  The fragilities (Tg-normalized temperature dependence of alpha-relaxation
times) of 33 glass-forming liquids and polymers are compared for isobaric, mP,
and isochoric, mV, conditions. We find that the two quantities are linearly
correlated, mP = (37 +/- 3) + (0.84 +/- 0.5)mV. This result has obvious
important consequences, since the ratio mV/mP is a measure of the relative
degree to which temperature and density control the dynamics, Moreover, we show
that the fragility itself is a consequence of the relative interplay of
temperature and density effects near Tg. Specifically, strong behavior reflects
a substantial contribution from density (jammed dynamics), while the relaxation
of fragile liquids is more thermally-activated. Drawing on a scaling law, a
physical interpretation of this result in terms of the intermolecular potential
is offered.
