  The formation of clusters in supercooled liquids close to the glass
transition temperature is described by rate equations in which the coefficients
are determined on physical grounds rather than in terms of thermodynamic
quantities such as free energies and surface tensions. In particular, the
density of free molecules in the liquid as a function of temperature is
determined self-consistently. Calculations for a very simple model indicate
that such rate equations are capable of producing physically reasonable
results. Our results suggest that the difference between strong and fragile
liquids may be associated with the strength of the binding of a surface
molecule to a cluster, and they also provide indications about the nature of
the glass transition and the structure of the resulting glass.
