  Viscous resistance to changes in the volume of a gas arises when different
degrees of freedom have different relaxation times. Collisions tend to oppose
the resulting departures from equilibrium and, in so doing, generate entropy.
Even for a classical gas of hard spheres, when the mean free paths or mean
flight times of constituent particles are long, we find a nonvanishing bulk
viscosity. Here we apply a method recently used to uncover this result for a
classical rarefied gas to radiative transfer theory and derive an expression
for the radiative stress tensor for a gray medium with absorption and Thomson
scattering. We determine the transport coefficients through the calculation of
the comoving entropy generation. When scattering dominates absorption, the bulk
viscosity becomes much larger than either the shear viscosity or the thermal
conductivity.
