  We calculate the velocity of sound in an ideal gas of massive hadrons with
non-vanishing baryon number. The gas is in thermal and chemical equilibrium.
Also we show that the temperature dependence $T(\tau) \cong T_{0} \cdot
({\tau_{0} \over \tau})^{c_{s}^{2}}$ is approximately valid, when the gas
expands longitudinally according to the Bjorken law.
