The soot cloud in a luminous diffusion flame contains carbon monoxide and other unburnt gases, and both soot and gases oxidize when additional oxygen diffuses into the flame. According to the literature, the relative oxidation rates satisfy d log [soot] ~ 0"5 x 10 -6 d log [CO] O
where D is the diameter of the soot particles in centimetres. Particles with D ~-1 x 10 -6 cm (a size often observed in flames) should bum up with the gases. In order to survive the oxidation of the gases and give smoky flames, the particles must have considerably larger diameters. Plots of log [soot] versus log [CO], as both oxidized, were made for flames containing various mole fractions of soot. Mole fraction was defined as atoms of carbon precipitated as soot per gas molecule in the soot cloud. At sufficiently large mole fractions, the oxidation rate of soot decreased relative to the oxidation rate of CO until, eventually, soot survived to give smoky flames. This was taken as evidence for coagulation to larger particles the greater the mole fractiorL Increasing the pressure increased the mole fraction of soot precipitated, and also increased the coagulation of a given mole fraction. A simple calculation suggests:
(1) When the log/log plots just failed to give evidence for coagulation, soot particles did not exceexl 2.2 ร 10 -6 cm diameter. {2) g ~ :~ the flames first became smoky, the particles could have grown as large as 6 x l0 -6 cm diameter