[Physics FAQ] - [Copyright]  Updated PEG January 1997.  Original
by Philip Gibbs October 1996, with thanks to many who
contributed their knowledge and references.  Is glass liquid or
solid?   To answer the question "Is glass liquid or solid?" we
have to understand its thermodynamic and material properties.
Thermodynamics of glass  There is still much about the molecular
physics and thermodynamics of glass that is not well understood,
but we can give a general account of what is thought to be the
case.  Many solids have a crystalline structure on microscopic
scales.  The molecules are arranged in a regular lattice.  As
the solid is heated the molecules vibrate about their position
in the lattice until, at the melting point, the crystal breaks
down and the molecules start to flow.  There is a sharp
distinction between the solid and the liquid state, that is
separated by a first order phase transition, i.e. a
discontinuous change in the properties of the material such as
density.  Freezing is marked by a releas