  The fluid equations for the baryon-electron system in an expanding universe
are derived from the Boltzmann equation. The effect of the Compton interaction
is taken into account properly in order to evaluate the photon-electron
collisional term. As an application, the acoustic motions of the
baryon-electron system after recombination are investigated. The effective
adiabatic index $\gamma$ is computed for sound waves of various wavelengths,
assuming the perturbation amplitude is small. The oscillations are found to be
dumped when $\gamma$ changes from between 1 (for an isothermal process) to 5/3
(for an adiabatic process).
