Third sound and 3He-4He mixture films

The fundamental ides of third sound are surveyed, and the historical importance of third sound as a technique for the investigation of helium-film systems is emphasized through the discussion of several examples. The desire to study 3He-4He mixture films has led to the development of new methods for detecting third sound and these are discussed. We then describe some of the interesting properties of 3He-4He mixture films that have been discovered by third-sound techniques and by recent preliminary third-sound and nuclear magnetic resonance investigations of the mixture films conducted in this laboratory. (32 refs., 13 figs.)