Oxides of manganese, if heated with caustic alkalies in a current of air, or with potassium or sodium nitrate, are converted into manganate ; the manga-nate, however, is much more easily decomposed than the chromate, and, indeed, is stable only in presence of excess of alkali. Manganic acid is incapable of existence ; an attempt to liberate it, by addition of an acid to its sodium salt, results in the formation of a permanganate and a manganous salt, thus: 5Na9MnO4.Aq + 6H2SO4.Aq = 5Na2SO4.Aq + Mn8O4. Aq + 4HMnO4.Aq + 4H2O.