  The temperature dependence of the viscosity of glass is a major concern in
the field of glass research. Strong deviations from the Arrhenius law make the
interpretation of the activation energy difficult. In the present study, a
reasonable interpretation of the apparent activation energy is demonstrated
along similar lines as those adopted in solid-state physics and chemistry. In
contrast to the widely held view that phase transition in glass occurs at the
reference temperature $T_{0}$ according to the Vogel--Fulcher--Tammann formula,
in the present work the transition observed at the glass-transition temperature
$T_{g}$ is regarded as a phase transition from the liquid to solid phases. A
distinct feature of glass is that the energy barrier significantly changes in
the transition range with width $\Delta T_{g}$. This change in the energy
barrier alters the manner in which the apparent activation energy constitutes
the Arrhenius form. Analysis of available experimental data showed that the
actual energy barrier is significantly smaller than the apparent activation
energy, and importantly, the values obtained were in the reasonable range of
energy expected for chemical bonds. The overestimation of the apparent
activation energy depends on the ratio $T_{g}/\Delta T_{g}$, which explains the
existence of two types of glasses strong and fragile glasses. The fragility can
be re-interpreted as an indication of the degree of increase in the energy
barrier when approaching $T_{g}$ from high temperatures. Since no divergence in
viscosity was observed below $T_{g}$, it is unlikely that a transition occurs
at $T_{0}$.
