  Several papers have recently presented results of measurements of physical
aging by studying the behavior of glassy materials quenched from temperatures
above their glass transition temperature $T_g$. The evolution of the aging
process is usually followed by plotting the relaxed enthalpy versus the
accompanying decrease in volume. Here, we focus on the slope of such plots,
which are found to be similar to the inverse value of the isothermal
compressibility close to $T_g$. An explanation of this empirical result is
attempted in the frame of a model that interconnects the defect Gibbs energy
with properties of the bulk material.
