In thermodynamics, Carl von Holtzmann (c.1815-c.1875), sometimes referred to as "Karl Holtzmann", was a German school teacher and competent mathematician, who in 1845 published a pamphlet on the heat of gases and vapors titled “On the Heat and Elasticity of Gases and Vapors” in which he calculated a value for the mechanical equivalent of heat. [1] Holtzmann says that he learned of French physicist Sadi Carnot’s heat theories, as discussed in his 1824 Reflections On the Motive Power of Fire, through the 1834 publications of French physicist Émile Clapeyron. [2]

Holtzmann’s paper is significant in that it is cited on the opening pages of German physicist Rudolf Clausius’ 1850 paper “On the Motive Power of Heat”, where he states that it is incorrect for both Holtzmann and Clapeyron to assume that the “quantity of heat” (in reference to indestructible particles of caloric) is constant. Correctly, as Clausius would show over the next fifteen years, “the accomplishment of work requires not merely a change in the distribution of heat, but also an actual consumption of heat (referring to lost system heat as molecules do internal work on each other, otherwise known as entropy) and that, conversely, heat can be developed again by the expenditure of work (referring to the mechanical equivalent of heat)”. [3]

References
1. Holtzmann, Carl von. (1845). “Ueber die Wärme und Elasticität der Gase und Dämpfe” (“On the Heat and Elasticity of Gases and Vapours”), Mannheim: Taylor’s Scientific Memoirs, iv. 189; also Pogg. Ann., vol. 72a.
2. Cardwell, Donald Stephen Lowell. (1971). From Watt to Clausius: The Rise of Thermodynamics in the Early Industrial Age, (pg. 225-26). Cornell University Press.
3. 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.