The disclosure relates to an apparatus for measuring the size of gas entrained particles by detecting the rate of incandescent burning of the particles. Entrained particles are passed at a controlled velocity through a heated zone where they are gradually consumed. The radiant energy emitted by the burning particles is detected at varying locations along the path of flow through the heated zone. An indication of particle size distribution is obtained by deriving the third derivative of detected radiant energy intensity with respect to distance of flow through the heated zone.