Everything about The Caloric Theory totally explained
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 The "caloric theory" was superseded by the mid-19th century in favor of the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.
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.
The study of
crystals in modern
solid-state physics reflects a shadow of the abandoned caloric theory.
Lattice vibrations of crystals, which carry 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.
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 doesn't result in a loss of its ability to produce heat, and therefore no loss of
caloric. This suggested that
caloric couldn't 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 is the
kinetic energy of molecules.
Further Information
Get more info on 'Caloric Theory'.
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