"Réflexions sur la puissance motrice du feu" (Carnot)
A work of 1824 that showed that the motive power of a heat engine depends only on the temperatures of the hot and cold bodies, not on the working substance.
The young French engineer Sadi Carnot reasoned by analogy with falling water: work is produced by the 'fall of caloric' from a high temperature to a low one, while the quantity of caloric is conserved. The work was ignored until 1834, when Emile Clapeyron restated it graphically.
Despite the false premise (caloric as a weightless conserved fluid), Carnot's principle became the basis of the second law of thermodynamics.
Why this date
exact
What it grew from
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Caloric (calorique) as a weightless conserved fluid
influenced · Confidence: high
Carnot builds his argument inside the caloric theory, assuming that the quantity of caloric is conserved while the engine works.
Dispute: Britannica notes that "Carnot himself had doubts about it even while he was writing". The posthumous notes (published by his brother Hippolyte in 1878) show that around 1830 Carnot abandoned caloric in favour of heat as motion (Mason, A History of the Sciences, 1956, quoted via Wikiquote). Chemistry World goes further and writes "He implicitly rejected the idea of caloric", but this is a minority view and contradicts the 1824 text.
Link sources
- Mendoza E., «Sadi Carnot», Britannica (2024) Verified 20 Sep 2026
He worked within the framework of the caloric theory of heat, assuming that heat was a gas that could be neither created nor destroyed.
- Carnot S. «Reflections on the Motive Power of Heat», R. H. Thurston (New York: Wiley, 1897), archive.org id reflectionsonmot00carnrich (1824)
The production of motive power is then due ... not to an actual consumption of caloric, but to its transportation from a warm body to a cold body
- de Oliveira, arXiv:2508.17027 (2025) Verified 20 Sep 2026
heat is a conserved quantity, which is the fundamental law of heat theories prevalent before the emergence of thermodynamics, called caloric theory, implicit in the Carnot theory.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Mendoza E., «Sadi Carnot», Britannica (2024) Verified 20 Sep 2026
Links under review
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The two laws of thermodynamics (Clausius 1850, Thomson 1851)
influenced · Confidence: high
The claim rests on a summary in the Wikipedia article. A verbatim quotation is needed from the work it cites: Fox R. (ed.), «Sadi Carnot: Réflexions sur la puissance motrice du feu», critical edition, 1986.
Carnot's principle (efficiency depending only on temperatures) was retained by Clausius and Thomson after caloric was abandoned and became the second law of thermodynamics.
Link sources
- Mendoza E., Britannica (2024) Verified 20 Sep 2026, quotation assembled from fragments
Though the assumption was incorrect and Carnot himself had doubts about it even while he was writing, many of his results were nevertheless true. ... Eventually Carnot's views were incorporated by the thermodynamic theory as it was developed by Rudolf Clausius in Germany (1850) and William Thomson (later Lord Kelvin) in Britain (1851).
- Daub E. E., DSB «Clausius» (1971) Verified 20 Sep 2026
rearranged the traditional argument so that any denial of the premise would require that heat be transferred from a colder to a warmer body, a conclusion which Clausius stated would be an obvious deviation from the normal behavior of heat.
- de Oliveira, arXiv:2508.17027 (2025) Verified 20 Sep 2026, quotation assembled from fragments
The Carnot principle was used by Clausius in formulating the second law of thermodynamics ... Clausius rejected part 1 and retained part 2
- Wikipedia «Reflections on the Motive Power of Fire» (2024) Verified 20 Sep 2026
Despite the fact that the caloric theory of heat was incorrect, Carnot's work brought together three insights that remain relevant
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Mendoza E., Britannica (2024) Verified 20 Sep 2026, quotation assembled from fragments
Node sources
- Mendoza E., «Sadi Carnot», Britannica (2024) Verified 20 Sep 2026
He saw that, in a steam engine, motive power is produced when heat “drops” from the higher temperature of the boiler to the lower temperature of the condenser, just as water, when falling, provides power in a waterwheel. He worked within the framework of the caloric theory of heat, assuming that heat was a gas that could be neither created nor destroyed. Though the assumption was incorrect and Carnot himself had doubts about it even while he was writing, many of his results were nevertheless true, notably the prediction that the efficiency of an idealized engine depends only on the temperature of its hottest and coldest parts and not on the substance (steam or any other fluid) that drives the mechanism. Although formally presented to the Academy of Sciences and given an excellent review in the press, the work was completely ignored until 1834, when Émile Clapeyron, a railroad engineer, quoted and extended Carnot’s results.
- Mendoza E., «Sadi Carnot», Britannica (2024) Verified 20 Sep 2026
Eventually Carnot’s views were incorporated by the thermodynamic theory as it was developed by Rudolf Clausius in Germany (1850) and William Thomson (later Lord Kelvin) in Britain (1851).
- Carnot S. «Reflections on the Motive Power of Heat», R. H. Thurston (New York: Wiley, 1897), archive.org id reflectionsonmot00carnrich (1897) Verified 5 Oct 2026, quotation assembled from fragments
The production of motive power is then due... not to an actual consumption of caloric, but to its transportation from a warm body to a cold body... to its re-establishment of equilibrium.
- Carnot S. «Reflections on the Motive Power of Heat», R. H. Thurston (New York: Wiley, 1897), archive.org id reflectionsonmot00carnrich (1897) Verified 5 Oct 2026
It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated.
- Carnot S. «Reflections on the Motive Power of Heat», R. H. Thurston (New York: Wiley, 1897), archive.org id reflectionsonmot00carnrich (1897) Verified 5 Oct 2026, quotation assembled from fragments
Wherever there exists a difference of temperature... it is possible to have also the production of impelling power.
- de Oliveira M. J. «Reflections of Sadi Carnot», arXiv:2508.17027 (2025) Verified 5 Oct 2026, quotation assembled from fragments
The Carnot theory was developed analytically by Clayperon in the paper Mémoire sur la puissance motrice de la chaleur published in 1834. In this paper, Clapeyron introduces the pressure-volume diagram to represent the states of the system. ... The Carnot principle was used by Clausius in formulating the second law of thermodynamics.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.
- Fox R. (ed.) «Sadi Carnot: Réflexions sur la puissance motrice du feu», édition critique, Vrin (1978)