(1) In accordance with theoretical considerations, it is proved experimentally that hydrogen and bromine combine to a small extent when exposed to the light of the aluminium spark, which radiates strongly at λ=185 μμ. At equilibrium the vapour pressure of hydrobromic acid is 1·4 to 1·8 per cent, that of the bromine, and the equilibrium state is rapidly reached.
(2) The absorption coefficient of bromine increases slowly with decreasing wave-length for wave-lengths shorter than 254 μμ, but the gas is still very transparent even at 185 μμ.
The authors wish to acknowledge their indebtedness to Professor Sir James Walker and Professor Kendall for their interest and very helpful advice during the progress of the work, and also to Imperial Chemical Industries, Ltd., for a grant covering the expense of some of the apparatus used.