The caloric theory is an obsolete 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 Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

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 The "caloric theory" was superseded by the mid-19th century in favor of the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 theory of heat The history of thermodynamics is a fundamental strand in the history of physics, the history of chemistry, and the history of science in general. Owing to the relevance of thermodynamics in much of science and technology, its history is finely wov ...

, but nevertheless persisted in some scientific literature—particularly in more popular treatments—until the end of the 19th century.

Early history

In the

history of thermodynamics The history of thermodynamics is a fundamental strand in the history of physics, the history of chemistry, and the history of science in general. Owing to the relevance of thermodynamics in much of science and technology, its history is finel ...

, the initial explanations of heat were thoroughly confused with explanations of

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

. After

J. J. Becher Johann Joachim Becher (; 6 May 1635 – October 1682) was a German physician, alchemist, precursor of chemistry, scholar and adventurer, best known for his development of the phlogiston theory of combustion, and his advancement of Austrian camera ...

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 Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

Joseph Black Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of Glas ...

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. In his paper "Réflexions sur le phlogistique" (1783), Lavoisier argued that

phlogiston theory The phlogiston theory is a superseded scientific theory that postulated the existence of a fire-like element called phlogiston () contained within combustible bodies and released during combustion. The name comes from the Ancient Greek (''bur ...

was inconsistent with his experimental results, and proposed a 'subtle fluid' called caloric as the ''substance of heat''. According to this theory, the quantity of this substance is constant throughout the universe, and it flows from warmer to colder bodies. Indeed, Lavoisier was one of the first to use a

calorimeter A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimete ...

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, some believed that cold was a fluid, "frigoric". 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 described a poor radiator to be a substance with a polished or smooth surface as it possessed its molecules lying in a plane closely bound together thus creating a surface layer of caloric which insulated the release of the rest within. He described a great radiator to be a substance with a rough surface as only a small amount of molecules held caloric in within a given plane allowing for greater escape from within. Count Rumford would later cite this explanation of caloric movement as insufficient to explain the radiation of cold becoming a point of contention for the theory as a whole. The introduction of the caloric theory was influenced by the experiments of

related to the thermal properties of materials. Besides the caloric theory, another theory existed in the late eighteenth century that could explain the phenomenon of heat: the kinetic theory. The two theories were considered to be equivalent at the time, but kinetic theory was the more modern one, as it used a few ideas from

atomic theory Atomic theory is the scientific theory that matter is composed of particles called atoms. Atomic theory traces its origins to an ancient philosophical tradition known as atomism. According to this idea, if one were to take a lump of matter ...

and could explain both combustion and calorimetry. Caloric theory's inability to explain evaporation and sublimation further led to the rise of kinetic theory through the work of Count Rumford. Count Rumford observed solid mercury's tendency to melt under atmospheric conditions and thus proposed that the intensity of heat itself must stem from particle motion for such an event to occur where great heat was not expected to be.

Successes

Quite a number of successful explanations can be, and were, made from these hypotheses alone. We can explain the cooling of a cup of tea in 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 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 law The gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between pressure, volume and temperature of a sample of gas could be obtained which would hold to approximation for all gases. Boyl ...

s. Sadi Carnot, who reasoned purely on the basis of the caloric theory, developed his principle of the

Carnot cycle A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodyna ...

, which still forms the basis of

heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...

theory. Carnot's analysis of energy flow in steam engines (1824) marks the beginning of ideas which led thirty years later to the recognition of the second law of thermodynamics. Caloric was believed to be capable of entering chemical reactions as a substituent inciting corresponding changes in the matter states of other substances. 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. Changes of state had gone virtually ignored by previous chemists making the caloric theory the inception point for this class of phenomena as a subject of interest under scientific inquiry. However, one of the greatest apparent confirmations of the caloric theory was

Pierre-Simon Laplace Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized ...

's theoretical correction of Sir Isaac Newton’s calculation of the speed of sound. Newton had assumed an isothermal process, while Laplace, a calorist, treated it as adiabatic. 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 Sir Benjamin Thompson, Count Rumford, FRS (german: Reichsgraf von Rumford; March 26, 1753August 21, 1814) was an American-born British physicist and inventor whose challenges to established physical theory were part of the 19th-century revolut ...

published ''

An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction ''An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction'' is a scientific paper by Benjamin Thompson, Count Rumford, which was published in the Philosophical Transactions of the Royal Society in 1798. The paper ...

'', a report on his investigation of the heat produced while manufacturing

cannon A cannon is a large-caliber gun classified as a type of artillery, which usually launches a projectile using explosive chemical propellant. Gunpowder ("black powder") was the primary propellant before the invention of smokeless powder during ...

s. 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.See for example Lavoisier, A.-L. de (1783). ''Mémoire sur la chaleur, lu à l'Académie royale des sciences, le 28 juin 1783, par MM. Lavoisier et de La Place''. In fact, to some of his contemporaries, the results added to the understanding of caloric theory. Joule's Apparatus (Harper's Scan)

Rumford's experiment inspired the work of

James Prescott Joule James Prescott Joule (; 24 December 1818 11 October 1889) was an English physicist, mathematician and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). Th ...

and others towards the middle of the 19th century. In 1850,

Rudolf Clausius Rudolf Julius Emanuel Clausius (; 2 January 1822 – 24 August 1888) was a German physicist and mathematician and is considered one of the central founding fathers of the science of thermodynamics. By his restatement of Sadi Carnot's princip ...

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 In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means that ...

. In this way, the caloric theory was absorbed into the annals of physics, and evolved into modern thermodynamics, in which heat is usually transfer of kinetic energy of particles (atoms, molecules) from a hotter to a colder substance. In later combination with the law of energy conservation, the caloric theory still provides valuable physical insight into some aspects of heat, for example, the emergence of

Laplace's equation In mathematics and physics, Laplace's equation is a second-order partial differential equation named after Pierre-Simon Laplace, who first studied its properties. This is often written as \nabla^2\! f = 0 or \Delta f = 0, where \Delta = \na ...

and

Poisson's equation Poisson's equation is an elliptic partial differential equation of broad utility in theoretical physics. For example, the solution to Poisson's equation is the potential field caused by a given electric charge or mass density distribution; with th ...

in the problems of spatial distribution of heat and temperature.

Notes

References

* * * {{DEFAULTSORT:Caloric Theory Obsolete theories in physics History of chemistry History of thermodynamics