Atomic-force pictures reveal a heterogeneous microstructure at the surface of glassy layers which should be similar to one of the many equivalent microstructures a liquid is running through. These microstructures are described with the aid of a kinetic model of reversible aggregation which goes back to formulations as used in the description of living polymerization or aggregation of polymers in solution. Aggregates are considered as dynamic subsystems wherein collective modes of motions are excited. Fluctuations of the aggregates, densely packed in a disordered pattern, leads to a broad size distribution which happens to be controlled by Boltzmann's factor. The disordered structure within the aggregates themselves is optimized, reduced aggregate energy and entropy should be equal. Symmetries are deduced which elucidate many universal properties of the dynamic microstructure of liquids. Thermodynamic properties like the specific heats of aggregation in liquids or the dependence of the glass transition of homologues of linear atactic polystyrene are consistently described. (C) 1997 American Institute of Physics. [S0021-9606(97)51043-9].