In chemistry, caloric theory was a postulate that heat was composed of a type of fluid made of indestructible particles called "caloric". The fluid was conceived as being able to pass into the pores of bodies, such that the more caloric that was in the space of the pores the more the body had a tendency to expand, due to the action of heat. The caloric theory was developed in the late 18th century by French chemist Antoine Lavoisier; it supplanted the older 17th century "phlogiston theory", developed German chemist Georg Stahl, in the explanation of many aspects of heat and light. [1] The caloric theory was superseded, in a general sense, by the mechanical equivalent of heat, during the years 1798-1843, and the concept of "entropy", during the years 1850 to 1865.

The caloric theory was established in the late 18th century by French chemist Antoine Lavoisier, who between 1768 and 1787 published over sixty papers leading to his theory of combustion, in which the process of combustion resulted in the release of "caloric" particles. [2] His caloric theory of combustion found its way to the physicists of the world principally through the famous publication of this 1789 textbook Elements of Chemistry.

In 1777, Lavoisier outlined the view that in every combustion there is a disengagement of the matter of heat (or igneous fluid) or of light. [3] In 1786, he proved that the “matter of heat” is weightless by showing that phosphorous burned in air in a closed flask, with no appreciable change in weight. [4] In 1787, French chemist Guyton de Morveau, working in coordination with Lavoisier, Savoyard chemist Claude Berthollet, and French chemist Antoine de Fourcroy, named the matter of heat “calorique” (caloric). [5]

In 1789, in reference to their work in reformulating the chemical nomenclature, Lavoisier states: [6]

“We have distinguished the cause of heat, or that exquisitely elastic fluid which produces it, by the term caloric.”

The start of the downfall of caloric theory was Benjamin Thompson's 1798 cannon boring experiment; then in 1824, the teetering point of the theory was reached in the work of French engineer Sadi Carnot who used the theory in his Reflections on the Motive Power of Fire, but began to recant his views in his personal notes. Caloric theory was eventually superseded or rather upgraded to entropy theory, beginning in 1850 by German physicist Rudolf Clausius.

1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Partington, J.R. (1957). A Short History of Chemistry. Dover.
3. Lavoisier, Antoine. (1777). “On Combustion in General”, Sept. 5th. Euvres, ii, 225.
4. Lavoisier, Antoine. (1786). Euvres, ii, 618.
5. Morveau, Guyton de. (1787). Méthode de Nomenclature Chimique (Method of Chemical Nomenclature), 31.
6. Lavoisier, Antoine. (1789). Elements of Chemistry (pg. 5). London: G.G. and J.J. Robinsons.

Further reading
● Callendar, H.L. (1910). “The Caloric Theory of Heat and Carnot’s Principle”, Proc. Phys. Soc. London: 23, 153-89.

External links
Caloric theoryEric Weisstein’s World of Physics.
Lavoisier’s Caloric Fluid Theory (early attempts to understand the nature of heat), physics department, University of Virginia.

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