A new thermodynamic analysis is given for the equilibrium between a liquid cluster and the surrounding supersaturated gas phase in a finite constant volume. It is shown that for constant total density and intermediate volume this equilibrium is stable, although it is unstable for very large volume. We show that observation of the critical cluster size l* then yields information on the surface free energy of the liquid cluster. The accuracy of previous approximate prescriptions for obtaining the free energy of physical clusters is investigated.