  The growth of bubbles in cosmological first-order phase transitions involves
nontrivial hydrodynamics. For that reason, the study of the propagation of
phase transition fronts often requires several approximations. A frequently
used approximation consists in describing the two phases as being composed only
of radiation and vacuum energy (the so-called bag equation of state). We show
that, in realistic models, the speed of sound in the low-temperature phase is
generally smaller than that of radiation, and we study the hydrodynamics in
such a situation. We find in particular that a new kind of hydrodynamical
solution may be possible, which does not arise in the bag model. We obtain
analytic results for the efficiency of the transfer of latent heat to bulk
motions of the plasma, as a function of the speed of sound in each phase.
