1820.] Scientific Intelligence. 333 



2. Being thus acquainted with the weight of peroxide of iron 

 in 20 gr. of the powder, I calculated how much ferrochyazic 

 acid was requisite to saturate this quantity of iron ; and how 

 much potash would be just sufficient to decompose 20 gr. of 

 prussiate of iron. This quantity of potash, together with the 

 20 gr. of the prussiate of iron and a sufficient quantity of water, 

 were put into a phial, and digested on the sand-bath for 24 

 hours. The whole was then thrown on a filter, and the peroxide 

 of iron remaining on the filter was washed quite clean with 

 distilled water. The liquid which passed through the filter had 

 a yellow colour, and the taste and properties of a solution of 

 prussiate of potash. Being evaporated to dryness, and exposed 

 to a heat of about 212°, there remained 19*3 grs. of pure prus- 

 siate of potash. Now 19'3 grs. of prussiate of potash dried at 

 the temperature of 212° contain 10"2 gr. of ferrochyazic acid. 

 This of consequence is the quantity of ferrochyazic acid contained 

 in the 20 gr. of prussiate of iron which I examined. 



From the preceding experiments, it follows that prussiate of iron 

 is composed of 



Ferrochyazic acid 10"20 51"0 



Peroxide of iron 7"66 37"8 



Water 2-24 11-2 



20-00 100-0 



Now if we consider (with Mr. Porrett) the weight of an atom 

 of ferrochyazic acid as 6-75, and that of an integrant particle of 

 peroxide of iron as 5, in that case prussiate of iron will be a 

 compound of one atom ferrochyazic acid and one atom of per- 

 oxide of iron. If we were to suppose the weight of an atom of 

 peroxide of iron to be 10, then in that case the salt would be a 

 compound of two atoms acid + one atom peroxide. But I am 

 disposed to embrace the first alternative in consequence of the 

 following fact which is easily verified. 



Dissolve protosulphate of iron in water, and mix the solution 

 with some sulphuretted hydrogen gas. Then drop into it prus- 

 siate of potash. A white powder is thrown down, which is a 

 neutral protoferrochyazate of iron, or a compomid of an atom of 

 ferrochyazic acid and an atom of protoxide of iron. Expose this 

 salt while moist to the air, and it is gradually converted into 

 perferrochyazate of iron (or prussian blue) simply by the absorp- 

 tion uf oxygen. 



It is well known to the manufacturers of prussian blue that 

 the pigment is at first of a dirty pale blue, and that it acquires 

 its intense blue colour by long and laborious washing in water. 

 The reason of this is, that a considerable proportion of the salt 

 made at first by them is in the state ofprotochyazate of iron, and 

 it slowly becomes perchyazate by absorbing oxygen from the 

 atmosphere. If we employ the pcrnitrate of iron instead of the 

 sulphate, wc form prussian blue, at once possessed of the requi- 



