330 Essay upon the Compounds of Cyanogen. 



they undergo when exposed to the action of water, being all convert- 

 ed into hydro-cyanates of oxides. These salts possess an alkaline 

 reaction which cannot be overcome by the addition of prussic acid 

 in excess. When their solutions are exposed to the air, they pass 

 gradually into carbonates, an effect which is due in part to the mu- 

 tual reaction of the elements of the water and of the acid, and in part 

 to the absorption of carbonic acid from the atmosphere. The cyan- 

 urets of the alkaline and earthy metals are all to a greater or less de- 

 gree soluble, while the others being with few exceptions insoluble, 

 may be obtained by adding hydro-cyanate of potassa to any of their 

 saline combinations. 



Ferro-cyanic acid. — Mr. Porret obtained this acid by dissolving 

 fifty grains of crystallized ferro-cyanate of potassa, in two drachms of 

 distilled water, and mixing this solution with another of fifty eight 

 grains of tartaric acid in the smallest quantity of alcohol capable of 

 dissolving it. The slightly soluble super-tartrate of potassa was pre- 

 cipitated, and the filtered liquid yielded, by evaporation, crystals of 

 pure ferro-cyanic acid. When newly prepared, these crystals are 

 white or of a pale lemon color, but on exposure to air, they gradually 

 acquire a bluish tint. They are soluble in water and in alcohol, and 

 when the aqueous solution is treated with the peroxide or a per-salt 

 of iron, prussian blue is immediately formed. From the experiments 

 of Porret, he inferred its constitution to be. 



Carbon, _ _ _ 24=4 atoms. 



Azote, , - - 14=1 " 



Iron, - - - 28 = 1 " 



Hydrogen, - - - 1 = 1 " 



A suspicion of inaccuracy in this statement of its composition, 

 would naturally arise from the difference it exhibits between the pro- 

 portion of the nitrogen and carbon, from that in which they are known 

 to exist in cyanogen ; and the more recent experiments of Robiquet 

 have proved satisfactorily that no reliance can be placed upon it. 

 According to this chemist, when the ferro-cyanic acid is heated with 

 great excess of peroxide of copper, carbonic acid and nitrogen are 

 evolved in the proportion of two volumes of the former to one of the 

 latter, and hence the relative quantities of nitrogen and carbon must 

 be necessarily the same as in cyanogen. The memoir of Robiquet 

 is inserted in the Ann. de Chimie, fee. tom. XII, and to it I take the 

 liberty of referring, as containing the most satisfactory information on 

 rhe subject of this acid. His analysis decides it to consist of 



