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but it does not alter the folutions of this metal.If the pure pruffiat of lime is added to the ful-phat of copper it firft precipitates a lemon-co-loured pruffiat of copper, which more of theliquor rediffolves, and this folution is foluble inammonia without turning it blue. If more ofthe fulphat of copper is added, the precipitatelikewife difappears.

Some acids diffolve only a portion of thisprecipitate, and the remainder is white, but themuriatic acid diffolves the whole.

The triple pruffiats alfo, whether with analkaline or earthy bafis, decompofe compleatlyall folutions of copper, and with more eafethan the Ample pruffiat, as indeed is the cafewith all the other metallic folutions. The pruf-fiat of copper thus prepared is of a beautifulbright-brown, which mixes well with oil, andin beauty and intenfity furpaffes every brownpaint now in ufe. It has been introduced intothe arts by Mr. Hatchett . 1 He prefers themuriat of copper precipitated by pruffiat of lime.It mult be well wafhed with cold water afterprecipitation, and dried without heat.

This fait is however in faft a pruffiat of cop-per and iron, for this acid appears to carry withit in all its combinations, except that of mercury,a certain, and even considerable, portion of oxydof iron. As a proof of this, when this pruffiatof copper is digefted with an alkali, the triplepruffiat of pot afi and iron is obtained, as will beprefently mentioned.

The other metallic pruffiats are formed in afimilar way with the above, but are of little im-portance.

Prussiated Alkalies and Earths.

Of thefe by far the molt important is the

Pruffiat of poiafb. It has been already men-tioned that though the pure pruffic acid is capable@f uniting with cauftic potalh to faturation, theaffinity between the two is extremely weak,fo that this acid is difplaced even by the car-bonic, hence when this folution is expofed tothe air, the alkali abforbs carbonic acid, andthe pruffic is expelled and evaporates. Butwhen a certain quantity of oxyd of iron isprefent the effect is very different, and a triplefait feparable by cryftaliization is formed, con-taining pruffic acid, potalh, and iron, to whichthe acid now adheres fo ftrongly, that noacid can expell it unlefs heat is employed, ex-cept t e oxymuriatic, which decompofes anddeftroys the pruffic acid. This triple combi-

nation of pruffic acid, oxyd of iron, and potalh,is ufually termed limply pruffiat of potafij, Whichwe fhall here retain.

A great many experiments have been madeby chemilts to prepare this fait pure and ofuniform compofiticn, as in this ftate it is Avaluable chemical left, having the property offorming a precipitate with all the metallic folu-tions except thofe of gold, platina, and antimony,but with none of the earthy folutions exceptmuriat of alumine. Thus the fame teft dulyapplied will tell the prefence of any metallic fait(with the exceptions above mentioned) and by thecolour of the precipitate two or three of the in-dividual metals may be difeovered, particularlyiron and copper; and where iron alone is prefent,the quantity may be found by drying, weighing,and igniting the precipitate, deducting fromthe refidue the proportion of iron effential tothe pruffiat added. On account however of thegreat difficulty of preparing an alkaline pruffiatwith an uniform proportion of iron, this teft isfeldom employed to afeertain the quantity ofiron in any folution, but only its prefence.

Two or three good methods have been givenfor preparing pruffiat of potaffi. They are alleffential ly the fame, and are founded on thefollowing principles : when an alkali is digeftedwith heat, upon Pruffian blue, a great part ofthe acid, and a fmall part of the oxyd of iron ofthe blue, unite in folution with the alkali, anda yellow pruffiat of iron with excefs of oxyd isleft, as has already been mentioned. It wouldfeem therefore at firft that no more was re-quired to prepare the pruffiat of potalh, than toevaporate the folution to the proper point and tocryftallize. This indeed, if the alkali were fatu-rated, would readily afford the cryftals required;but in this ftate if any acid is poured on them,a copious feparation of blue pruffiat will takeplace, and therefore it would lead to greaterrors in analyfis, as an indication of iron willbe given in the liquor examined by this teft,when it merely contained an excefs of acid.The reafon of the effect of acids on this unpuri-fied pruffiat of potalh in precipitating blue pruf-fiat, feems to be, that during the firft digeftionof the alkaline ley and Pruffian blue, a confide-rable portion of the fub-pruffiat of iron, of thefame nature as that left behind, is taken up (forthis is found to yield a fimilar blue precipitateby the Ample addition of an acid which diflolvesthe excefs of iron) and hence it becomes necef-fary to purify the pruffiat of potalh from the

Journ. Royal Inft. or Phil. Mag. vol. 4.

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