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.