Journal of Statistical Physics, Vol. 57, Nos. 3/4, 1989. p. 647-664.
Gas mixtures of heavy plus light molecules can apparently support several different sorts of sound waves (the most recent such prediction is that of Campa and Cohen). The origin of the first such predictions is reviewed, along with the development of the two-temperature hydrodynamic equations which govern these mixtures at moderate wavenumbers and frequencies. Light scattering in the two-temperature regime is also discussed. Experiments in both sound propagation and light scattering are shown to confirm the existence of a two-temperature regime, and two simultaneous sound modes, in these disparate-mass gas mixtures.