AN admirable account of the history and present position of photochemistry was given by Prof. A. J. Allmand, of King's College, London, in delivering the twenty-seventh Bedson Lecture at Armstrong Col lege, Newcastle-upon-Tyne, on October 27. Tracing it from the work of Cruikshank and Scheele to that of Planck, Einstein and Warburg, he gave a concise account of the interpretation of absorption spectra, and the conception of activation, along with the application of kinetics to photochemical reactions, with consideration also of sensitised reactions. He related that the German chemical warfare records mentioned the difficulty experienced in the complete ehlorination of methyl formate, in which the yield obtained varies apparently capriciously from eighty to about two per cent. This was actually due, as Luther had shown for the homologues of benzene, to the absence or presence of air, oxygen being a powerful inhibitor. Further, it has been shown that under various conditions reaction tends to vary as the square root of light intensity, instead of being directly proportional to it, and in the case of the decomposition of hydrogen peroxide a maximum is reached in the plot of concentration against rate. In the combination of hydrogen with chlorine, intensive drying does not in fact inhibit the reaction. In the sensitisation of the decomposition of ozone by chlorine, there is formation of the oxide C12O6. In the bromination of benzene in the light, the red or brown liquid residues were found to contain C6H6Br2, and possibly C6H6Br4. Summarising, Prof. Allmand said that photochemical reactions tend to be complex, consisting of consecutive interactions of the free groups or atoms which are the primary products.