Note 18 — Chapter XXI · CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE

The name ‘peroxide’ should only be retained for those highest oxides (and MnO2 stands between MnO and MnO3) which either by a direct method of double decomposition are able to give hydrogen peroxide or contain a larger proportion of oxygen than the base or the acid, just as hydrogen peroxide contains more oxygen than water. Their type will be H2O2, and they are exemplified by barium peroxide, BaO2, and sulphur peroxide, S2O7, &c. Such a dioxide as MnO2 is, in all probability, a salt—that is, a manganous manganate, MnO3MnO, and also, as a basic salt of a feeble base, capable of combining with alkalis and acids. Hence the name of manganese peroxide should be abandoned, and replaced by manganese dioxide. PbO2 is better termed lead dioxide than peroxide. Bisulphide of manganese, MnS2, corresponding to iron pyrites, FeS2, sometimes occurs in nature in fine octahedra (and cube combinations), for instance, in Sicily; it is called Hauerite.