This substance, though known to chemists so early as 1818, when it was discovered by Thenard, was recently, for the first time, brought prominently before the notice of the medical profession, through the experiments of Dr. Richardson, of London. It consists of one equivalent of hydrogen and two eqs of oxygen; thus differing chemically from water in containing one eq. more of oxygen. It is made from water by imparting to it this additional equivalent, which is furnished in the nascent state by peroxide of barium, decomposed by muriatic acid. This acid has a strong affinity for protoxide of barium, or baryta, and when presented to the peroxide takes from it baryta, and liberates the second eq. of oxygen. For the precise steps of the process necessary to procure the peroxide of hydrogen in a pure state, see the 12th edition of the IT. S. Dispensatory (p. 1579). Below 60° F., it is a colourless liquid, of the sp. gr. 1.452; but at this temperature it begins to give out oxygen, and at a higher heat parts with it rapidly, and sometimes even with explosion, being resolved again into water and oxygen. Diluted, however, with water, it remains undecomposed at any temperature under 100°; so that, thus diluted, it can be kept in well-stopped bottles for use. The readiness with which it parts with oxygen is its most important property in a medical point of view. Though there are several inflammable substances which resist the influence of peroxide of hydrogen, yet for the most part it yields oxygen to oxidizable bodies, and among others to the organic colouring principles, and to the substances resulting from organic decomposition; so that it is a powerful decolorizer and deodorizer, and may be employed, with an efficiency little if any inferior to that of permanganate of potassa, as a disinfectant.

Dr. Richardson suggests its use as an internal remedy in low forms of fever, and has found it beneficial in chronic and subacute rheumatism, scrofulous tumours, hooping-cough, chronic bronchitis with dyspnoea, phthisis as a palliative, and in dyspepsia. He has found it sometimes to salivate profusely. As a local remedy, he recommends it for the dressing of gangrenous ulcers; and generally, as a disinfectant, there is reason to believe that it is not inferior in efficiency to the permanganate of potassa, while it has the advantage over it of being colourless. Of the peroxide, containing ten volumes of oxygen, which may be estimated by the quantity of oxygen given up by the peroxide of barium in its preparation, Dr. Richardson recommends as a dose from one to four fluidrachms, freely diluted with water. As a disinfectant, it is peculiarly adapted for the purification of apartments, by being distributed through them in the form of spray. In other respects, it may be employed in the same manner as the solution of permanganate of potassa. The only objection to it is the costliness of its preparation.