The study of the photochemistry of gases entered on a new phase with the work of Franck and Dymond on the fluorescence of iodine vapour in 1925. According to Franck (1) the primary process on the absorption of light by a molecule is excitation of the molecule. In the case of a homopolar molecule (e.g. halogen molecules) absorption of light of wavelength corresponding to the region of continuous absorption results in excitation followed immediately by dissociation without the intervention of a collision. Of the resulting atoms, one is in the normal state the other in the electronically excited state. At the convergence limit the atoms separate with negligibly small kinetic energies. It is interesting to note the results recently obtained by Hilferding and Steiner in an examination of the recombination of bromine atoms making use of the photochemical hydrogen bromine combination. They found that over the pressure range 258mm. to 1308mm. the relation between the velocity and the total pressure agrees with the results obtained Iby Jost. They were able to determine the rate of recombination of bromine atoms in triple collision with various foreign molecules. They found that at high pressures the number of effective triple collisions approximates to the theoretical. Again at pressures less than 258mm. the recombination of atoms on the walls is appreciable.