1813.] a Chemical Knowledge of Manganese. 265 



heat is applied. The liquid becomes quite colourless without 

 any precipitation taking place; and when an alkalie is added, a 

 precipitate falls in the state of a white powder. 



The solution of black oxide is precipitated by the pui e alkalies 

 dark brown; by alkaline carbonates, dark reddish brown ; and 

 by the prussiates, yellowish brown. These precipitates become 

 darker when exposed to the air. The precipitate thrown down 

 by the alkaline carbonates contains no carbonic acid. 



The solution of black oxide in sulphuric acid is incapable of 

 crystallizing. 



When the black oxide of manganese is digested for some hours 

 in diluted sulphuric acid, an amethyst-red solution is obtained, 

 in which there is an excess of acid. It cannot be made to yield 

 regular crystals. Alkalies occasion a brownish red precipitate 

 when mixed with this solution; a proof that the manganese con- 

 tained in it is in a higher degree of oxidation than in neutral 

 solution. 



Some very interestingexperiments which Giobert made known 

 in the year 1790 on this subject in the Memoires des Sciences de 

 Twin, t. 6, An. 1790 et 1791, p. 23—35, and which he pro- 

 duced as a proof that sulphuric acid treated with black oxide of 

 manganese attracted a portion of oxygen from the oxide, and by 

 that means is converted into oxysulphuric acid, dissolving a por- 

 tion of oxide, and by that means acquiring properties similar to 

 oxymuriatic acid, as far as the destruction of colours is con- 

 cerned — these experiments I have not repeated with the requisite 

 care. 



If we mix one part of pure black oxide with 1^- parts of con- 

 centrated sulphuric acid in a coated retort connected with a 

 pneumatic apparatus, it is well known that by the application of 

 a heat, at first gentle, but gradually raised to redness, we obtain 

 pure oxygen gas. In this case it is very evident that the affinity 

 of the protoxide for the acid facilitates the evolution of the 

 oxygen gas. When the black oxide is heated by itself we obtain 

 also, as in the present case, only pure oxygen gas. In this case 

 also the metal is reduced to the state of protoxide, in which state 

 it is completely soluble in acids. 



Water passes over into the receiver mixed with some sulphuric 

 acid ; and there remains in the retort, when the red heat has 

 been continued for an hour, a white porous mass, which dissolves 

 in water by the assistance of heat. This solution exhibits die 

 same properties as that of metallic manganese in sulphuric acid. 

 I have never been able to obtain (as some chemists assert) 

 colourless crystals by evaporating this solution. Single crystals, 

 indeed, may appear nearly colourless ; but when they are in 

 groups they appear evidently of a light rose-red colour. Perhajx 

 the mass ibund in the retort, which has quite a white colour, 



