  The mass of a black hole is interpreted, in terms of thermodynamic
potentials, as being the enthalpy, with the pressure given by the cosmological
constant. The volume is then defined as being the Legendre transform of the
pressure and the resulting relation between volume and pressure is explored in
the case of positive pressure. A virial expansion is developed and a van der
Waals like critical point determined. The first law of black hole
thermodynamics includes a PdV term which modifies the maximal efficiency of a
Penrose process. It is shown that, in four dimensional space-time with a
negative cosmological constant an extremal charged rotating black hole can have
an efficiency of up to 75%, while for an electrically neutral rotating back
hole this figure is reduced to 52%, compared to the corresponding values of 50%
and 29% respectively when the cosmological constant is zero.
