  A thermodynamic measure of the fragility of liquids has recently (Ito et al
ref.1) been defined in terms of the temperature dependence of the excess
entropy of liquid over crystal, scaled by the excess entropy at the glass
transition temperature, and it has been correlated with kinetic fragility. The
correlation has been vigorously contested [2] using dielectric relaxation data
on some molecular liquids. We put this conflict in perspective using an
extensive data set covering all classes of liquids while retaining the cases of
ref 2. The excess entropy of the liquid, used in the thermodynamic quantity
being correlated, has both configurational and vibrational components. This is
the source of a second conflict which concerns the relative importance of these
two components. Experimental tests supporting the Adam-Gibbs "entropy" theory
of the temperature dependence of transport in viscous liquids have mostly been
made using the excess entropy, but recent simulation studies on the hard sphere
fluid [3] SPC-E water [4] and mixed LJ [5,6] have shown, as in the A-G theory,
a correlation with the configurational component. We show why, in laboratory
studies, the correlation should be with both, inextricably. In the course of
this demonstration we identify a key role for excess vibrational entropy in
causing fragile behavior in ambient pressure liquids.
