  A detailed study of published experimental data for a variety of materials on
the incremental variation of heat capacity, thermal expansion, and
compressibility at glassy freezing reveals a striking dependence of the
Prigogine-Defay ratio R on the fragility index m. At high m, R approaches
values of ~1, the Ehrenfest expectation for 2nd order continuous phase
transitions, while R reaches values > 20 for low fragilities. We explain this
correlation by the degree of separation of the glassy freezing temperature from
a hidden phase transition into an ideal low-temperature glass.
