The black oxide is mainly used in the preparation of chlorine, liberation of which it brings about when treated with hot hydrochloric acid, or with a mixture of common salt and sulphuric acid. The quantity of chlorine which is obtained depends upon the proportion of dioxide present;[78] and in assaying may either be measured by its equivalent of iodine liberated, or by the oxidising effect on an acid solution of ferrous sulphate. When the ore also carries substances which have a reducing effect (such as ferrous compounds), such assays will give, not the total dioxide (MnO_{2}), but less, by the amount required to oxidise these impurities; and this is exactly what is required in valuing such an ore for commercial purposes. Manganese compounds are characterised by the readiness with which they may be converted into highly-oxidised bodies. Solution of manganese in hydrochloric acid, rendered alkaline with ammonia, yields a clear solution,[79] which rapidly takes up oxygen from the air, forming a brown precipitate of the oxide (Mn_{2}O_{3}). The addition of bromine or chlorine to such a solution determines the precipitation of a still higher oxide (approximately MnO_{2}). On treating a compound containing manganese with nitric acid and dioxide of lead (PbO_{2}), the oxidation is carried still further, a purple-coloured solution of permanganic acid (HMnO_{4} or H_{2}O.Mn_{2}O_{7}) being formed. On fusing minerals containing (even traces of) manganese with sodium carbonate in an open crucible, a green "melt" is obtained which owes its colour to sodium manganate (Na_{2}MnO_{4} or Na_{2}O.MnO_{3}). This salt is soluble in water, forming a green solution; which, when rendered acid, rapidly changes into the permanganate with the characteristic purple colour. Permanganate of potash is a salt much used in assaying, with some properties of which the student will have already become familiar. Compounds of manganese, on boiling with strong hydrochloric acid, yield manganous chloride[80] (MnCl_{2}). The properties given above serve for the detection of manganese; the higher oxides are distinguished by causing the evolution of chlorine (with its peculiarly suffocating smell) when acted on with hydrochloric acid; while the green "melt," with sodium carbonate, can be relied on for the recognition of manganese itself. There is no dry assay of manganese ores. WET METHODS. Strong hydrochloric acid is the best solvent for ores of manganese; but where the proportion of dioxide (MnO_{2}) is required, the solution is effected during the assay. The ore should be in a very fine state of division before treatment with acids.