On bromine chloride

It is curious, in view of the close attention which has been paid to many of the compounds formed between the halogens, that our knowledge of bromine chloride should be so vague that its very existence has frequently been denied. In 1826 Balard described a compound between bromine and chlorine formed by direct union, and a few years later Loewig claimed to have isolated a hydrate. BrCl. 5H2 O, from solutions. Berthelot threw some doubt upon this early work on account of the small heat of solution of chlorine gas in liquid bromine. Lebeau found that the crystals prepared in accordance with Balard’s directions were variable in their composition and concluded from this and from the melting point curve of chlorine-bromine mixtures that no compound really exists. The same conclusion was reached by Karsten and has been very generally adopted, except that the behaviour of bromine-chlorine mixtures in certain organic reactions, has occasionally led to suspicions of compound formation. One very cogent argument in favour of compound formation was touched upon by Balard, but has ever since been neglected—the marked diminution in the colour of bromine produced by admixture of chlorine. Our attention was drawn to this phenomenon by Mr. G. M. B. Dobson, of the Clarendon Laboratory, Oxford, who observed it in preparing filters for ultra-violet light from mixtures of the two gases and who found further that the ultraviolet absorptions are not additive in these mixtures. It is with his kind permission that we have now investigated the matter more fully. The obvious explanation of the colour change is the formation of a compound between the two halogens, which is more transparent in the visible than is bromine; though some physical influence of the chlorine on the bromine cannot immediately be ruled out as a hypothesis. The experiments described in the present communication were designed to test the chemical explanation, and to determine, if possible, the formula of the compound and the amount of it formed.