  Heat capacities at fixed volume and pressure as a function of the degree of
ionization are graphically depicted as functions of reciprocal temperature and
ionization degree. The polytropic index is calculated as a function of the same
variables; as it is not constant a partially ionized plasma can be only
approximately polytropic fluid. In parallel, it is launched the idea that
Alfv\'en waves can be used to heat the plasma in a propulsion jet with magnetic
nozzle.
