DURING the Great War, synthetic phenol was manufactured by sulphonating benzene, but a continuous process for vapour phase chlorination has been employed at the Dow Chemical Co.'s works in the United States for some time now. The chlorobenzene is afterwards hydrolysed with caustic soda and some diphenyl oxide is obtained as a by-product. Diphenyl oxide has a melting point of 27° C. and in order to be useful as a commercial fluid for heat transmission it must not set solid in the piping when the plant is allowed to cool. Although diphenyl has a melting point of 70° C., yet a eutectic mixture, known as 'Dowtherm', consisting of 26·5 per cent diphenyl with diphenyl oxide, melts at 12° C. This compound can be used for heating purposes in a similar manner to steam but with the additional advantage of higher temperatures for the same pressure. The boiling point at atmospheric pressure is about 258° C. and at 135 Ib. per square inch gauge pressure it gives a temperature of 400° C. against 181° C. for steam. At 370° C. the temperature can be controlled to within about 1° C. by simply manipulating a throttle valve. The use of condensing vapour for heating results in considerable economy in plant design since the hot liquids in use hitherto have very low heat transfer coefficients. The vapours are not toxic, although leaks can be readily detected since the vapour is exceedingly pungent. It gives a means of accurately controlling the reaction temperature in the manufacture of dye intermediates and is used in the distillation of fatty acids and glycerine, the deodorization of edible oils, the evaporation of caustic soda, the processing of varnishes, the saponifying of greases and the melting of asphalts and bitumen. Owing to the low surface tension and viscosity of Dowtherm, all joints have to be made extremely care fully. The licence for development in Great Britain is held by Messrs. W. J. Fraser and Co., Ltd., of Dagenham, Essex.