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Caloric theory
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The caloric theory is an obsolete scientific theory that heat consists of a fluid called caloric that flows from hotter 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 The "caloric theory" was superseded by the mid-19th century in favor of the theory of heat but nevertheless persisted in scientific literature until the end of the 19th century.
he history of thermodynamics, the initial explanations of heat were thoroughly confused with explanations of combustion.
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The caloric theory is an obsolete scientific theory that heat consists of a fluid called caloric that flows from hotter 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 The "caloric theory" was superseded by the mid-19th century in favor of the theory of heat but nevertheless persisted in scientific literature until the end of the 19th century.
Early 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 caloric theory was introduced by Antoine Lavoisier. Lavoisier had discovered the explanation of combustion in terms of oxygen in the 1770s. In his paper "Réflexions sur le phlogistique" (1783), Lavoisier argued that phlogiston theory 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 to measure the heat changes during chemical reactions, for example.
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.
The introduction of the Caloric theory was also influenced by the experiments of Joseph Black related to the thermal properties of materials. Besides the caloric theory, another theory existed in the late eighteenth century that could explain the phenomena of heat: the kinetic theory. The two theories were considered to be equivalent at the time, but caloric theory was the more modern one, as it used a few ideas from atomic theory and could explain both combustion and calorimetry.
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 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. 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.
Original caloric theory worked with an image of a continuous fluid. In quantum mechanical context, some aspects of discreteness extend the picture. For example, in the study of crystals in modern solid-state physics. Lattice vibrations of crystals, which are formally equivalent (but not identical) to some thermal energy, are quantized, and consequently have wave-particle duality. The particle representation of a lattice vibration is called a phonon, by analogy with the photon. Note that the concepts of heat and mechanical vibration, which are often confused due to their equivalence, show fundamental differences in spectroscopy and other physical phenomena.
Later developments
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.
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.
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. In this way, the caloric theory was absorbed into the annals of physics, and evolved into modern thermodynamics, in which heat may formally be put equivalent to kinetic energy of some particles (atoms, molecules) of the substance. However, there is a principal difference between the concept of heat and a mechanical movement of particles, which shows in spectroscopy. While sharp spectral lines correspond to mechanical movements of the particles, the heat shows spectrosopically as a noise with some spectral distribution.
In later combination with the law of energy conservation, the caloric theory still shows a very valuable physical insight into some aspects of heat. For example, the emergence of Laplace's equation and Poisson's equation in the problems of spatial distribution of heat and temperature. The caloric theory is now also remembered for the naming of a unit of heat, the calorie.
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