  The dependence of the bulk viscosity with the relativistic parameter
$z=kT/mc^{2}$, obtained through the complete Boltzmann equation [1,2], is
thoroughly analyzed. A complete and rigorous examination of the relevant
non-relativistic and ultra-relativistic limits is carried out in the case of a
hard disk model and compared with the results obtained in the relaxation time
approximation. The modifications of the non-vanishing bulk viscosity to the
Rayleigh-Brillouin is briefly discussed.
