In mercury porosimetry of limestone two hystereses are generated. A PV-diagram for initial intrusion and extrusion reveals one hysteresis as entrapment of mercury in large cavities that are connected to the exterior through narrow channels. The other hysteresis is a loop (HL) enclosed between the extrusion and reintrusion curves which indicate narrow channels. An HL is unique to a stone. The thermodynamic work (W) calculated as SIGMAPDELTAV, is W_I, W_E, W_RE, W_HY, and W_T for the initial intrusion, extrusion, reintrusion, the hysteresis loop, and the entrapment, respectively. The pore-potential (U) defined as U=W_I- W_E=W_T+W_HY regulates the volume and rate of movement of water through the pores which in turn determines the stone's durability. Microporous stones are highly susceptible to weathering and have a large value for W_HY. W_HY in association with the W_T can be related to the durability factor (DF) as DF=W_T/(W_T+W_HY). The DF values, thus obtained, accurately reflect the observed condition of the strata of the Sphinx. -- CAL