always contains more oxyd, or (what is thefame thing) lefs acid than the blue pruffiat, theprecife proportion of its conftituent parts isfound to vary confiderably, whereas thofe ofthe blue pruffiat are much more conftant.When any acid is poured on this fub-pruffiat,it difl'olves the excefs of oxyd, and immediatelybrings it to the ftate of blue pruffiat, which againmay be decompofed by alkalies as at firft, andanother but fmaller quantity of fub-pruffiat isleft; and thus by fucceffive applications of acidand alkali the whole may be decompofed.

It has been mentioned that a blue pruffiatmay be made, fimply by adding pruffic acid toa foluticn of iron, and expofure to light. Thistherefore can contain nothing but pruffic acidand oxyd of iron, probably in proportions whichvary but little, if at all. But .when that pruffiatis prepared in the ufual way, by adding pruffiatof potafh to the fulphat or any other fait of iron,the blue precipitate always carries down with ita portion of alkali, with which it firmly unites,fo as to render the alkali infolubie in acids or inwater. Hence it is that though the actualquantity of alkali in the pruffiat of potafh ismore than fufficient to faturate the acid of theiron folution, the latter always remains acidulousafter the blue precipitate has fubfided. f Thiswill explain the great weight of fuch precipi-tates compared with the quantity of metallicoxyd contained in the folution which has beendecompofed.

liefides the fpecies of pruffiats of iron alreadymentioned, that is, the blue and the yellow, orfub-pruffiat, which differ in the proportions ofoxyd of iron which they contain, another isdefcribed by Prouft, the white prujjint , whichis ftated to have the fame proportion of acidand iron as the blue pruffiat, but in whichthe metal is in a much lower ftate of oxydation.The method of procuring the white pruffiat isthe following : g take a folution of green fulphatof iron, or that in which the metal is at thelowed; ftate of oxydation (and wdiich is pre-pared from the common fulphat by adding ironfilings, or tin, or fulphuretted hydrogen, andhas been defcribed under the article Iron ) add•to it -a folution of pure pruffiat of potafh, rathermore than is fufficient to decompofe it, andcork the bottle immediately. A copious whiteprecipitate falls down, which is the white pruf-fiat of iron, and foon becomes green, but ifthe bottle remains corked this colour does not. deepen, nor alter by expofure to light. Afterftanding for forne hours, it becomes covered by' Berthollet, Chem. Statics . t, Aa.

a yellow liquid, which is a mixture of pruffiatand fulphat of potafh, together with a little ofthe white pruffiat, which is held in folution,and hence if expofed to the air it becomes blue,and a blue pruffiat is precipitated.

The white pruffiat therefore, according tothis ingenious chemilt, differs from the blueonly in containing lefs oxygen, which it hasa very ftrong tendency to abforb from theair, or from all bodies that yield this fubftance,and thus to pafs into the ftate of blue pruffiat.Thus the oxymuriatic acid effefts this changeimmediately, the nitric acid more {lowly. Butthe fulphuric and muriatic acids have no effeft.On the other hand, the de-oxygenating fub-ftances, fuch as fulphuretted hydrogen, have noeffeft on the white pruffiat, but bring to thisftate the blue pruffiat when long kept in contaftwith it. The fame change is produced whenthe blue pruffiat is kept in water in a well-clofedbottle, in contact with bits of iron or tin. Con-formably with his theory concerning the Taltsof iron, Prouft concludes therefore, that in thewhite pruffiat the metal contains 27 per cent.of oxygen; in the blue, when fully oxygenated,it contains 48 per cent, and he fuppofes thatthere is no intermediate ftate of Oxygenation,except what may be produced by the meremechanical mixture of one oxyd with another inany given mafs.

This however he fuppofes is actually the cafefor the moft part with the white pruffiat, thatis to fay, it fcarcely ever can be obtained, or atleaft preferved, for any time perfectly white,but from the prefence of atmofpherical air, orother fources of oxygen, a fmall and varyingportion is always converted into the blue oxyd,and in mingling with the whole mafs, it givesthofe different fhades of blue-green which areobferved.

Such is the theory given by Prouft of thedifference between thefe two pruffiats, whichhowever is called in queftion by Berthollet -, kjOn account of the great rapidity with which theblue pruffiat is formed in various circumftanceswhere the operation of the external air canhardly be fuppofed'to aft. The fafts given byBerthollet are the following : if a mixture ofthe green or leaft oxydated fulphat of iron bemixed with pruffiat of potafh, though the latteris in large proportion, the liquor retains thevitriolic tafte, fhewing that the whole : of thefulphat of iron is not decompofed, and the fub-ftance which Prouft calls the white pruffiat isprecipitated ; but if diftilled water is added to.. Cliira. tom. 23. h Chem. Statics .