Editing History of physics (section)

===Laws of thermodynamics===
{{further|History of thermodynamics}}

[[File:Baron Kelvin 1906.jpg|thumb|upright|{{nowrap|[[William Thomson, 1st Baron Kelvin|William Thomson (Lord Kelvin)]]<br>(1824–1907)}}]]

In the 19th century, the connection between heat and mechanical energy was established quantitatively by [[Julius Robert von Mayer]] and [[James Prescott Joule]], who measured the mechanical equivalent of heat in the 1840s. In 1849, Joule published results from his series of experiments (including the paddlewheel experiment) which show that heat is a form of energy, a fact that was accepted in the 1850s. The relation between heat and energy was important for the development of steam engines, and in 1824 the experimental and theoretical work of [[Nicolas Léonard Sadi Carnot|Sadi Carnot]] was published. Carnot captured some of the ideas of thermodynamics in his discussion of the efficiency of an idealized engine. Sadi Carnot's work provided a basis for the formulation of the [[first law of thermodynamics]]&nbsp;– a restatement of the [[law of conservation of energy]]&nbsp;– which was stated around 1850 by [[William Thomson, 1st Baron Kelvin|William Thomson]], later known as Lord Kelvin, and [[Rudolf Clausius]]. Lord Kelvin, who had extended the concept of absolute zero from gases to all substances in 1848, drew upon the engineering theory of [[Lazare Carnot]], Sadi Carnot, and [[Émile Clapeyron]] as well as the experimentation of James Prescott Joule on the interchangeability of mechanical, chemical, thermal, and electrical forms of work to formulate the first law.
[[File:Rudolf Clausius 01.jpg|thumb|upright|{{nowrap|[[Rudolf Clausius]] (1822–1888)}}]]
Kelvin and Clausius also stated the [[second law of thermodynamics]], which was originally formulated in terms of the fact that heat does not spontaneously flow from a colder body to a warmer one. Other formulations followed quickly (for example, the second law was expounded in Thomson and [[Peter Guthrie Tait]]'s influential work ''Treatise on Natural Philosophy'') and Kelvin in particular understood some of the law's general implications. The second Law – the idea that gases consist of molecules in motion – had been discussed in some detail by Daniel Bernoulli in 1738, but had fallen out of favor, and was revived by Clausius in 1857. In 1850, [[Hippolyte Fizeau]] and [[Léon Foucault]] measured the [[speed of light]] in water and found that it is slower than in air, in support of the wave model of light. In 1852, Joule and Thomson demonstrated that a rapidly expanding gas cools, later named the [[Joule–Thomson effect]] or Joule–Kelvin effect. [[Hermann von Helmholtz]] put forward the idea of the [[heat death of the universe]] in 1854, the same year that Clausius established the importance of ''dQ/T'' ([[Clausius's theorem]]) (though he did not yet name the quantity).