The history of development of the theory of light scattering and dispersion in gases is outlined, from the works of Rayleigh, Planck, and Mandelstam to those of Lorentz and Einstein. It is shown that of central concern in these studies was actually the problem of thermal fluctuations in a medium. A formula for the permittivity ε(ω) taking account of radiation friction forces is derived for the case of an isotropic medium.