Caloric theory

Caloric theory treated heat as an invisible, weightless fluid that flowed from warmer bodies to cooler ones. It was displaced by the view that heat is energy associated with microscopic motion.

Caloric: A hypothetical, weightless fluid once proposed to account for heat and temperature changes. This imagined substance was the central ingredient in the theory’s account of heat.
Antoine Lavoisier: French chemist who helped establish modern chemical nomenclature and advanced the conservation of mass. He included caloric among the simple substances in his chemical framework.
Kinetic theory of heat: The view that heat arises from the motion of particles making up matter. It explains heating through motion rather than transfer of a weightless fluid.
Carnot cycle: An ideal reversible cycle that sets the maximum efficiency of a heat engine operating between two temperatures. Carnot derived its efficiency using caloric flow between hot and cold reservoirs.
Heat capacity: The amount of heat required to raise a substance’s temperature by a specified amount. Caloric theory described differing heat capacities as differing capacities to hold caloric.
Pierre-Simon Laplace: French mathematician and physicist whose work shaped celestial mechanics and mathematical physics. He developed caloric explanations of heat alongside Lavoisier.
Mechanical equivalent of heat: The quantitative relationship between mechanical work and the amount of heat produced. Joule’s measurements linked work and heat in ways caloric conservation could not explain.
Carnot's theorem: The result that no engine operating between two fixed temperatures can outperform a reversible engine. The theorem survived the abandonment of the caloric interpretation behind Carnot’s reasoning.
Latent heat: Energy absorbed or released during a phase change without changing temperature. The theory treated latent heat as caloric absorbed or released during changes of state.
Sadi Carnot: French engineer whose analysis of heat engines founded a key part of thermodynamics. His 1824 engine analysis used caloric flow to derive limits on efficiency.

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