of the Oxides of Manganese, 31 



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 Arfwed- 

 son has described under the name oiOxidum manganoso-man- 

 ganicuto (Annals of Philosophy, N. S. vol. vii. p. 267), and which 

 is uniformly produced when the nitrate, peroxide, or deut- 

 oxide 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 composition. This opinion origi- 

 nated in the remark, that, on exposing the peroxide of man- 

 ganese to a white heat, the quantity of oxygen lost by different 

 portions of it, though agreeing perfectly in some experiments, 

 differed widely in others ; and that, on one occasion, I pro- 

 cured the green oxide almost in a state of purity. I subse- 

 quently discovered, however, that the disagreement in the re- 

 sults was occasioned by the want of a free current of air within 

 the furnace. In some of the experiments the draft was un- 

 guardedly cut offj and consequently an atmosphere of car- 

 bonic oxide gas, collecting around the heated manganese, re- 

 duced it more or less nearly to the state of protoxide. On 

 avoiding this source of fallacy, the results were no longer dis- 

 cordant ; and I am now quite satisfied that the red oxide 

 formed at a white heat and with free exposure to atmospheric 

 air, is uniform in its composition. The accuracy of this in- 

 ference is established by the occurrence of the red oxide in 

 nature, as will appear in the sequel of the present communi- 

 cation. 



The red oxide, when formed at a white heat and rubbed in 

 a mortar to the same degree of fineness, is always of a brownish- 

 red colour when cold, and nearly black while warm. The 

 powder of the native red oxide has a reddish-brown tint, and 

 the colour of the red oxide prepared by exposing the precipi- 

 tated protoxide or the carbonate to a moderate red heat, has 

 most commonly an admixture of yellow, something like rhu- 

 barb, though of a deeper hue ; but both of these acquire the 

 red colour when heated to whiteness. 



The red oxide manifests little tendency to pass into a higher 

 degree of oxidation by abstracting oxygen from the atmo- 

 sphere, even by the aid of heat. Thus a portion of the red 

 oxide, preserved for an hour at a low red heat, and freely ex- 

 posed to the air at the same time, did not acquire any appre- 

 ciable addition to its weight. The protoxide of manganese 

 precipitated from the sulphate by an excess of pure potash, 

 collected on a filter and washed, fully exposed to the air in its 



moist 



