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freed from fuperfluous oxyd of iron. It is, ina fe.w words, the following: when the folutionof pruffiat of potafh has been thoroughly puri-fied by fulphuric acid, inftead of evaporating itto crylfallize, pour into it a folution of fulphatof copper in warm water as long as a reddifhbrown precipitate appears. Wafli this pruffiatof copper thoroughly, and add it by degrees toa folution of pure potafh. The pruffic aqidthen quits the copper and unites with the pot*afh, ftill all along carrying with it a confider-able portion of iron with which it finally cryf-tallizes. Then tell the folution with barytes,and evaporate and crylfallize in the way al-ready mentioned. The fait thus obtained is freefrom fulphuric acid and contains no more oxydof iron than is abfolutely neccflary to its com-pofition, and therefore none of the oxyd is unfa-turated but is held in fuch firm union withboth the pruffic acid and potafh as not to bedetached by any fimple affinity unlefs with theaffiftance of heat.

Pruffiat of potafh is a fait of a pale amberyellow colour, and eafily foluble in four or fivetimes its weight of water. The alkali is per-fedfly faturated, fo that it does not alter vegeta-ble colours. When the fait, either folid or infolution, is long heated or expofed to the lightfor fome time it depofits blue pruffiat of iron,and hence to keep it for chemical purpofes itfhould be put into an opake bottle, or in a darkplace. It is not very eafy to explain precifelythe caufe of this change. Probably it is in thefirft inftance produced by the diffipation of aportion of' the pruffic acid in the part immedi-ately expofed to light, which leaves the re-mainder of this part in thofe proportions inwhich the blue pruffiat is conftituted; but asthe latter contains fcarccly any alkali, it is ne-ceffary that the corrcfponding portion of potafhfhould alfo be detached from the part that hasloft fome of its acid before the blue pruffiat canappear. What are the affinities that producethis effect p

The pruffic acid will alfo unite to foda andammonia by a fimilar procefs to that alreadydefcribed.

It is not abfolutely neceffary in forming anyof the pruffiated alkalies that the alkali fhouldhe in a cauftic ftate, but it very much affiftsand fhortens the operation.

Pruffiat of Lime. When pruffian blue isboiled with lime-water the fame difcoloratiotitakes place as with potafh, and the lime infaturating itfelf with the pruffic acid becomes

* PM. Journ. 4to. vol.

of a clear yellow colour. This liquor alfo de-pofits blue pruffiat by expofure to light. It isan ufeful teft for iron and copper, but on ac-count of the lime it contains it is decompoledpartially by fulphuric acid in many forms.

Pruffiat of Barytes . This fait is formed inthe following way, and has been examined par-ticularly by Mr. W. Henry. 1 Make a folutionof pure barytes in boiling water, and add by ■degrees pure pruffian blue in. fine powder (prc-vioufly waffied with hot water) till it ceafes tobe difcoloured. Filter the folution, and afterftanding a few hours fmall yellowifh cryftalswill appear, which are the pruffiated barytes.From the remaining folution a further quantityof cryftals may be obtained by evaporation.Their form appear^ to be a rliomboidai paral-lelopiped.. This fait is very fparingly foluble inwater, 4 oz. of which at 65° diiTolve only onegrain, and of boiling water 5 or 6 grains. Itis totally foluble in dilute muriatic and nitricacid, and without decompofition. When heatedto rednefs the pruffic acid burns off, and therefidue contains carbonat of barytes. Sulphuricacid, or any fulphat added to the folution ofthis fait decompofes it compleatly, and fulphatof barytes is precipitated. It is alfo decompofedby carbonat of potafh, carbonated barytes fallsdown, and the folution contains pruffiat of pot-afh in great purity. This would be a fliort andunexceptionable way of preparing this latterfait, but very expenfive. Pruffiat of barytesmay alfo be formed by the double decompofi-tion of pruffiat of potafh and muriated barytes.When thefe falts are added together, if no ful-phuric acid in any form be prefent, and thefolution rather dilute, no immediate change inthe tranfparency takes place; but after ftandingtogether for a few hours the bottom of the vef-fel becomes covered with minute cryftals,which are pruffiated barytes. It is this cir-cumftance of fome time being required to caufethis precipitation that gives the opportunity ofpurifying pruffiat of potafh by barytic folutionfrom fulphuric acid, without decompofing thepruffiat of potafh, as the fulphat of barytes falls,down immediately , but the pruffiat requires fometime, fo that the addition of the barytes fhouldceafe when the folution no longer gives an im-mediate precipitate. Barytic water added topruffiat of lime decompofes it and cryftals ofpruffiated barytes gradually form.

Prufftat of Strontian .“

This fait is prepared like the former by digeft-ing pruffian blue with the pure earth. But this

3 and 4* k Ditta.