1819.] some neiv Combinations of Prussic Acid. 443 



It cannot be doubted that it the charcoal could be presented 

 to the prussic acid in a state of perfect solution, the combination 

 would take place instantly, as happens with the gums. 



When the prussiate of charcoal is dried, it has a fine brilliant 

 blue colour, like prussian blue containing alumina, which 

 appears singular, if we consider the black colour of charcoal. It 

 dissolves in cold sulphuric acid of commerce without losing its 

 colour. The solution appears green when the acid begins to 

 act, as is the case with indigo ; and when it is diluted with 

 water, the prussiate precipitates. 



If we employ very concentrated sulphuric acid, the colour 

 disappears on solution, and the liquid becomes yellowish ; but 

 when water is added, the prussiate precipitates of its original 

 colour. It is insoluble in water and in muriatic acid, which 

 merely divides it. 



These properties of prussiate of charcoal belong likewise to 

 those of starch, gum arabic, and cochineal, which are probably 

 of the same nature. 



The analogy which exists between this prussiate and indigo is 

 remarkable. They have both the same colour. The absorption 

 of oxygen makes them both pass from green to blue. They are 

 both soluble in sulphuric acid, and finally they are composed of 

 the same principles ; namely, of a great proportion ol carbon 

 combined with azote, hydrogen, oxygen, and iron ; but in pro- 

 portions, or, perhaps, only in an order of composition which gives 

 them different properties. 



After having formed blue prussiates with the oxide of carbon 

 and several of its compounds, I thought of subjecting to the 

 same process the earths which had not yet been united to this 

 acid, and the simple substances which I could procure. 



V. Prussiate of Sulphur. 



As sulphur unites easily with potash, I formed a sulphuret 

 with 10 gr. of flowers of sulphur and 20 gr. of prussiate of potash 

 in a small matrass, which I heated by a spirit of wine lamp. 

 When I thought that the sulphuret was formed, I broke the 

 matrass in a vessel of boiling water mixed with muriatic acid. 

 The sulphuret, which had acquired no colour, was decomposed 

 in the water, and the prussiate of sulphur made its appearance 

 in fine green flocks in the acid liquid. A few days are sufficient 

 to give it a fine deep blue colour. 



This prussiate has a green colour only when the sulphuret is 

 made in close vessels. If it be made in an open crucible, and 

 if it be allowed to bum for some time stirring it with a glass rod, 

 the prussiate is blue the instant it is developed in the muriatic 

 acid ; so that the portion of the sulphur which burns is sufficient 

 to bring the rest to the point necessary for forming prussian 

 blue. 1 shall prove afterwards that no prussiate becomes blue 

 unless oxygen andiron enter as constituent purts of prussic acid. 



