180 Contributions towards [Sept, 



to separate manganese in solution from other substances with 

 which it is in combination, as has been proposed by some che- 

 mists, is therefore in the highest degree improper. When water 

 containing the greatest possible quantity of carbonate of manga- 

 nese in solution is exposed to the air, it can hardly be said to 

 cover itself with a skin. This erroneous opinion has probably 

 originated from confounding manganese with magnesia. When 

 a solution of manganese in an acid (the acid being in excess) is 

 decomposed by a carbonated alkali, and the filtered liquid is 

 diluted with water, if it be left exposed to the air, or placed in 

 an artificial warmth, it is soon covered all over with a thin skin; 

 but it is impofsible from this to estimate the qualifies of car- 

 bonate of manganese, or to conclude its great solubility in car- 

 bonic acid water ; for when this skin is accurately examined, 

 we find that it is a salt composed of carbonic acid, oxide of man- 

 ganese, the alkali employed in producing the precipitation, and 

 probably of the acid likewise in which the metal was previously 

 dissolved. 



In order to precipitate manganese completely from a solu- 

 tion, it is necessary that there should be no excess of acid pre- 

 sent. Only the neutral solutions are completely decomposable. 



The fat oils dissolve carbonate of manganese by the applica- 

 tion of heat, and form with it a compound having the consistence 

 of plaster. This observation was first made by Scheele. 



Estimate of the Proportion of the Constituents of Carbonate of 



Manganese. 



I first estimated, by dissolving a hundred grains of carbonate 

 of manganese, the loss of weight sustained by the escape of 

 carbonic acid. By this method I determined the proportion of 

 acid to amount to 34< , 25 per cent. 



To determine the other constituents, I filled a very small good 

 coated retort with carbonate of manganese, of which 120 grains 

 were requisite. The beak of the retort passed into a glass ball, 

 from which proceeded a tube to allow the gas to escape. By the 

 application of a heat, at first gentle, but gradually so high as to 

 make the retort red-hot, the gas and the water were expelled 

 from the salt. After continuing a red heat for two hours, the 

 apparatus was taken to pieces, and the weight of the different 

 products obtained determined. The water in the glass ball 

 weighed 12 grains; and the oxide remaining in the retort, and 

 which had acquired a greenish grey colour, weighed 67 grains. 



If we subtract the weight of the oxide and water thus obtained 

 from the 120 grains employed in the experiment, there remains 

 a loss of 41 grains. As the gas which made its escape during 

 the distillation was pure carbonic acid, this lois gives us 41 

 grains for the amount of that acid in the salt. This agrees very 



