~Oxides of manganese.~ The following oxides of manganese are known: MnO, Mn_{2}O_{3}, Mn_{3}O_{4}, MnO_{2}, and Mn_{2}O_{7}. Only one of these, the dioxide, needs special mention. ~Manganese dioxide~ (_pyrolusite_) (MnO_{2}). This substance is the most abundant manganese compound found in nature, and is the ore from which all other compounds of manganese are made. It is a hard, brittle, black substance which is valuable as an oxidizing agent. It will be recalled that it is used in the preparation of chlorine and oxygen, in decolorizing glass which contains iron, and in the manufacture of ferromanganese. ~Compounds containing manganese as a base-forming element.~ As has been stated previously, manganese forms two series of salts. The most important of these salts, all of which belong to the manganous series, are the following: Manganous chloride MnCl_{2}.4H_{2}O. Manganous sulphide MnS. Manganous sulphate MnSO_{4}.4H_{2}O. Manganous carbonate MnCO_{3}. Manganous hydroxide Mn(OH)_{2}. The chloride and sulphate may be prepared by heating the dioxide with hydrochloric and sulphuric acids respectively: MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl, MnO_{2} + H_{2}SO_{4} = MnSO_{4} + H_{2}O + O. The sulphide, carbonate, and hydroxide, being insoluble, may be prepared from a solution of the chloride or sulphate by precipitation with the appropriate reagents. Most of the manganous salts are rose colored. They not only have formulas similar to the ferrous salts, but resemble them in many of their chemical properties. ~Compounds containing manganese as an acid-forming element.~ Manganese forms two unstable acids, namely, manganic acid and permanganic acid. While these acids are of little interest, some of their salts, especially the permanganates, are important compounds. ~Manganic acid and manganates.~ When manganese dioxide is fused with an alkali and an oxidizing agent a green compound is formed. The equation, when caustic potash is used, is as follows: MnO_{2} + 2KOH + O = K_{2}MnO_{4} + H_{2}O. The green compound (K_{2}MnO_{4}) is called potassium manganate, and is a salt of the unstable manganic acid (H_{2}MnO_{4}). The manganates are all very unstable. ~Permanganic acid and the permanganates.~ When carbon dioxide is passed through a solution of a manganate a part of the manganese is changed into manganese dioxide, while the remainder forms a salt of the unstable acid HMnO_{4}, called permanganic acid. The equation is 3K_{2}MnO_{4} + 2CO_{2} = MnO_{2} + 2KMnO_{4} + 2K_{2}CO_{3}. Potassium permanganate (KMnO_{4}) crystallizes in purple-black needles and is very soluble in water, forming an intensely purple solution. All other permanganates, as well as permanganic acid itself, give solutions of the same color.