  Newton' viscosity law for the momentum flux and Fourier's law for the heat
flux define Navier-Stokes hydrodynamics for a simple, one component fluid.
There is ample evidence that a hydrodynamic description applies as well to a
mesoscopic granular fluid with the same form for Newton's viscosity law.
However, theory predicts a qualitative difference for Fourier's law with an
additional contribution from density gradients even at uniform temperature. The
reasons for the absence of such terms for normal fluids are indicated, and a
related microscopic explanation for their existence in granular fluids is
presented.
