PERSALTS OF IRON. 565 



rated without the decomposition of the compound. In its 

 formation 3 eq. of ferrocyanide of potassium react upon 2 eq. of 

 a persalt of iron. Thus supposing the ferrocyanide of potassium 

 and perchloride of iron to be mixed : 



6K + 3(Cy 3 Fe) and 4Fe + 6Cl = 4Fe + 3(Cy 3 Fe) and 6K Cl. 



In precipitating prussian blue, care should be taken to avoid an 

 excess of the ferrocyanide of potassium, as the precipitate is 

 apt to carry down a portion of that salt. The combination of 

 prussian blue and peroxide of iron, called basic prussian blue, 

 was noticed at page 557. 



Although there is no carbonate of the peroxide of iron, the 

 hydrated peroxide is dissolved by alkaline bicarbonates, under 

 certain conditions, which are not well understood, and a red 

 solution is formed. 



% Per sulphates of iron. The neutral persulphate, Fe 2 O 3 , 3SO 3 , 

 is formed by adding to a solution of the protosulphate half as 

 much sulphuric acid as it already contains, and peroxidising by 

 nitric acid. It gives a syrupy liquid, without crystallizing. 

 This salt is found native in Chili, forming a bed of considerable 

 thickness. It is generally massive, but forms also six sided 

 prisms, with right summits, which are colourless, and contain 

 9HO, (Rose.) The persulphate of iron is soluble in alcohol. 

 It may be made anhydrous by a low red heat; but after ignition 

 dissolves in water with extreme slowness, like calcined alum. 



When hydrated peroxide of iron is digested in the neutral 

 sulphate, a red solution is formed, which, according to Maus, is 

 the compound Fe 2 O 3 , 2SO 3 . The rusty precipitate which is 

 formed in a solution of the protosulphate from the absorption 

 of oxygen, is another subsulphate, of which the empirical for- 

 mula is SO 3 -f2Fe 2 O 3 . 



A double persulphate of iron and sulphate of potash, or iron 

 alum, is formed by evaporating a solution of the mixed salts to 

 their point of crystallization. It is colourless and quite analo- 

 gous in composition to ordinary alum, (page 515.) Its formula 

 is KO,SO 3 + Fe 2 O 3 , 3SO 3 + 24HO. 



Another double sulphate is formed, which crystallizes in large 

 six-sided tables, and of which the formula is 2(KO,SO 3 ) +Fe 2 O 3 , 

 2SO 3 -f 6HO, (Maus), when potash is added gradually to a con- 

 centrated solution of persulphate of iron, till the precipitate 



