Fluid theory of heat

In science, fluid theory of heat, heat-fluid theory, "substance theory of heat" (Planck, 1949), "material theory of heat" (Caldwell, 1971), or "matter theory of heat", which can be contrasted with the "kinetic theory of heat", is a now-defunct theory, which argued that heat is a type of conserved elastic fluid, being either weightless (caloric) or with weight (e.g. phlogiston or terra pinguis), that was able to insinuate the interstices or pores of bodes causing volumetric expansion.

Massive or massless
The views on whether or not heat had mass were varied: [2]

Herman Boerhaave (c.1724) found no change in the weight of a mass of iron when heated.
Georges Leclerc (c.1774) found in increase in weight of iron when heated.
John Roebuck (1775) found an increase in weight when iron was cooled.
John Whitehurst (1776) found an increase in weight when iron was cooled.

The work of Roebuck and Whitehurst indicated that heat had “levity”, a property attributed to “element of fire” by Aristotle.

In 1786, French chemist Antoine Lavoisier proved, via experiment, 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] Lavoisier then introduced the "caloric theory of heat".

English scientist Henry Cavendish, beginning in the mid to late 18th century, did a number of experiments on heat, publishing a number of articles and papers, in which he, supposedly, rejected the fluid theory of heat, and in its place, attempted to formulate a so-called "mechanical theory of heat". [5] This, however, was not likely the name used by Cavendish to describe his theory, being that the term mechanical theory of heat was first introduced by German physicist Rudolf Clausius, in his 1850s German term mechanische wärmetheorie.

The fluid theory of heat, during the years 1798 to circa 1849, was supplanted by the “atomic motion theory of heat” (1798, Benjamin Thompson), and then eventually by the “mechanical equivalent of heat” (Robert Mayer, 1842; James Joule, 1844), then the mechanical theory of heat (Rudolf Clausius, 1850, and thereafter). [1] Into the late 19th century, the study of radiation, and radiant heat, led to the “wave theory of heat”, so to speak, and eventually to radiation thermodynamics. [2]

The fluid theory of heat was used by French physicist Sadi Carnot in 1824 when he initiated the science of thermodynamics. [3] In the decade to follow, however, Carnot, in his personal notes, recanted on his adherence to the fluid theory of heat.

The following are related quotes:

“I was led to suspect, that elastic fluids, when they were mechanically expanded, would attract or absorb heat from the bodies in their vicinity; and that, when they were mechanically condensed, the fluid matter of heat would be pressed out of them, and disused among the adjacent bodies.”
Erasmus Darwin (1788), “Frigorific Experiments on the Mechanical Expansion of Air” [6]

“One day, I happened to come across the treatises of Rudolf Clausius, whose lucid style and enlightening clarity of reasoning made an enormous impression on me, and I became deeply absorbed in his articles, with an ever increasing enthusiasm. I appreciated especially his exact formulation of the two laws of thermodynamics, and the sharp distinction which he was the first to establish between them. Up to that time, as a consequence of the theory that heat is a substance, the universally accepted view had been that the passing of heat from a higher to a lower temperature was analogous to the sinking of a weight from a higher to a lower position, and it was not easy to overcome this mistaken opinion.”
Max Planck (1949), Scientific Autobiography (pg. 16)

See also
‚óŹ Kinetic theory of heat

1. Asimov, Isaac. (1988). Understanding Physics, Volumes 1-3 (atomic motion theory of heat, pg. 235). Barnes & Noble.
2. Hankins, Thomas L. (1985). Science and the Enlightenment (wave theory of heat, pg. 80). Cambridge University Press.
3. Fermi, Enrico. (1936). Thermodynamics. Prentice-Hall.
4. Lavoisier, Antoine. (1786). Euvres, ii, 618.
5. (a) McCormmach, Russell. (2004). Speculative Truth: Henry Cavendish, Natural Philosophy, and the Rise of Modern Theoretical Science (pgs. 123-24). Oxford University Press.
(b) Cavendish, Henry. (2011). The Scientific Papers of the Honourable Henry Cavendish, F.R.S. (Cavendish Papers on Heat, pgs. 326-89). Cambridge University Press.
(c) McCormmach, Russell. (1988). “Henry Cavendish on the Theory of Heat” (abs), Isis, 79(1): 37-67.
6. (a) Erasmus, Darwin. (1788). “Frigorific Experiments on the Mechanical Expansion of Air” (abs) (pg. 43), Philosophical Transactions of the Royal Society, 78:43-42, Jan 01.
(b) Cardwell, Donald S.L. (1971). From Watt to Clausius: the Rise of Thermodynamics in the Early Industrial Age (pg. 58). Cornell University Press.

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