Finite-mass heat reservoirs and the second law

It is proved that a simple system composed of a pure substance, at its triple point, can exchange energy while passing through stable-equilibrium states at constant volume and temperature, and, therefore, represents a heat reservoir with finite mass. Moreover, it is proved that the behaviour of a pair of identical simple systems composed of the same substance, each in a state of the triple point, requires a slight modification of some recent statements of the second law of thermodynamics. © 1995 Società Italiana di Fisica.