In the history of thermodynamics, Sadi Carnot (1796-1832) was a French engineer who founded the science of thermodynamics with the self-published 1824 treatise Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power a short booklet on the generalized energetics of heat engines.

Overview
Sadi was born June 1796 in Paris to French mathematical engineer Lazare Carnot and Sophie Dupont, the daughter of a well-off family of Saint-Omer. In August of 1807, Lazare decides to take care of the education of his two sons, Sadi and younger brother Hippolyte, and teaches them math, science, languages, and music. In 1812, Sadi entered the École Polytechnique, graduating two years later.

In 1824, Sadi published his treatise Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power. In this work, Carnot's overall aim, stimulated by the earlier work of his father, on the universal nature of geometric operation and force constraints of machines, was to outline the general principles and laws of the phenomenon of the production of motion by heat not only to steam engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated. Terms or concepts introduced by Carnot include: working substance, motive power, "re-establishment of equilibrium in the caloric", Carnot engine, Carnot cycle, thermodynamic efficiency, among others.

In 1834, Carnot's Reflections was put on a graphical footing by French physicist Émile Clapeyron. [2] It was through Clapeyron that Carnot's theories were introduced to German physicist Rudolf Clausius who, beginning in 1850, was thus stimulated to spend 15-years writing the monumental Mechanical Theory of Heat, through which "entropy" and the second law of thermodynamics were introduced.

The key point of Carnot's work that goaded Clausius to such an effect was Carnot's postulate, based on the faulty caloric theory, that "no change occurs in the condition of the working body" in a complete heat engine cycle. Clausius sought to remidy this statement by introducing a new variable to science dQ/T called "entropy" that in any heat engine process (cycle), which is in any way possible, increases, meaning that energy is used by the molecules of the working body on each other as they change their physical condition of state or configuration during each step of an expansion or contraction due to the action of a heat gradient.

Essence of thermodynamics
The paragraph that interested Clausius to such an effect, via his reading of Clapeyron's 1834 paper, in reference to the process in which work is done by heat through the medium of a working body in alternate contact with a heat gradient, is Carnot’s postulate that (as quoted by Clausius): [4]

“No heat is lost in the process, but that the quantity of heat remains unchained … this fact is not doubted; it was assumed at first without investigation, and then established in many cases by calorimetric measurements. To deny this would overthrow the whole theory of heat, of which it is the foundation.”

This paragraph is the core of the science of thermodynamics. The term "quantity of heat" here refers to French physicist Antoine Lavoisier's 1787 theory of particles of caloric, which were considered to be indestructible entities responsible for the expansion of bodies by the action of heat. [5]

References
1. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Clapeyron, Emile. (1834). “Memoir on the Motive Power of Heat”, Journal de l’Ecole Polytechnique. XIV, 153 (and Poggendorff's Annalender Physick, LIX, [1843] 446, 566).
3. Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
4. Clausius, Rudolf. (1850). "On the Motive Power of Heat, and on the Laws Which can be Deduced From it for the Theory of Heat." Poggendorff's Annalen der Physik, LXXIX, 368, 500.
5. Lavoisier, Antoine. (1787). Elements of Chemistry. (English ed.) Edinburgh: G.G. and J.J. Robinsons.