Essay upon the Compounds of Cyanogen. 333 



cessively engaged the attention of BerthoUet, Prout, Vauquelin, 

 Robiquet and others, and its true character may be now regarded as 

 definitively ascertained. 



Pure Prussian blue is best obtained by adding ferro-cyanate of 

 potassa to per-muriate of iron, and washing the precipitate with hot 

 water. The intensity of its color is equal to that of indigo. It is 

 insipid, inodorous, and insoluble in water or in alcohol, under ordi- 

 nary circumstances, though when recently precipitated it communi- 

 cates a blue color to the former, which cannot be separated by the 

 filter. When heated to the temperature of 400° F. in contact with 

 air, it takes fire and burns ; and when distilled in a close retort, it 

 yields hydro-cyanate and carbonate of ammonia, carbonic acid and 

 an inflammable gas, and leaves a residue of minutely divided char- 

 coal and metallic iron, which possesses the properties of a pyropho- 

 rus. Concentrated sulphuric acid destroys the color of prussian 

 blue, rendering it nearly white, an effect which must arise from the 

 abstraction of water, since no gas is evolved during the change, no 

 trace of iron can be discovered in the acid when decanted, and the 

 original color is restored by the addition of that liquid. Muriatic 

 acid, in great excess, also affects its color, changing it at first to green 

 and afterwards to yellow; and by the repeated action of large quan- 

 tities of the acid upon small portions of the salt, M. Robiquet suc- 

 ceeded in completely decomposing, and in obtaining the ferro-cyanic. 

 acid in an insulated state. 



The constitution of this salt has been the subject of much dispute, 

 but after a diligent experimental investigation, and a comparison of 

 mv own with the results of other chemists, I have arrived at the fol- 

 lowing conclusions. 



1. That Prussian blue always contains water, and is indebted to 

 it for its color. 



2. That it contains carbon and nitrogen in the same proportion as 

 that in which these elements exist in cyanogen. 



3. That it contains iron, one portion of which is in the state of 

 red oxide, and may be separated by muriatic acid, leaving a residue 

 of ferrocyanic acid equal to nearly 72 per cent, of the salt employed. 



4. That if oM the iron which it contains be converted by the ac- 

 tion of nitric acid into peroxide, a quantity is obtained whose propor- 

 tion to that separated from an equal portion by muriatic acid is as 

 5 to 2. 



