  We show that, in the framework of Carath\'eodory's approach to
thermodynamics, one can implement black hole thermodynamics by realizing that
there exixts a quasi-homogeneity symmetry of the Pfaffian form $\deq$
representing the infinitesimal heat exchanged reversibly by a Kerr-Newman black
hole; this allow us to calculate readily an integrating factor, and, as a
consequence, a foliation of the thermodynamic manifold can be recovered.
