  Irreversible thermodynamics of simple fluids have been connected recently to
the theory of dynamical systems and some interesting assumptions have been made
about the nature of the associated invariant measures. We show that the tests
of validity of these assumptions are insufficient by exhibiting observables
that are incorrectly sampled with the proposed invariant measures. Only
observables belonging to the "high temperature phase" of the thermodynamic
formalism are insensitive to the sampling methods. We outline methods that are
free of these deficiencies.
