1821. J the Composition of Prussiates. .441 



possess when differently prepared, and which may become of 

 some utihty in the arts. It is sokible in pure water, but not in 

 water which contains a certain quantity of any neutral salt. It 

 is on this account that it does not dissolve when it is washed, 

 and even the greater part of the saline matter is separated from 

 it. A clear blue solution then passes through the filter ; but it 

 again precipitates prussian blue, if it falls into the liquid which 

 had previously passed through. 



I made use of a solution of sal ammoniac to separate it from 

 other salts. I washed it with a small quantity of water, until it 

 began to be coloured ; and I afterwards dried the precipitate by 

 pressure between paper. The blue solution is precipitated by 

 the addition of muriatic acid, but the precipitate does not lose 

 its solubility in pure water. The blue solution is not rendered 

 turbid by alcohol. This solubility of prussian blue is not always 

 the same. Sometimes the whole of the precipitate rendered 

 blue by oxidation in the air is soluble ; at other times, more or 

 less of it remains insoluble. Ebullition produces no change in 

 its solubility. This property in a substance, which under other 

 circumstances is so insoluble, appears to be of the same nature 

 as the solubility of the oxides ojf tin and titonium, and also of 

 silica, the solubility of which is evidently different in its nature 

 from that of salts. As the addition of excess of acid does not 

 deprive prussian blue of its solubility in pure water, it is evident 

 that this property does not depend upon an excess of base. 



In order to compare these phenomena with those which occur 

 in insoluble salts of protoxide of iron, I examined the changes 

 produced by oxidation in some of the latter. A great number 

 of them may be preserved without undergoing alteration, or they 

 become yellow by the formation of a salt with peroxide in excess. 

 But there are two, viz. the phosphate and the arseniate of pro- 

 toxide of iron (the acids of which combine with bases, according 

 to a different law from other acids), which, by absorbing oxygen 

 from the atmosphere, change their white colour into a darker 

 one, and form salts with excess of base differing as much from 

 salts of the protoxide as of the deutoxide. The phosphate 

 becomes blue ; it is even found in nature partly blue and partly 

 white ; but in this latter case, it becomes blue in the air in a few 

 days. The arseniate, on the contrary, becomes of a deep green 

 colour. The two varieties of the arseniate appear also to be 

 found in nature. The neutral salt has been lately found in 

 Saxony : it is called scorodite ; it is a salt which contains water, 

 and has the same coloiu' as protosulphate of iron : the other has 

 been long known ; it is the cubic arseniate of iron. 



There is, however, an essential difference between these salts 

 and prussian blue. They do not form double salts with other 

 bases ; and when they are decomposed by caustic potash, they 

 do not yield hydrate of deutoxide of iron, like prussian blue. On 



