  Relativistic field equations for a gas in special and general relativity are
determined from the Boltzmann equation. The constitutive equations are obtained
from the Chapman-Enskog methodology applied to a relativistic model equation
proposed by Anderson and Witting. Two applications in general relativity are
considered: one refers to a gas in a homogeneous and isotropic Universe where
irreversible processes are present during its evolution; in the other it is
analyzed a gas under the influence of a spherically symmetrical non-rotating
and uncharged source of the gravitational field.
