The light yield for the Swan bands is measured for a number of hydrocarbons. For C2H2 + 2½O2 flames 3 x 10-5 photons are emitted for each O2 consumed at 1 atm. pressure, rising to 13.5 x 10-5 at 3.5 mm. pressure. For propane the effect of pressure is more marked. The effect of mixture strength and of diluents is studied. For partial replacement of hydrocarbon by H2 or CO (the mixture being held stoichiometric) the yield falls with surprising rapidity, being proportional to a high power of the hydrocarbon concentration (square to sixth power). Preliminary results for CH are similar. The C2 emission does not appear to be due to true chemiluminescence, but there is abnormally high excitation by some active species in the flame, and the concentration of C2 exceeds the values found by Brewer, Gilles & Jenkins for equilibrium with solid carbon at the flame temperature. Polymerization and formation of unsaturated compounds appears to be important. The relation between C2 emission and the formation of solid carbon is discussed; polymerization of C2 cannot account quantitatively for the formation of solid particles.
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Gaydon et al. (1950) studied this question.