PRU

P R U

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the mixture it immediately becomes of a deepblue, with a greenifh tinge. Muriatic acid givesit a deep blue colour, and fo fpeedily, that nofenfible action of the atmofpheric air can befufpedted.

Again, if fome of the white pruffiat be putinto a flalk filled with muriatic acid, andinftantly corked, it becomes perfedlly blue, andthe fame takes place with the fulphureous andphofphoreous acids. Alfo if white pruffiat beformed in a glafs, and fulphuric acid poureddown upon it, the whole becomes blue that isin the reach of the acid, whilft the liquor at topremains colourlefs. Thefe three laft experi-ments are diredtly in contradiction to Prouft,who aflerts that the acids which do not impartoxygen, fuch as the fulphuric and muriatic, donot change the white pruffiat. Berthollet ex-plains thefe phenomena in the following way :Jn experiments on the metallic folutions it isfound that the oxyd of iron adheres much moreftrongly to the fulphuric acid when it is a littleoxydated than when it is much fo; thereforethe green fulphat is not decompofed by pruf-fiat of potaih when both folutions are con-'centrated, but yet thefe two fubftances exercifea mutual action which fimply feparates themfrom the fmall quantity of water which theycontain. If however this quantity is increafed,the adtion of the fulphuric acid on the oxyd isproportionably leffened, and then the prufficacid begins to combine with the iron and pro-duce the blue pruffiat, which mixes with thewhite compound and colours it, but as thepruffiat has an excefs of oxyd of iron, thecolour is greenifh. , Hence is explained theeffeCt of Ample dilution with water on thewhite mafs, which Prouft fuppofes to arife fromthe atmofpheric air that the water may contain.If belides, an acid is added (of whatever kindit be, whether fuch as is liable to yield oxygen,or to abforb it) this combines with the alkaliprefent in the white mafs, and thus increafesthe aCtion of the pruffic acid on the oxyd ofiron, and enables it to decompofe the fulphat.This alfo is ftill farther augmented, if the acidaifo imparts oxygen to. the oxyd, inafmuch asby fo doing, it weakens the affinity betweenthe oxyd and l'uiphuric acid, and renders it moreeafily decompofed by the pruffic acid. Berthol-let however acknowledges that there is fomedifference between the blue pruffiats, accordingto their Bate of oxydation, which gives differentffiai! s of blue, but he denies the exiflence of♦he white pruffiat as defcribed by Prouft.

'• Scheele.

58 )

From all that has hitherto been obferved there-fore we cannot reckon with certainty more thanthree pruffiats of iron, viz. the green, blue, andyellow. In the two fir ft the proportion of metalto pruffic acid appears to be the fame, but thegreen is the molt oxydated, and is foluble inacids. The blue pruffiat, which is the modknown and the molt interefting, is infoluble inwater or in acids, but may be decompofed byalkalies, and in the procefs the yellow or fub-pnffiat is left, which contains an excefs ofmetallic oxyd, probably in the fame degree ofoxydation as in the blue pruffiat, arid may bebrought to the blue ftate by any acid whichfimply diffolves the excefs of oxyd. Thefepruffiats however, and particularly the blue,are in general triple falts, owing to a fmallquantity of alkali which they retain, but theymuft not on that account be confounded withthe cryftallized pruffiat of potaffi and iron.

Pruffiat of Mercury. The pruffic acid exer-cifes fo ftrong an adtion on the oxyd of mercury,that even corrofive muriat is decompofed byPruffian blue, and all the oxyds of this metal dothe fame, as has been already obferved whendefcribing the method of obtaining this acidpure. The pruffiat of mercury duly evaporatedcryftallizes in tetrahedral prifms. It is theonly fimple combination of this acid and a me-tallic oxyd which will cryftallize. When pruf-fiated lime 1 (meaning the combination of purepruffic acid with lime, and not the triple faitformed by digefting Pruffian blue with lime,for this contains iron) is added to nitrated mer-cury, a black precipitate falls down, which isthe mercury reduced, and therefore feparatedfrom the pruffic acid.

Pruffiat of Silver . The pure pruffic acidwhen added to nitrated filver, precipitates awhite pruffiat of filver. When this metal isprevioufly feparated from its folution by car-bonat of foda, this acid unites with it, andforms white pruffiat as before, and expells thecarbonic acid which it contains. If the purepruffiat of lime as above defcribed is added toany folution of filver, it forms a white curdyprecipitate. k If more of the liquor is added,the precipitate rediffolves, and this folutioncannot then be decompofed by muriatic acidor muriat of ammonia, which is a Angular ano-maly. The precipitate itfelf is infoluble inacids.

Pruffiat of Copper . The pure pruffic aciddigefted with the carbonated oxyd of coppereffervefces and turns it of a faint lemon colour,

* Jbid,