is a very nteresting substance nearly related to water, and differing therefrom, in that its molecule contains 1 atom more oxygen. It can be prepared in a number of ways, one of which consists in mixing barium peroxide 2 with dilute sulphuric acid, when the following interaction takes place: Another method consists in treating barium peroxide in the presence of water with carbon dioxide under pressure. The dilute solution of hydrogen dioxide thus resulting, can be concentrated to some extent by evaporation, or it can be purified by distilling at a low pressure and concentrated up to 90 per cent, by an appliance known as the “sulphuric acid concentrator,” and from this solution 100 per cent, peroxide can be obtained by fractional freezing. The pure peroxide is quite stable if kept at o degrees C.; its freezing point is 17 degrees C., and its sp. gr. at o degrees C. is 1 4633. As ordinarily prepared, it is often unstable and apt to decompose with explosive violence. There is also a process for making this compound by electrolyzing sulphuric acid, and the subsequent conversion of the persulphuric acid thus produced into hydrogen dioxide. It is stated that sodium persulphate can be produced to a greater degree of concen tration than the free acid, and that distillation of the once recrystallized sodium salt gives reasonably high yields of the peroxide. It is always found amongst the products which result when phosphorus or turpentine, and terpenes generally, are exposed to air and moisture. It also occurs in nature, being found present in the air following lightning discharges and under some other circumstances. Hydrogen dioxide readily parts with its second atom of oxygen and is a very valuable sanitary and oxidizing agent, being one of the constituents of the disinfecting fluid known commercially as “Sanitas” Fluid. Solutions of it in water are largely used for bleaching straw, hair, and other articles, also in the practice of surgery, on account of its antiseptic character and its power to destroy septic matter in wounds by oxidation. It is odourless and colourless like water, to which substance it is reduced when it parts with its active oxygen: It is soluble in alcohol, and a dilute solution of it in water can be used to restore oilpaintings which have become discoloured owing to the formation of lead sulphide, by reason of its power of oxidizing the black sulphide into white lead sulphate.

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