t»RUSSlAN BLUE. 101 



sulphate of potassa, if all the potassium combined in the pre- 

 cipitate were to combine with it. The water was acid to the 

 taste and reddened litmus paper, but the materials being left 

 together five months, and shaken every day or two, the pre- 

 cipitate became of a very dark blue, and the water no longer 

 gave any indications of acidity, and was found to contain 

 sulphate of potassa, instead of sulphuric acid. The next thing 

 to be ascertained was to see how much of a salt of peroxide of 

 iron was requisite to decompose 100 grs. of ferro-prussiate of 

 potassa; for which purpose I dissolved 100 grs. of prussiate of 

 potassa, took a given quantity of proto-sulphate of iron, to 

 which I added the necessary quantity of sulphuric acid to 

 constitute the per-sulphate, the iron was oxidated by adding 

 nitric acid, then evaporating to dryness and redissolving in 

 water. After several trials, I found that 100 grs. of prussiate 

 of potassa required to decompose it, a quantity of per-sulphate 

 of iron containing the same quantity of iron as that contained 

 in 86 grs. of crystallized proto-sulphate of iron, that is to say, 

 86 grs. of proto-sulphate of iron converted into per-sulphate of 

 iron were just sufficient to decompose 100 grs. of prussiate of 

 potassa. From this we are led to conclude that prussian 

 blue, made by decomposing prussiate of potassa with a salt of 

 joro^oxide of iron, and washing the precipitate with acidulated 

 water until it becomes completely peroxidated is identical 

 chemically with prussian blue obtainedin decomposing prussiate 

 of potassa with a salt ofjoe^'oxide of iron; but if the precipitate 

 is washed with water only, it will contain an excess of oxide 

 of iron, and in both cases if dried before it has become com- 

 pletely oxidated, it will contain potassium or potassa as one of 

 its constituents. Having incinerated some prussian blue made 

 with a salt of protoxide of iron, and some made with a salt of 

 peroxide of iron, I f)und that on washing with hot water the 

 oxide of iron obtained from the incineration of blue made 

 with a salt of protoxide of iron, the water became strongly 

 alkaline ; wliereas on washing the oxide obtained from the 

 incineration of blue made with a salt of peroxide of iron, the 

 water became very slightly alkaline, from which I infer that 

 prussian blue owes its beauty to the presence of a little potas- 

 sium or potassa as one of its constituents. Berzelius states 

 that prussian blue made with a salt of protoxide of iron differs 



