178 Contributions towards [Sept. 



tion to this observation. This is the case, in an especial man- 

 ner, with carbonic acid. 



If some metallic manganese in the state of powder be thrown 

 into water impregnated with carbonic acid, and the vessel be 

 left for some days covered up, we shall find the metal, upon 

 examination, changed into a green oxide. If we allow the 

 vessel to continue untouched for some weeks, the oxide (pro- 

 vided the quantity of carbonic acid be sufficient) is changed into 

 white carbonate of manganese; and remains for the greatest 

 part at the bottom of the vessel in that state. Only a small 

 portion of it is dissolved in the water. 



Carbonic acid gas acts in the same manner upon the metal. 

 I placed a small porcelain vessel, containing some powdered 

 manganese, in a large receiver, free from water, and filled with 

 carbonic acid gas, and let it remain eight days. The metal 

 was converted into green oxide, and some days after contained 

 white carbonate of manganese mixed with it. This peculiar 

 oxidation of manganese appeared at first truly astonishing. 

 Was the carbonic acid decomposed, and was the oxidation 

 produced by its giving out oxygen to the metal ? Not a single 

 drop of water was perceptible in the receiver, to which we 

 could have ascribed the change. Or was it produced by a por- 

 tion of water mixed with the gas in a state of vapour? The 

 last opinion seems most probable, and will have the greatest 

 number of supporters. But is the first opinion quite impro- 

 bable ? I shall not venture to adopt it till a decisive method of 

 ascertaining when a gas is free from water shall have convinced 

 me that the last opinion is inconsistent with truth. 



(B.) On the brown oxide. — Manganese, which from exposure 

 to the air has been changed into a light brown oxide, is also 

 altered by carbonic acid, and partly converted into carbonate 

 of manganese. I put the oxide into a small glass, and suspended 

 it for some weeks in a large flask filled with carbonic acid gas. 

 At the end of that time it had assumed a greyish brown colour. 

 Nitric acid disengaged from it carbonic acid gas, dissolved a 

 portion of it, and left another portion in the state of black oxide. 

 From this 1 conclude that, from the action of the carbonic acid, 

 one portion of the brown oxide had given out oxygen to another 

 portion, and had thus converted it into black oxide", while the 

 portion deprived of a part of its oxygen combined with the car- 

 bonic acid. 



Remarks on the Carbonate of Manganese. 



All solutions of the salts of manganese are precipitated by the 



Alkaline carbonates. When the protoxide of manganese is 



■ thrown down in this manner, we obtain a snow-white precipitate, 



