IRON. 357 



tained, while protosulphate remains undissolved. 

 From this and several other facts stated by Gay- 

 Lussac in his Memoir, it is evident that this 

 substance cannot be considered as a peculiar 

 oxide, but as a combination of protoxide, and 

 peroxide in the proportions above stated. 



From the phenomena which appear during Seems to 



' exist in 



the manufacture of Prussian blue, there are Prussian 

 strong reasons for considering the iron which 

 that pigment contains as in the state of Gay- 

 Lussac's oxide, or as a mixture or compound of 

 1 part of protoxide with 2 parts of peroxide. 

 For if we attempt to make Prussian blue by 

 means of persulphate of iron at once, we get 

 a pigment apparently of the finest colour ; but 

 so intense that it appears quite black when dry. 

 The manufacturers always find it necessary to 

 employ protosulphate of iron. The Prussian 

 blue, when first thrown down, is an exceedingly 

 light blue powder. This powder is continually 

 washed with water for a fortnight or three 

 weeks. The shade gradually deepens during 

 the washings, and when it is judged sufficiently 

 deep, the water is drained off and the pigment 

 allowed to dry. From my knowledge of this 

 process, I am disposed to think that protoxide 

 and peroxide of iron are capable of uniting in 

 the proportion of 1 atom of the former to 2 

 atoms of the latter. But, with the exception of 

 Prussian blue, I am not aware of any combina- 



Z3 



