Magnesium, magnesia nigra, and siderea, manganese oxyde Hauy iv. 243, is of a black brown colour, with occasionally a little of the metallic splendour, of a specific gravity from 3.70 to 4.75. Some varieties which stain the fingers are much lighter. It is divisible in rhomboidal prisms of about 100° and 80°, and colours borax of a violet hue when exposed to the blow pipe. It was long supposed to be an ore of iron, and though shown in 1770 by Kaim to contain a peculiar metal, it was only completely reduced by Gahn about the year 1774. Manganese attracts oxygen more rapidly than any other body, except phosphorus and the black oxide; that most commonly employed contains sixty parts of the metal and forty of oxygen. In this variety the oxygen seems in excess, and the excess only is yielded in decomposition; for the white manganese retains its oxygen with great obstinacy.
It is needless to enlarge on the preparation of manganese, as its chief use is in preparing the oxygenated acids and salts. M. Alyon has recommended an ointment composed of the black oxide of manganese with axunge, in the proportion of one drachm to an ounce, which he recommends in a variety of external diseases, where the oxygen is apparently deficient, but whose boasted merits experience in this country has not supported. In the preparation of oxygen for respiration, manganese is chiefly used, and it is the principal ingredient in Guy ton's box to secure from contagion. Those, however, who prepare oxygen gas should recollect the observation of M. Seguin, that some manganese at first yields a portion of azotic gas.