The caloric theory is a superseded scientific theory that heat consists of a self-repellent fluid called "caloric" that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids. The caloric theory was superseded by the mid-19th century in favor of the mechanical equivalent of heat, but nevertheless persisted in some scientific literature—particularly in more popular treatments—until the end of the 19th century.
Caloric theory held that heat was a conserved, weightless fluid called caloric that flowed from hotter objects to colder ones.
Early history
150px|right|thumb|The world's first ice-calorimeter, used in the winter of 1782–83, by [[Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat involved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. These experiments mark the foundation of thermochemistry.]]
Phlogiston theory is replaced by combustion in oxygen
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.
There is one version of the caloric theory that was introduced by Antoine Lavoisier. Prior to Lavoisier's caloric theory, published references concerning heat and its existence, outside of being an agent for chemical reactions, were sparse only having been offered by Joseph Black in Rozier's Journal (1772) citing the melting temperature of ice. In response to Black, Lavoisier's private manuscripts revealed that he had encountered the same phenomena of a fixed melting point for ice and mentioned that he had already formulated an explanation which he had not published as of yet. Lavoisier developed the explanation of combustion in terms of oxygen in the 1770s.
Igneous fluid
On 28 June and 13 July 1783, Lavoisier read his two-part manuscript Reflections on phlogiston () at the Royal Academy of Sciences in Paris. In this paper Lavoisier argued that the phlogiston theory was inconsistent with his experimental results, and proposed a 'subtle fluid' he named “igneous fluid” as the substance of heat. Lavoisier argued that this “igneous fluid” is the cause of heat, and that its existence is necessary to explain thermal expansion and contraction.
Caloric
thumb|right|200px|1787 copy of "Méthode de Nomenclature Chimique," featuring work by [[Louis-Bernard Guyton de Morveau|Morveau.]]
Caloric vs. heat
The term “caloric” was not coined until 1787, when Louis-Bernard Guyton de Morveau used calorique in a work he co-edited with Lavoisier. The word “caloric” was first used in English in a 1788 translation of Guyton de Morveau's essay by James St. John.
In his influential 1789 textbook Traité Élémentaire de Chimie, Lavoisier clarified the concept of caloric and introduced it to a wider audience. The book defined an element as a single substance that can't be broken down by chemical analysis and from which all chemical compounds are formed. He published his discovery that fermentation produces Carbon dioxide (carbonic gas) and spirit of wine, saying that it is "more appropriately called by the Arabic word alcohol since it is formed from cider or fermented sugar as well as wine", and publishing the first chemical equation "grape must = carbonic acid + alcohol", calling this reaction "one of the most extraordinary in chemistry", noting "In these experiments, we have to assume that there is a true balance or equation between the elements of the compounds with which we start and those obtained at the end of the chemical reaction."
Lavoisier emphasized that caloric was the cause of heat and therefore could not be equated with heat, i.e. not be the cause of itself. As for a definition of heat, Lavoisier offered just a simple, dictionary-style explanation:
Caloric theory
Lavoisier was one of the first to use a calorimeter to measure the heat released during chemical reaction. Lavoisier presented the idea that caloric was a subtle fluid, obeying the common laws of matter, but attenuated to such a degree that it is capable of passing through dense matter without restraint; caloric's own material nature is evident when it is in abundance such as in the case of an explosion.
In the 1780s, Count Rumford believed that cold was a fluid, "frigoric", after the results of Pictet's experiment. Pierre Prévost argued that cold was simply a lack of caloric.
Since heat was a material substance in caloric theory, and therefore could neither be created nor destroyed, conservation of heat was a central assumption. Heat conduction was believed to have occurred as a result of the affinity between caloric and matter thus the less caloric a substance possessed, thereby being colder, attracted excess caloric from nearby atoms until a caloric, and temperature, equilibrium was reached.
Chemists of the time believed in the self-repulsion of heat particles as a fundamental force thereby making the great fluid elasticity of caloric, which does not create a repulsive force, an anomalous property which Lavoisier could not explain to his detractors.
Radiation of heat was explained by Lavoisier to be concerned with the condition of the surface of a physical body rather than the material of which it was composed. Lavoisier explained that the caloric quantity of a substance, and by extent the fluid elasticity of caloric, directly determined the state of the substance. Thus, changes in state were a central aspect of a chemical process and essential for a reaction where the substituents undergo changes in temperature. 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 became more precise.
Later developments
In 1798, Count Rumford published "An Inquiry 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.
His results were not seen as a "threat" to caloric theory at the time, as this theory was considered to be equivalent to the alternative kinetic theory. In fact, to some of his contemporaries, the results added to the understanding of caloric theory.
thumb|Joule's apparatus for measuring the [[mechanical equivalent of heat.]]
Rumford's experiment inspired the work of James Prescott Joule and others towards the middle of the 19th century. In 1850, Rudolf Clausius published a paper showing that the two theories were indeed compatible, as long as the calorists' principle of the conservation of heat was replaced by a principle of conservation of energy. Although compatible however, the theories differ significantly in their implications. In modern thermodynamics, heat is usually a transfer of kinetic energy of particles (atoms, molecules) from a hotter to a colder substance.
Although caloric theory was replaced during the nineteenth century by the mechanical theory of heat and modern thermodynamics, several mathematical methods developed within the caloric framework remained in use. In particular, equations such as Laplace's equation and Poisson's equation continued to appear in the study of steady-state heat conduction and temperature distributions.
See also
- Chemical nomenclature
- Chemical revolution
- Energeticism