  Using Landau theory, it is shown that eight phenomenological parameters are
needed to describe and distinguish the twelve viscosity coefficients of a
biaxial nematic phase, or the five viscosity coefficients of a uniaxial nematic
phase. The dependence of the coefficients on the macroscopic uniaxial and
biaxial order parameters is established. Since these order parameters are
determined by the anisotropies of the dielectric constant, we show that it
should be possible to determine values for all eight of the phenomenological
parameters of the theory from measurements of the temperature dependence of the
five viscosities of a uniaxial phase.
