The magisterial book, The Mechanical Theory of Heat (1867, 1879), by Rudolph Clausius[aut]Clausius, Rudolph (1822-1888) is discussed in detail. In the Introduction (12.2) ClausiusClausius’ dicta and principles presents a clear summary of the basic concepts of classical mechanics. In Chapter 1 (12.3), The concepts of work and heat are developed in complete detail. In Chapter 2 (12.4) the differential relationships between heat, work and the variables of state (P,V,T) are developed in complete detail. In Chapters 3 and 4 (12.5), the principles of the Thermodynamics of cyclic processes are presented. In Chapter 5 (12.6), Clausius presents a “master class” in the Thermodynamics of many variables and analyzes the Clapeyron[aut]Clapeyron, Émile (1799-1864) Equation and the Carnot functionCarnot function. In Chapter 6 (12.7), Clausius analyzes the Thermodynamics of a system composed of liquid and its vapor. In Chapter 7 (12.8), Clausius treats a solid and its corresponding liquid. In Chapter 8 (12.9) Clausius considers the Thermodynamics of Natural Rubber. In Chapter 9 (12.10) Clausius develops the key equations for the energy and the entropyEntropy. In Chapter 10 (12.11) Clausius develops the Thermodynamics of Non-equilibrium processes. In Chapter 12 (12.12) Clausius discusses the Thermodynamics of Radiant HeatRadiant heat. In Chapter 13 (12.13) A few concluding remarks are discussed.