Bernoulli (1738) (c)
Diagram from Swiss physicist Daniel Bernoulli's Hydrodynamica, showing that pressure is the result of the average collisions of gas particles hitting the wall of the container.
In physics, kinetic theory, or "kinetic theory of gases" or "kinetic theory of heat", as opposed to the "material theory of heat" (or matter theory of heat), is a statistical model which links the temperature of a gas to the velocity of its particles.

In this kinetic theory, the particles, atoms or molecules, are considered to be spherical, to have non-correlation of velocities, and to have perfectly elastic collisions. [1] The principle founder of the kinetic theory of gases, according to Scottish physicist James Maxwell, is German physicist Rudolf Clausius. [6] Maxwell referred to the theory, which explained the properties of gases, as the "theory of the collisions of molecules". [6]

The term ‘kinetic theory of gases’, according to the 1898 view of Scottish chemist William Ramsay, concerns the attempts by Daniel Bernoulli, in 1738, to explain Boyle’s law (1662) in terms of the nature and motion of particles, followed by similar attempts by John Herapath (1821) and James Joule (1851), whose ideas were systematized in 1857 by Clausius under the name ‘Kinetic Theory of Gases.’ [9] Another pioneer of kinetic theory is Scottish civil engineer John Waterston (1843).

In 1716, Jacob Hermann, a student of Jacob Bernoulli, put forward the following view:

Heat, other things being equal, is proportional to both the density of a hot body and to the square of the agitation of its particles.”
Jacob Hermann (1716), Phoronomia [11]

In 1738, Daniel Bernoulli, in his Hydrodynamica, possibly being influenced by either his father which laid the basis for the kinetic theory of gases. This publication, in which Bernoulli’s gave an explanation of Boyle’s law, is said to have marked the initiation of the kinetic theory of gases. [7] In this work, Bernoulli positioned the argument, still used to this day, that gases consist of great numbers of molecules moving in all directions, that their impact on a surface causes the gas pressure that we feel, and that what we experience as heat is simply the kinetic energy of their motion. [2] Bernoulli also stated that: [8]

“The weight P [weight of the mass + atmospheric pressure] will vary as the square of the particle velocity ().

In 1856, August Kronig, German chemist and physicist, published his "A General Theory of Gases", which outlined a simple gas-kinetic model that considered the translational motion of the particles. [3] In 1857, German physicist Rudolf Clausius published his “On the Nature of the Motion which we call Heat”, in which, according to his own words independently of Krönig, developed a similar, but much more sophisticated version of the theory which included translational and contrary to Krönig also rotational and vibrational molecular motions. In this same work he introduced the concept of mean free path of a particle. [4]

In 1859, after reading Clausius' paper, James Maxwell formulated what is known as the "Maxwell distribution" of molecular velocities, which gave the proportion of molecules having a certain velocity in a specific range. [5]

Human molecules
William Ramsay, in his 1898 “The Kinetic Theory of Gases and Some of its Consequences”, likened gas molecules to football players.

In 1991, Chinese writer Ben She.Yi Ming, associated with Beijing Science and Technology, published a book entitled Kinetic Theory of Human Molecules; though it remains to be seen as to what its contents are. [10]

1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Laidler, Keith. (1993). The World of Physical Chemistry, (142-60). New York: Oxford University Press.
3. Krönig, A. (1856), "Grundzüge einer Theorie der Gase" (A General Theory of Gases), Annalen der Physik 99: 315-322.
4. Clausius, R. (1857), "Über die Art der Bewegung, die wir Wärme nennen" (About the Nature of the Movement, Which we call Heat), Annalen der Physik 100: 353-379.
5. Mahon, Basil (2003). The Man Who Changed Everything – the Life of James Clerk Maxwell. Hoboken, NJ: Wiley.
6. (a) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
(b) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (II)”, (pgs. 278-81). Nature, Feb. 07.
7. Wilson, William. (1950). A Hundred Years of Physics (pg. 62). Read Books.
(a) Bernoulli, Daniel. (1738). “On the Properties and Motions of Elastic Fluids, Especially Air” (Hydrodynamica, Section 10) in: The Kinetic Theory of Gases of Gases (pgs. 57-65), 2003, by Stephen G. Brush, Nancy S. Hall. Imperial College Press.
(b) Bernoulli, Daniel. (1738). Hydrodynamica, Sive Vivibus et Motimus Fluidorum Commentarii. Sectio Decima: “De affectionibus atque botimus fluidorum elasticorum, praecipue autem aeris.” (pgs. 200-204). Argentorati, Sumptibus Johannes Reinholdi Dulseckeri.
9. Ramsay, William. (1898). “The Kinetic Theory of Gases and Some of its Consequences”, Science, 8: 768-76.
10. Anon. (1991). Kinetic Theory of Human Molecules (Amz). Beijing Science and Technology.
11. (a) Talbot, G.R. and Pacey, A.J. (1966). “Some Early Kinetic theories of Gases: Herapath and his Predecessors”, British Journal for the History of Science, 3:133.
(b) Cardwell, Donald S.L. (1971). From Watt to Clausius: the Rise of Thermodynamics in the Early Industrial Age (pg. 24). Cornell University Press.

Further reading
● Brush, Stephen. (1965). Kinetic Theory: the Nature of Gases and of Heat: Volume One. Elsevier, 2016.

External links
Kinetic theory – Wikipedia.

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