234 On the Oxidation of Metals in general. 



Six prussiates of iron therefore exist, which are well cha- 

 racterized ; three of them are neutral, and three with an ex- 

 cess of oxide; the excess of oxide may be more or less con- 

 siderable, which varies in a singular manner the colour of 

 these prussiates. These six prussiates of iron may be com- 

 bined with different quantities of prussiate of potash, and 

 witliout doubt also with the other alkaline prussiates, in such 

 a manner that this genus of salts is very numerous in species. 

 It is generally observed that those which contain much alkaline 

 prussiate are soluble; those, on the contrary, which contain 

 much prussiate of iron are insoluble. We know that Prus- 

 sian blue becomes green by exposure to the air, and that it 

 then forms red oxygenated prussiate of iron. When the or- 

 dinary prussiate of potash is boiled with Prussian blue, which 

 is thus oxygenated in whole or in part, the oxygenated 

 prussic acid acts upon the potash, and the prussic acid com- 

 bined with it acts upon the oxide of iron in such a manner, 

 that the filtered liquor precipitates the red nitrate or muriate 

 of iron of a green colour. This is the reason why certain 

 solutions of prussiates, made by means of Prussian blue, 

 precipitate the sulphate of iron little oxidated in a white co- 

 lour instead of a blue one. 



Tt is this variety presented by prussiates in their compo- 

 sition, which renders their preparation so difficult. This, 

 however, is not the only obstacle we meet with in the ma- 

 nufacture of Prussian blue ; there are still a great many 

 others, which were only surmounted after a long time, and 

 some of them even exist at this moment. One of the per- 

 fections necessary to be attempted in this art, would be to 

 bave for other uses the great quantity of carbonate of am- 

 monia formed by calcination, which would admit of the 

 prussiate of iron being used with much more adv^antage in 

 commerce. I am convinced, by experiments made with 

 great care, that on calcining potash with animal matters, 

 as much carbonate of ammonia and prussiate of potash is 

 obtained as when such animal matters are distilled by them- 

 selves ; so that as XTJuch prussiate of iron is formed with the 

 carbon resulting from their distillation, as with all their prin- 

 ciples united. We may thus make sal-ammoniac at the 



same 



