The fact that a large number of chemical reactions have been shown to be dependent on the presence of aqueous vapour has led to many experiments being made on the union of hydrogen and oxygen. Some years ago one of us made experiments which showed that an electric spark would fire ordinary electrolytic gas, whether in the dried or the moist state; and experiments on the rate of detonation in electrolytic gas seemed to show that, once an explosion-wave was started, no influence was exerted on the propagation of the wave by aqueous vapour, except a slightly retarding one. More recently, H. B. Baker carried out a series of experiments with very pure hydrogen and oxygen obtained by the electrolysis of a solution of highly purified barium hydroxide. His results show that the initiation of the flame by a heated wire is largely influenced by the purity of the gases.