456 FJ^as of Ossgtne in Pnjuw Blue, It.. 



colour ispetfc£l at the inllant of precipitation, anJ cxpofurc to the atmorpherc adds no- 

 thing to its intenfitjr. 



PrufTian blue is, in a word, tliat prufllate, whofc bifis contains -f-l^ of oxygeiic. It is the 

 fame with refped to the white pruffiate as the red fulpliate is to the green. Thefe two 

 prufliates do not dillcr witli regard to their acid. Their dilliniSlive charafters arife from the 

 diltcreiit oxidation of thuir bafcs. 



The blue pruflrate is not altered by acids. The oxygenated muriatic acid changes it, by 

 rendering it green, at the f.ime time that itfelf undergoes alteration, as Berthoilet has difco- 

 vcred. But the aclion of tliis deftruflive acid is cxercifed on the pruffic acid, and not on 

 theoxide, which can receive no greater dofe of oxygene than it has already a, quired from 

 the nitric acid, the air, &c. 



The acids which are ufed to brighten fuch PrufTian blues as are imperfed, .me of no other 

 life than to diflblve the great quantity of carbonate of iron, precipitated h\ the pot-afh 

 which is not fatutated with prufRc acid, and fuper-abounds in ill prepared Uxiviums. If 

 white pruffiate were to exift in fuch precipitates, it would be in no refpeft changed by the 

 acid, and would acquire a blue colour only by abforbing from the atmofpheric .lir the quan- 

 tity of oxygene necelTary for that purpofe. 



In order to (hew that oxygene is the principle which in the blue pruffiate of iron aSbrds 

 the diflinftive colour, nothing more is neceflary than to remark the colour of the oxide 

 when precipitated by alkalis. In the green fulphate the precipitate was black ; after the 

 precipitation from Pruffian blue it is red. No other principle but oxygene could have oc- 

 cafioned this difference in thefe oxides. 



Mr. Proufl having fpoken indifferently of the yellow and the red oxide of iron, as being 

 completely faturated with oxygene, obferves, that he ufes thefe expreffions without diftinc- 

 tion, becaufe a number of fa£ls have convinced him that there is no difference between 

 them. Everv red oxide, fays he, when diffolved in any acid whatever, is precipitated of a 

 yellow colour by alkalis, whether pure or faturated with carbonic acid. This laft acid oc- 

 cafions no difference in the precipitate, becaufe it has no tendency to unite with iron at that 

 degree of oxidation. The red oxides when dried are brown, obfcure and often black, accord- 

 ing to the degree of drynefs or denfity they may have acquired. But if they be pounded in a 

 mortar, the charadlerittic colour foon appears. Jt is affirmed, that thefe o ides have the 

 power of decompofing ammoniac -, but our author kept them for feveral years in ammoniac, 

 without obferving that they were in any refpect changed. He had no better fuccefs with 

 rtie oxide of manganefe at the ordinary temperature of the nir. 



The folution of fulphuratcd hydrogene gas, when kept in a veffel with the blue pruffiate, 

 isdecompofed. It feizes from the oxide that portion of oxygene which conlUtutes the dif- 

 ference between the blue and the white pruffiates ; and the pruffiate thus rendered white 

 exhibits the fame phenomena with alkalis, as if it had be..n obtained immediately from the 

 creen fulphate. The white pruffiate kept under hepatic water undergoes no alteration ; in 

 which refped it rtfembles the green fulphate. tither of thefe readily yield to the hydro- 

 gene diffolved in this water all the oxygene they poffefs, exceeding 27 per cent. 



The fame theory explains, why the red fulphate and the nitrate of iron dccompofe ful- 

 phuratcd hydrogene. The oxide of iron burns the hydrogene, the lulphurisdcpofited, and 

 riie fupcrnatant liquor, inflead of affording a red precipitate by alkalis, exhibits a green. 



This 



