Caloric theory: Encyclopedia - Caloric theory

Caloric theory

The caloric theory of heat is an obsolete scientific theory in thermodynamics, developed mostly during the 18th and 19th centuries.

Caloric theory - History

In the history of thermodynamics, the initial explanations of heat were thoroughly confused with explanations of combustion. After J. J. Becher and Georg Ernst Stahl introduced the phlogiston theory of combustion in the 17th century, phlogiston was thought to be the substance of heat.

The calorific theory was introduced by Antoine Lavoisier. Lavoisier had discovered the explanation of combustion in terms of oxygen in the 1770s. In his book Reflexions sur le phlogistique (1783), Lavoisier argued that phlogiston theory was inconsistent with his experimental results, and proposed caloric as the substance of heat.

Caloric theory - Theory

The theory held that changes in temperature are due to the transfer of an imponderable fluid, invisible and weightless, called caloric.

The theory originally hinged on two key assumptions:

1. That heat was a 'self-repulsive' (or 'elastic', or 'expansive') substance, while it was attracted to ordinary matter; and
2. That temperature was the density of caloric^[1]

Later calorists were also generally committed to the claim that since heat was a material substance, it could neither be created nor destroyed; i.e., that heat was conserved^[2].

Caloric theory - Successes

Quite a number of successful explanations can be, and were, made from these hypotheses alone. We can understand why a cup of tea cools at room temperature: caloric is self-repelling, and thus slowly flows from regions dense in caloric (the hot water) to regions less dense in caloric (the cooler air in the room).

We can explain the expansion of air under heat: caloric is absorbed into the molecules of air, which increases its volume. If we say a little more about what happens to caloric during this absorption phenomenon, we can explain the radiation of heat, the state changes of matter under various temperatures, and deduce nearly all of the gas laws.

Sadi Carnot developed his principle of the Carnot cycle, which as of 2005 still forms the basis of heat engine theory, solely from the caloric viewpoint.

However, one of the greatest confirmations of the caloric theory was Pierre-Simon Laplace's theoretical correction of Sir Isaac Newton’s pulse equation. Laplace, a calorist, added a constant to Newton’s equation, which we refer to today as the adiabatic index of a gas^[3]. This addition not only substantially corrected the theoretical prediction of the speed of sound, but also continued to make even more accurate predictions for almost a century afterward, even as measurements of the index became more precise.

Caloric theory - Demise

In 1798, Count Rumford published An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction, a report on his investigation of the heat produced while manufacturing cannons. He had found that boring a cannon repeatedly does not result in a loss of its ability to produce heat, and therefore no loss of caloric. This suggested that caloric could not be a conserved "substance" though the experimental uncertainties in his experiment were widely debated.

Rumford's experiment led to the work of James Prescott Joule and others in the 19th century, and a competing theory began to gain popularity, characterising heat as a kind of motion, the kinetic theory.

In 1850, Rudolf Clausius published a paper showing that the two theories were compatible, as long as the calorists' principle of the conservation of heat was replaced by a principle of conservation of energy. In this way, the caloric theory was absorbed into the annals of physics, to be replaced by modern thermodynamics, in which heat is the kinetic energy of molecules.

Caloric theory - Significance in the philosophy of science

The caloric theory has recently reemerged as a point of consideration in the debate over scientific realism in the philosophy of science. Realists tend to suggest that the success of a theory is dependent on the truth of that theory. However, the caloric theory is considered by some^[4] to be a counterexample to this claim.

Caloric theory - Notes

1. ^ Chang, 2003
2. ^ See, for example, Reflexions on the motive power of fire, by Sadi Carnot.
3. ^ Laplace, 1816
4. ^ For example, see Lauden, 1981

Categories: Obsolete scientific theories | Thermodynamics | History of science | History of chemistry | History of ideas

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