion of the lixivium was boiled forfome minutes with the green oxyd produced bydecompofing vitriolated iron by an alkali addedwhen ftill wet, and this was expofed to the airfor fome time, but t ftill retained its power offorming Pruffian blue. On the other hand,when another portion was boiled with the fullyoxydated calx of iron from a nitrous folution,none of the metal was taken up, and the lixi-vium then loft its Pruflic acid by fhort expofureto air. Hence the important fat! is proved,that the prefence of iron, is neceflary to givefixity to the pruffic acid though already com-bined with an alkali, and to prevent it frombeing fpeedily diffipated by the weakeft agents,even by the carbonic acid of the air.

To dete£t more fully the volatilization ofthe pruffic acid when not detained by iron, thischemift fufpended over the Ample lixiviumfome paper firft dipped in vitriolated iron, andthen pencilled with alkali, and after two hoursexpolure on dropping muriatic acid on thefpot, it became covered with the fineft Pruffianblue.

Thefe experiments led to the attempt to ob-tain the prulfic acid perfe£!ly free from allcombination. For this purpofe the lixivium fanguinis (without iron) was fuperfaturatedwith fulphuric acid, and diftilled irom a glafsretort with a gentle heat. When one-third hadpalled over into the receiver it was examined.It was a watery liquor with ^ peculiar tafteand im e ll like peach-flowers, but it had de-tained only a part of the prulfic acid expelled,as t ie air in and around the velfeis was flronglyimpregnated with the volatile acid, and' turnedpaper blue that had been dipped in an ironfolution and alkah, as in the cafe already men-tioned. But enough of the acid was detainedby the water to be clearly detected by the iron

teft. The pruffic acid of this aqueous folutionwas ftill more volatile than in the lixivium, asmight be expected, and was entirely diffipatedby a fhort expofure to air. In cliftilling thisliquor the procefs may be flopped after about athird has come over, as this contains all thepruflic acid. It alfo holds a little of the ful-phuric acid (as detefted by precipitating a ba-rytic folution) fo as to redden litmus, whichthe pure pruffic acid will not do.

It was then attempted to procure this acidfrom the pruffian blue itfelf, or rather from thatyellow cryllallized fait obtained by digeftingthe blue with cauftic alkali in the way thatwill be afterwards deferibed. This is a triplefait compofed of pruffic acid, potafh, and oxydof iron, and therefore one in which the ironferves as a bond of union between the acid andalkali. It contains lefs oxyd of iron than thepruffian blue, and hence whenever any portionof its pruffic acid is abftradled, the remainderis liable to pafs into the ftate of pruffian blue.This fait fhould be termed in ftri£lnefs prujjiatof potafh and iron, to diftinguifh it from thefimple pruffiat of potafh, fuch as exifts in thelixivium fatiguin'" but as the latter is feldontprepared, the triple pruffiat above mentionedhas got the name fimply of pruffiat of potaf b,which we fhall here retain.

To obtain the acid from this fait, an ounceof it was diffolved in four parts of water, towhich were added three drachms of fulphuricacid, and the whole was put into a retort witha clofe receiver. As foon as the mafs beganto boil, it grew thick, and a great quantity ofpruffian blue was feparated. The diftillationwas continued till an ounce cf liquor had,palled over, which proved to be pure prulficacid fimply diflolved in water, and all the vef-fels were alfo full of the vaporized pruffic acid.The' blue mafs in the retort was then filtered,by which a clear liquor (which contained merelyfulphat of potafh) was feparated, and tire pruf-fian blue remained on the filter. This bluewas again boiled with cauftic potafh, to con-vert it into pruffiat of potafh, and diftilled withfulphuric acid as before, during which precifelythe fame phenomena took piace as at firft,pruffiated water and gas were diftilled, andfome pruffian blue and fulphat of potafh wereformed. The fame was repeated a third time,with fimilar appearances, only that the quan-tity of regenerated blue was now very minute,fo that it appears that by a fuccelfion of thefeoperations all the pruflic acid maybe expelledfrom the pruffiat.

But this method of obtaining pure pruffic