The motion of small suspended particles in a gas or gas mixture containing gradients of temperature, pressure, or composition is derived as a special case of the Chapman-Enskog kinetic theory of gases, by formally treating the suspended particles as large molecules. Gas molecules colliding with the suspended particles are considered to rebound elastically, but a fraction f rebound in random directions and the remainder rebound specularly. The results check, in an indirect way, the calculations of Waldmann by a momentum transfer method on a slightly different model, in which the randomly rebounding molecules also have a random distribution of speeds. Significantly different results are predicted by the two models only in the presence of a temperature gradient (thermal diffusion), which has interesting implications concerning thermal diffusion in polyatomic gases.