FERROCYANIDE OF POTASSIUM, ETC. 
311 
of iron, some remarkable reactions are observable which 
are not common to all the salts of iron, and which, as far 
as I know, have not been previously mentioned. 
The neutral tartrate of iron, and the neutral citrate, are 
precipitated by the alkaline ferrocyanides, as all others of 
the per salts of iron, but the precipitate formed has not all 
the appearances of ordinary prussian blue, and its colour is 
often very much modified, without arising from an excess 
of one of the reacting salts. 
The tartrate of iron with excess of tartaric acid, on the 
contrary, is not precipitated by a solution of ferrocyanide 
of potassium, and the solution becomes immediately blue 
and remains limpid. After the lapse of a long time, a slight 
precipitate occurs, which is not so dense as prussian blue ; 
the liquid remaining meanwhile clear and blue. 
This blue liquid, obtained by mixing the solution of 
tartrate of iron with excess of tartaric acid and the ferro- 
cyanide, when evaporated to dryness, does not change its 
properties ; it is concentrated by heat without decomposing, 
the product of the evaporation remains a beautiful deep 
blue and redissolves in water. 
Ammonia destroys the blue colour of the above reaction. 
This decoloration, effected by ammonia on the soluble blue, 
(of the acid tartrate of iron and ferrocyanide) is explained 
by the immediate formation of the double tartrate of iron 
and ammonia, which, as we already know, for the other 
double tartrate of iron and the alkalies is without action on 
the double cyanide of iron and potassium. 
The citrate of the peroxide of iron with excess of citric 
acid, and the ferrocyanide of potassium, form a soluble blue, 
equally decomposable by ammonia. 
I have not essayed the action of the ferrocyanide of pot- 
assium, except on these two salts ; but it is probable that 
it applies equally to all the salts of peroxide of iron with 
organic acids. 
Prussian blue precipitated from a salt of the peroxide of 
