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feebly impregnated with the tinging matter, and hence the precipitate is blue. It is plain, therefore, that the Berlin blue, fo I call the Pruffian blue of the fhops, is not pure Prulliated iron, but a mixture of this with em- bryon alum ; that is, with alurn which is barely ftripped of its excefs of acid, together with coal, fulphur, and other impurities of the blood lye. Macquer firft difcovered that Pruflfxated pron, or Berlin blue, might be ftripped of the tinging matter by digeftion with alkalis ; and that thefe, when faturated, do not decotnpofe iaturate folu- tions of any earth, though, by means of a dou ble affinity, they decompole and precipitate nie- tallic folution; an alkalinelye, thus faturated with the Pruffian acid, or Pruffiated iron, is called JMacquer’s Lye or Teft. It is to be remarked, howcvcr, that though an alkali may thus be neutralized, it will not long continue neutral, becaule it takes up much of the yellow calx (which originates frotn the heat neceflarily employed to impregnate the whole of the alkali) ; and this calx, adhering Üghtly, f>on falls in the form of a blue calx, particularly if expol'ed to the air, frorn which the alkali may derive the aörial acid ; fo alfo mineral acids, or even weaker acids, decompofe it, if not immedi- ately, at leaft after a few hours, an effeeft which has often been falfely aferibed to the iron con- tained in thofe acids ; hence earthy iolutions that contain an excefs of acid, as thofe of alum and barytes, are foon prccipitated by it ; yet if thefe folutions contain a large excefs of acid they may efeape precipitation, as, after faturating the weakly combined part of the alkali, enough of the excefs may ftill rcrjiain to keep the earths in folution,