In thermodynamics, heat Q is a transfer quantity that may be defined as the energy which, exchanged by a system, has the effect of modifying its temperature (sensible heat) or inducing a change of state (latent heat). [1] Heat can also be defined as the process of energy transfer (conduction, convection, or radiation), energy in transit, or a flow of energy from one body or system to another as a result of a difference in temperature. [2] In differential terms, dQ is an amount of energy transferred as a result of an interaction between two systems differing in temperature. [3]

Matter theory of heat
The early history of theories of heat is a long and convoluted subject, beginning with Aristotle viewing heat as fire element (350BC); Geber explaining heat in terms of sulphur or the ‘stone with burns’ (c.790); Paracelsus explaining heat in terms of a mixture of Aristotle’s four element theory and Geber’s three principle theory (1524); Johann Becher, modifying Paracelsus’ theory to arrive at the concept of heat as terra pinguis (1669); Georg Stahl, a student of Becher, modifying the terra pinguis theory to arrive at the phlogiston theory of heat (1703); these all tended to revolve around what was called the "matter theory of heat".

Caloric theory of heat
The next big change came when in the late 18th century French chemist Antoine Lavoisier began to conduct heat weighting experiments during combustion reactions; the result of which is that he disproved the phlogiston theory through experiment, and replacing it with the “caloric theory” (1787).

Motion theory of heat
The next big change in thought about heat came when in 1798, American-born English physicist Benjamin Thomson (who eventually married Lavoiser's wife, after Lavoisier was guillotined) disproving Lavoisier’s caloric theory, via his famous cannon boring experiment, and followup publication “An Inquiry Concerning the Source of Heat which is Excited by Friction”, to arrive at the motion theory of heat (1798); Thomson, in turn, then appointed the like-minded English physician-physicist Thomas Young to lecture at the Royal Institute, particularly based on the fact that Young was one of the first to embrace his avant-garde idea that heat was the manifestation of atomic motion. Young in addition, however, was the first to experimentally prove the wave theory of light (c.1803), and thus he added to Thomson’s argument that heat must be a combination of motion and light. Young’s description of heat, as presented in his lectures on natural philosophy, published in 1807, is one of the most profound and modern descriptions of heat to be found, even in modern times (aside from the elastic medium part): [5]

“If heat is not a substance, it must be a quality; and this quality can only be motion. It was Newton’s opinion, that heat consists in a minute vibratory motion of the particles of bodies, and that this motion is communicated through an apparent vacuum, by the undulations of an elastic medium, which is also concerned win the phenomena of light. If the arguments which have lately been advanced, in favor of the undulatory nature of light, be deemed valid, there will be still stronger reasons for admitting his doctrine respecting heat, and it will only be necessary to suppose the vibrations and undulations principally constituting it, to be larger and stronger than those of light, while at the same time the smaller vibrations of light, and even the blackening rays [ultraviolet light], derived from still more minute vibrations, may, perhaps, when sufficiently condensed, concur in producing the effects of heat. These effects, beginning from the blackening rays, which are invisible, are a little more perceptible in the violet, which still possess but a faint power of illumination; the yellow green afford the most light; the red give less light, but much more heat, while the still larger and less frequent vibrations [infrared light], which have no effect on the sense of light, may be supposed to give rise to the least refrangible rays, and to constitute invisible heat.”

This is quite an ingenious description, to say the least. Young incorporates the experimental findings of the motion theory of heat, the experimental findings of the double-slit experiment, and the view that light is one part of the electromagnetic spectrum (a theory completed by James Maxwell in 1873, based on Young's work), to give one of the most cogent descriptions of heat ever presented, even for modern times.

Mechanical equivalent of heat
The next big idea on heat came about through the experiments of English physicist James Joule (among others) who experimentally introduced the mechanical equivalent of heat (1843);

Entropy formula of heat
In 1850, Joule's work on the mechanical equivalent of heat was taken up by German physicist Rudolf Clausius, who, over the next 15-years, combined the “motion theory of heat” with the “mechanical equivalent of heat” with the Euler reciprocity relation to arrive at the exact differential state function formulation of what he called the “equivalence value” of heat:

Equivalence value of heat =

and by 1865 this equivalence value came to be known as entropy, symbol S. This, in turn, opened the door to numerous mathematical "entropy formulation" varieties of heat, in the years and decades to follow.

Human chemistry
In 1948, American author Thomas Dreier gave the following crude description of “heat” generated in the context of human chemical reactions: [4]

“What is democracy but a successful formula for controlling the chemical reactions of our 145,000,000 people, and turning the friction and heat generated by our living together into production and progress?”

In this sense, the definition of heat, in human thermodynamics, is the same, however, the terminological transfer and the understanding of generalized state terms, such as "energy" or temperature", and conceptions such as "system", e.g. social system, or "latent heat" used in reference to human social systems is a new area of research. How does the sexual heat of reproduction, for instance, related to the definition of heat as energy in transfer? There are many who will argue that the term "heat" used in reference to human life processes is only metaphor. When human systems are defined as consisting of substrate-attached systems of human molecules, however, according to which heat from the sun falls through a temperature gradient to the body of the cold night sky and thereby drives the daily production of human work, the standard definition of heat finds clarification.

See also
● Thermal words

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. (a) Daintith, John. (2005). Oxford Dictionary of Science. Oxford University Press.
(b) Schroeder, Daniel V. (2000). An Introduction to Thermal Physics, (pg. 18). Addison Wesley Longman.
(c) Baierlein, Ralph. (1999). Thermal Physics, (pg. 21). Cambridge University Press.
3. Gyftopoulos, Elias P. and Berretta, Gian-Paolo. (2005). Thermodynamics - Foundations and Applications, (pg. 226). New York: Dover.
4. Dreier, Thomas. (1948). We Human Chemicals: the Knack of Getting Along with Everybody (pg. 86). Updegraff Press.
5. (a) Young, Thomas. (1807). Natural Philosophy. Publisher.
(b) Robinson, Andrew. (2006). The Last Man Who Knew Everything: Thomas Young, the Anonymous Genius who Proved Newton Wrong and Deciphered the Rosetta Stone, among other Surprising Feats (pg. 128). Plume Books.

Further reading
● Henry, William. (1803). “A Review of Some Experiments which have been supposed to disprove the Materiality of Heat”, Philosophical Magazine, 15:45-54.
● Kelland, Philip. (1837). Theory of Heat (182 pgs). London: John W. Parker.
● Maxwell, James C. (1872). Theory of Heat (313 pgs). London: Longmans, Green, and Co.
● Preston, Thomas. (1894). Theory of Heat (719 pgs). London: MacMillan and Co.
● Fuchs, Hans U. (1996). The Dynamics of Heat. Springer.

External links
● Heat – Wikipedia.
● History of heat – Wikipedia.