This volume is the study of the physical, thermal and structural properties of disordered solids such as glasses. These solids are usually referred to as amorphous or vitreous materials. The properties of these disordered solids are very different from similar ordered solids and the study of their properties has been limited by the lack of a suitable model. The Debye model neglects the effect of frequency on phase velocity by assuming a constant phase velocity in spite of quite different velocities of longitudinal and transverse acoustical waves and assumes a common cut-off frequency for...