1. Introductory chapter; Part I. Mathematical Theory of a Gas in a Steady State: 2. The method of collisions; 3. The method of statistical mechanics; 4. Comparison between the methods of the two preceding chapters; 5. General statistical mechanics and thermodynamics; Part II. Physical Properties of a Gas in a Steady State: 6. Temperature, pressure, etc.; 7. Mass motion, thermodynamics, calorimetry and dissociation; Part III. Mathematical Theory of a Gas not in a Steady State: 8. Boltzmann's equation, etc.; 9. Maxwell's theory; 10. The free path; Part IV. Physical Phenomena of a Gas not in a Steady State: 11. Viscosity; 12. Conduction of heat; 13. Diffusion; 14. The evidence of the kinetic theory as to the size of molecules; 15. Aerostatics and planetary atmospheres; Part V. Radiation and the Quantum Theory: 16. Statistical mechanics and the partition of energy in continuous media; 17. Radiation and the quantum theory; 18. Quantum dynamics.