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It is shown that two-centre defects, closely related to intimate valence-alteration pairs (IVAPs), display the same statistical features as those suggested in the tunnelling-atom theory for two-level, low-energy excitations. Comparisons are made with measured short- and long-time relaxation effects in vitreous silica. The possibility that two-level systems originating from IVAPs and from tunnelling can co-exist in a-SiO2 is discussed, with particular reference to very-long-time relaxation effects.

The effects of the internal energy at the glass transition are studied by means of the Prigogin-Defay ratio. It is first shown that the jumps of the heat capacities at constant pressure and at constant volume must be comparable in magnitude. In addition, special systems are studied, where the Prigogine-Defay ratio turns out to depend only on the heat-capacity jumps. Finally, it is shown that the kinetic nature of the glass transition is necessarily related to an excess of internal energy in the melt.

The problem of the glass transition is approached from the thermodynamical viewpoint, with particular reference to the Prigogine-Defay ratio Γ. Some controversial points concerning the condition Γ>1 are shown to be irrelevant from the phenomenological viewpoint. The importance of the internal energy at the glass transition is emphasized. Accordingly, a thermodynamical model, based on fluctuating internal stresses, is proposed and applied to a class of GexSe1-x liquids.