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simple ionic crystals. Recently Stirling using a similar model has satisfacto­ rily fitted the experimental measure­ ments for SrTIO3. The theory has been extended by Cowley and Bruce to include anharmonic effects. This is the area which is currently of greatest in­ terest to those concerned with struc­ tural phase transitions, and which must eventually succeed In connect­ ing the lattice dynamics approach to the more orthodox approaches to the theory of phase transitions. The theory of a new phenomenon in acoustics called “phonon echoes” was given by the very interesting paper of J. Joffrin, Université Paris VI. Laboratoire d’Ultrasons, in terms of a classical presentation. The essential properties which are the time reversal, the node to node coupling, and the existence of a non-linear interaction of phonons with an external electric field were described. The applications for the physicist and for the engineer were also briefly reported. K. Dransfeld, Max-Planck-Institut für Festkörperforschung HochfeldMagnetlabor Grenoble, summarized in his talk the new results of the dyna­ mical properties of disordered mate­ rials at very low temperatures (below 1°K). All these observations are con­ sistent with a new type of low lying amorphous materials but which are absent in perfect crystals. Both light and neutron scattering methods are extensively used to study phonon spectra of solids. A compari­ son between the two methods was made by N. Kroó, Central Research Institute for Physics, Budapest, in order to show the similarities and differences, with typical examples given mainly in those fields where the applied method is superior. The physics and chemistry of solid surfaces exposed to vacuum or to gases are proceeding very fast. G. Heiland’s (2. Physikalisches Institut der Rheinisch-Westfälischen Techni­ schen Hochschule, Aachen) valuable and detailed report on the new results in this field was therefore of topical interest. Reliable methods for the pre­ paration of clean surfaces and new ways of characterization and investi­ gation provide meaningful data on a much wider scale than a few years ago. Stimulated by recent experi­ mental results, the theoretical activity has grown considerably. Driving forces for research on surfaces originate not only from theory, but mainly from applications in solid state electronics and in heterogeneous catalysis. New experimental methods of surface re­ search are the following:

1st EPS Conference of the Nuclear Physics Division on Nuclear Interactions at Medium and Low Energies