### Abstract:

These lectures were given by the visiting Scientist, Dr. Franz Mohling, from University of Colorado during 1964. Lecture- I deals with Free Bose Gas; The formula for the average number of quanta in a single state could also be used to derive thermodynamical properties of black body radiation by elementary means. Here a Bose particle is defined to be any particle which can occupy a quantum state independently of how many other (identical) Bose particles are already in that state. A Fermi particle is one which cannot occupy a quantum state if another(identical) Fermi particle is already in that state. Hence elementary particles obey either Bose - Einstein statistics, or Fermi -Dirac Statistics. The principal result obtained in lecture I was an equation for the grand partition function in X- ensemble formulation of quantum statistics: The objective of the second Lecture is to show how the grand potential may be expressed in terms of the basic two-particle interactions of a Bose system. Self-Energy Problem, Momentum Space Ordering, and Quasi-Particles are discussed in the Third Lecture.