100 COMPOSITION OF 



and ferro-prussiate of potash in quantities proportional to their 

 equivalents, not wishing to have either in excess. Thinking 

 that tliere might have been some mistake in the weight of 

 the materials, I weighed carefully 300 grs. of ferro-prussiate of 

 potassa and 390 grs. of crystalhzed proto-sulphate of iron, 

 dissolved them separately and poured the solutions together, 

 and obtained precisely the same result, a large excess of proto- 

 sulphate of iron. Satisfied as to the accuracy of the experi- 

 ment, I then dissolved 100 grs. of ferro-prussiate of potassa to 

 ascertain how much crystallized proto-sulphate of iron was 

 requisite to produce a complete mutual decomposition. After 

 several trials, 1 found that S6 grs. were required, instead of 

 130 grs. the equivalent of 100 of ferro-prussiate of potassa; so 

 that a portion of potassium is retained in the precipitate, which 

 may be considered as a double cyanide of iron and potassium, 

 containing much less potassium than the common prussiate of 

 potassa. The presence of potassium in this precipitate was 

 discovered by Proust, who did not ascertain its quantitative 

 composition; but from this experiment it is evident that one- 

 third of the ferro-prussiate of potassa is retained in combination 

 with the precipitate, which instead of being FeUy^ or FP^, is 

 llFe Cy -|- 4P (Jy, that is, a double cyanide of iron and potas- 

 sium, in v/hich the cyanogen combined with the potassium, 

 is just one-third of that which was originally combined with 

 it in the ferro-prussiate of potassa. When this precipitate is 

 washed with, water, the potassium it contains is dissolved, in 

 the form of ferro-prussiate of potassa, as the iron becomes 

 oxidated, but if the precipitate is washed with acidulated 

 water, the precipitate becomes blue much sooner and no ferro 

 prussiate of potassa is separated; but as the iron becomes 

 oxidated, its capacity of saturation is increased one-third, and 

 the hydro-ferro-cyanic acid combines with it, and the potassa 

 resulting from the potassium, combined with the acid used to 

 acidulate the water. To ascertain whether this was really 

 the case, I dissolved 100 grs. of ferro-prussiate of potassa, 

 precipitated the solution with 86 grs. of proto-sulphate of iron, 

 and after the precipitate had subsided, I drew off the water, 

 then filled up the bottle with water and added 16 grs. of 

 >sulphuric acid, a quantity just sufficient to form a neutral 



