The small-amplitude disturbances in an arbitrary mixture of radiation and gas-dust are studied. The results are valid for arbitrary radiation density, gas density, gas pressure, absorption coefficient, scattering coefficient, and wavelength of the disturbance. The dispersion relation in one equation contains both the radiation instability of Spitzer and the damped waves of Lifshitz and Silk. The instability occurs only if radiation pressure exceeds gas pressure and the wavelength exceeds a critical value. The maximum growth rate is reduced (by effects of finite optical depth) from the value calculated by Spitzer. The instability, which may occur near the Sun, compresses gas in the same way as gravitation, with an efolding time of l0 years. In the interior of the Galaxy the time may be only a few million years. Contrary to the suggestion of Gamow, in big-bang models the instability occurs with only a negligible growth rate, making it unimportant for cosmology. The proper frequency and damping rate of nearly adiabatic waves is included for both optically thin and optically thick disturbances. The results for optically thick waves agree with those of Peebles when the absorption is neglected, but contain a new damping term when absorption is present. This new damping term, however, is numerically unimportant for cosmology.