P R U

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P R U

from the fire. Then take out the contents ofthe crucible, and boil them for half an hour withtwo quarts of water and filter the liquor. Addto this immediately a folution of two ounces ofvitriol of iron and eight ounces of alum in twoquarts of boiling water, which will caufe agreat effervelcence, .and a copious depofit of ablue-green precipitate. Separate this latter byfiltration, and add to it a few ounces of muri-atic acid with much ftirring, which will imme-diately change the colour from blue-green to adeep blue. The quantity of acid is to be de-termined by the effect produced. The preci-pitate after (landing for a day is then thoroughlywafhed and flowly dried, and is the Pruilianblue.

This with fome variations in the proportionsof ingredients and in the exact manipulation islaid to be the ulual way of preparing this pig-ment in manufacture, and will always fucceedto a certain degree. Neverthelefs fevernl che-mists who have exactly followed it in the largeway for the purpofe of fale, have been difap-pointed in the quantity yielded, and have notalways fucceeded in the quality of the article, fothat it is probable that there are fome particu-lars in the manipulation, &c. which are onlyknown to the manufacturers.

It is immaterial whether the alkali ufed inthe calcination with blood is cauftic or carbon-ated, fo that the latter is always employed.Any good pearlalh will anlwer the purpofe,and it is better to ufe from two to three timesas much dried blood.

This procefs confilts effentially of two ope-rations, one, the impregnation of the alkaliwith that peculiar principle contained in theblood (and in rnoft other animal matters) whichgives the power of ftriking a blue colour withiron, and is called the Prujjic Acid ; and theother, of partially decompofing this alkalinepruffiat by the mixed l'ulphats of iron and alu-mine, and uniting thereto the oxyd of iron andthe alumine ieparated by the alkali. The ufeof the muriatic acid in finilhing the procefs,appears to be (as we {hall prefently mention)chiefly to diflblve out the fuperfluous oxyd ofiron, and bring the whole to that fine bluecolour for which this fubltance is fo much ad-mired.

'The firft lixivium of the alkali after beingcalcined with the blood, was formerly termedPhlogijlh'a'ed Alkali.

Pruffian blue as commonly prepared is avery compounded fubltance, confilling of the

‘ Schecle’s Eflays

Pruflic acid, of oxyd of iron, generally of pot-afli and of alumine, but the two lall of thefeare in no degree effential to the Pruffian blue,chemically {peaking, as the folution of fulphatof iron alone will produce it of as fine a co-lour, and as perfebt a chemical compound.The alumine may be entirely put out of thequeltion, as the fubjedt is fufficiently compli-cated without it, and its ufe feems chiefly tobe to give a body to the colour (as in preparingthe alum lakes) and perhaps to render it moreconvenient for the painter’s ufe.

The inveltigation of the properties of thismolt Angular fubltance, is perhaps, one of themolt perplexing in chemiltry, as the fubltancecalled Pruflic acid, is quite fui generis , and ex-ercifes affinities which are remarkably compli-cated, and fcarcely to be explained by analogywith any other known body.

Macquer in a very elaborate enquiry ex-plained fome of the molt important combina-tions of this fubltance, but little was knownconcerning the intimate nature of Pruflic acid,before the molt admirable refearches of Scheele ,who, with an intelligence and fagacity peculiarto himfelf, devifed and performed-a number ofexperiments on this Angular fubltance whichimmediately explained many of its molt im-portant properties, and opened the way to thefine experiments of lierthollet and fucceedincchemilts.

Pruffian blue is infoluble in water and inacids. Though the lixivium from which it isformed is made at a red heat, the Pruffian blueitfelf is entirely decompofed at this temperaturegiving out carbonated hydrogen and carbonicacid, and the oxyd of iron and alkali are leftbehind.

We cannot follow a better order in our ac-count of this lingular fubltance, than by firftgiving a Ihort abltradt of the experiments ofthe iilnltrious Swedifh chemift. 1 He firft ex-amined the lixivium Janguinis , or liquor ob-tained by dilTolving in water all the folublepart of the mafs of calcined blood and alkali,before the folution of vitriol of iron is added.This lixivium, compofed of Pruflic acid andpotalh, foon lofes its power of forming Pruffianblue with folution of iron when it has beenexpofed to air. This the author conjecturedmight be owing to the carbonic acid of theatmofphere, which fuppofition was confirmedby confining a little of it in an atmofphere offixed air for a day, after which the oxyd ofiron produced by adding vitriol of iron to it,on Pniflian Blue.