of the Oxides of Manganese. 155 



speedily disappears ; because the red oxide, which is dissolved in 

 small quantity by the sulphuric acid, is then also converted into 

 the protoxide with the evolution of oxygen gas. The red colour 

 disappears gradually even without the aid of heat ; for the solu- 

 tion will be found after a few days to be almost and sometimes 

 quite colourless, while a minute quantity of red oxide has sub- 

 sided to the bottom. On applying a very gentle heat, the red 

 oxide is redissolved, and the acid acquires a lively amethyst-red 

 colour. It is easy, by operating in this way, to obtain satisfactory 

 proof, that a minute portion of red oxide suffices to communicate 

 a rich colour to a considerable quantity of sulphuric acid. The 

 acid may be made to retain its red colour, either by diluting it 

 with water, or by keeping it in contact with undissolved oxide. 



On the Red Oxide. 



I have followed the usage of most chemists in applying the 

 term Red Oxide to that compound which ARFWEDSON has de- 

 scribed under the name of Oxidum Manganoso-wianganicum, (An- 

 nals of Philosophy, N. S. vii. 267), and which is uniformly pro- 

 duced when the nitrate, peroxide, or deutoxide of manganese is 

 exposed to a white heat. In my early experiments on this oxide, 

 I entertained considerable doubt as to the uniformity of its com- 

 position. This opinion originated in the remark, that, on ex- 

 posing the peroxide of manganese to a white heat, the quantity 

 of oxygen lost by different portions of it, though agreeing per- 

 fectly in some experiments, differed widely in others ; and that, 

 on one occasion, I procured the green oxide almost in a state of 

 purity. I subsequently discovered, however, that the disagree- 

 ment in the results was occasioned by the want of a free current 

 of air within the furnace. In some of the experiments the draft 

 was unguardedly cut off, and consequently an atmosphere of 



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