  Given a thermodynamic process which carries a system from one equilibrium
state to another, we construct a quantity whose average, over an ensemble of
microscopic realizations of the process, depends only on these end states, even
if at intermediate times the system is out of equilibrium. This result: (1) can
be used to express the entropy difference between two equilibrium states in
terms of an irreversible process connecting them, (2) leads to two statistical
statements of the Clausius-Duhem inequality, and (3) can be generalized to
situations in which the system begins and/or ends in nonequilibrium states.
