[9 bis] Peroxide of barium also gives oxygen at the ordinary temperature in the presence of the solutions of many substances in a higher degree of oxidation. In this respect we may mention that Kassner (1890) proposes to obtain oxygen for laboratory purposes by mixing BaO_{2} with FeK_{3}(CN)_{6} (red prussiate of potash, Chapter XXII.): the reaction proceeds with the evolution of oxygen even on the addition of a very small quantity of water. In order to ensure a gradual evolution of gas the author proposes to introduce both substances into the reaction, little by little, instead of all at once, which may be done with the following arrangement (Gavaloffsky): finely powdered peroxide of barium is placed in an ordinary flask and sufficient water is added to fill the flask one-third full. The cork closing the flask has three holes; (1) for the gas-conducting tube; (2) for a rod to stir the BaO_{2}; and (3) for a glass rod terminating in a perforated glass vessel containing crystals of FeK_{3}(CN)_{6}. When it is desired to start the evolution of the oxygen, the vessel is lowered until it is immersed in the liquid in the flask, and the BaO_{2} is stirred with the other rod. The reaction proceeds according to the equation, BaO_{2} + 2FeK_{3}(CN)_{6} = FeK_{4}(CN)_{6} + FeK_{2}Ba(CN)_{6} + O_{2}. The double salt, FeBa_{2}(CN)_{6}, crystallises out from the mother liquor. To understand the course of the reaction, it must be remembered BaO_{2} is of a higher degree of oxidation, and that it parts with oxygen and gives the base BaO which enters into the complex salt FeK_{2}Ba(CN)_{6} = Fe(CN)_{2} + 2KCN + Ba(CN)_{2}, and this latter = BaO + 2HCN-H_{2}O. Moreover, FeK_{3}(CN)_{6} contains the salt Fe_{2}(CN)_{6} which also corresponds to the higher degree of oxidation of iron, Fe_{2}O_{3}, whilst after the reaction a salt is obtained which contains Fe(CN)_{2}, and corresponds to the lower degree of oxidation, FeO, so that (in the presence of water) oxygen is also set free on this side also, _i.e._ the reaction gives lower degrees of oxidation and oxygen. [10] Scheele, in 1785, discovered the method of obtaining oxygen by treating manganese peroxide with sulphuric acid.