  The impossibility of separating into work and heat the energy transmitted
between two subsystems through a movable piston is analyzed in this article.
The process here described, although "quasi-static", is not reversible. It is
shown that the First Principle, dU=dW+dQ, introduced by Clausius, does not
generally allow a physical identification of dW and dQ, although dU=-pdV+TdS is
verified along the equilibrium points of each subsystem.
