Presence of Anhydrous Persulphate qf Iron. 121 



rose-colour gradually weakens, and at last disappears entirely, 

 the liquid having all the properties of a diluted solution of 

 the protosulphate of iron. The second method is by oxidiz- 

 ing the iron to a maximum. This is most conveniently done 

 by a little peroxide of manganese, or of lead, or, still more ra- 

 pidly, by nitric acid. A high temperature answers the same 

 purpose ; for then the protoxide of iron decomposes a portion 

 of sulphuric acid, and is converted into the peroxide. 



These facts account very satisfactorily for the deposition 

 of the sulphate of the peroxide of iron in the concentrated sul- 

 phuric acid. The sulphur employed in the manufactory of 

 the acid commonly contains some sulphuret of iron, which is 

 converted into a sulphate during the combustion of the sul- 

 phur, and mechanically carried off by the rapid ascent of the 

 gaseous products. The sulphate of iron is held in solution 

 so long as the acid is weak ; but when the acid is concentrat- 

 ed by boiling, the iron, if at first in the form of protoxide, is 

 oxidized to a maximum, and subsides as a sulphate of the 

 peroxide. This deposition is quite free from the sulphate of 

 lead as would be expected ; for the same condition which 

 causes the persulphate of iron to subside, tends to preserve 

 the sulphate of lead, which is always present in the common 

 acid, in a state of solution. This salt is thrown down when 

 the acid is diluted. 



M.M. Bussy and Lecanu obtained the persulphate of iron 

 in a pure state by decanting off the supernatant liquid, and 

 washing away the adhering sulphuric acid by means of al- 

 cohol. It is composed according to their analysis of 



Peroxide of iron - - 40 



Sulphuric acid - - 60 



which is therefore the common persulphate of iron, — the per- 

 sesquisulphatc of Thomson, which contains one equivalent of 

 the oxide to one and a half of the acid. It is remarkable 

 that this salt acquires such a great degree of cohesion by long 

 contact with boiling sulphuric acid as to be dissolved with 

 difficulty by water or alcohol, though, when prepared in the 

 usual way, it is very soluble in both these liquids. 



