1st part: The Activity in Classical Thermodynamics Chapter 1: Thermodynamic systems Chapter 2: Gibbs and Helmhotz energies Chapter 3: Escaping tendency and molar Gibbs energy Chapter 4 : Partial molar properties Chapter 5: Chemical potential or partial molar Gibbs energy Chapter 6: An overview of the notion of activity Chapter 7: The concept of fugacity Chapter 8: Ideal solutions Chapter 9: Definitions of the activity Chapter 10: Activity of a gaz Chapter 11: Activities of non- electrolytes in solutions Chapter 12:Activities of electrolytes: definitions Chapter 13: Determination of the activity of non-electrolytes Chapter 14: Determination of the activity of electrolytes Chapter 15: Debye and Hückel 's relations Chapter 16: Excess Gibbs energies and activities Chapter 17: Chemical equilibrium constants : activities and Gibbs energies of reactions Chapter 18: Derivation of thermodynamic equilibrium constants- pH and its measure Chapter 19: General principles of calculations of ionic species concentrations in solutions involving activities 2nd part: The Activity in Statistical Thermodynamics Chapter 20: Statistical thermodynamics in brief Chapter 21: Concept of ensembles and postulates Chapter 22: The canonical ensemble: notion of distribution Chapter 23: Thermodynamic quantities in the framework of the canonical ensemble Chapter 24: Other ensembles Chapter 25: Systems of molecules and subsystems independent distinguishable and indistinguishable Chapter 26: Perfect gases Chapter 27: Classical statistical mechanics- Notion of configuration - Classical canonical partition function Chapter 28: The configurational partition function - Molecular distribution functions Chapter 29: The radial distribution function Chapter 30: Radial distribution function and thermodynamic quantities- calculations of the internal energy and of the pressure of a system Chapter 31: Radial distribution function and calculation of the isothermal compressibility coefficient of a system Chapter 32: The chemical potential and the radial distribution function . General formal introduction of the activity and of the activity coefficient Chapter 33: Imperfect gases - The virial coefficients in terms of interaction potential energies Chapter 34: A statistical expression of the activity. A relation between it and the concentration in the case of an imperfect gaz Chapter 35: Activities of gases in a mixture of imperfect gases Chapter 36: Chemical equilibria between gases and statistical thermodynamics Chapter 37: Activity coefficients of gases Chapter 38: Activities and concentrations of non-electrolytes in dilute liquid solutions - McMillan- Mayer's theory of their osmotic pressure Chapter 39: A study of dilute solutions of non-electrolytes at constant pressure and temperature Chapter 40: The activity coefficients of solutes in binary non-electrolyte solutions Chapter 41: Molecular distribution functions in binary mixtures Chapter 42: The Kirkwood-Buff's theory- Changes of chemical potentials of solutes with their concentrations at constant pressure and temperature Chapter 43: Expressions of the chemical potentials of the components of ideal solutions of non- electrolytes Chapter 44: Chemical potentials, activities and activity coefficients of the components of a non- ideal solution of a non-electrolyte Chapter 45: Expressions of the activities of non-electrolytes and of their coefficients Chapter 46: More on the Debye-Hückel 's theory Chapter 47: Distribution functions and the theory of electrolytes solutions Chapter 48: The activity notion in retrospect. Appendices About Some Authors Bibliography