Editing Conservation of energy (section)

===Mechanical equivalent of heat===
A key stage in the development of the modern conservation principle was the demonstration of the ''[[mechanical equivalent of heat]]''. The [[caloric theory]] maintained that heat could neither be created nor destroyed, whereas conservation of energy entails the contrary principle that heat and mechanical work are interchangeable.

In the middle of the eighteenth century, [[Mikhail Lomonosov]], a Russian scientist, postulated his corpusculo-kinetic theory of heat, which rejected the idea of a caloric. Through the results of empirical studies, Lomonosov came to the conclusion that heat was not transferred through the particles of the caloric fluid.

In 1798, Count Rumford ([[Benjamin Thompson]]) performed measurements of the frictional heat generated in boring cannons and developed the idea that heat is a form of kinetic energy; his measurements refuted caloric theory, but were imprecise enough to leave room for doubt.

[[File:SS-joule.jpg|thumb|left|130px|[[James Prescott Joule]]]]

The mechanical [[equivalence principle]] was first stated in its modern form by the German surgeon [[Julius Robert von Mayer]] in 1842.<ref>von Mayer, J.R. (1842) "Remarks on the forces of inorganic nature" in ''Annalen der Chemie und Pharmacie'', '''43''', 233</ref> Mayer reached his conclusion on a voyage to the [[Dutch East Indies]], where he found that his patients' blood was a deeper red because they were consuming less [[oxygen]], and therefore less energy, to maintain their body temperature in the hotter climate. He discovered that [[heat]] and [[mechanical work]] were both forms of energy, and in 1845, after improving his knowledge of physics, he published a monograph that stated a quantitative relationship between them.<ref>Mayer, J.R. (1845). ''Die organische Bewegung in ihrem Zusammenhange mit dem Stoffwechsel. Ein Beitrag zur Naturkunde'', Dechsler, Heilbronn.</ref>

[[File:Joule's Apparatus (Harper's Scan).png|thumb|right|[[James Prescott Joule|Joule]]'s apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate.]]

Meanwhile, in 1843, [[James Prescott Joule]] independently discovered the mechanical equivalent in a series of experiments. In one of them, now called the "Joule apparatus", a descending weight attached to a string caused a paddle immersed in water to rotate.  He showed that the [[gravitational energy|gravitational potential energy]] lost by the weight in descending was equal to the [[internal energy]] gained by the water through [[friction]] with the paddle.

Over the period 1840–1843, similar work was carried out by engineer [[Ludwig A. Colding]], although it was little known outside his native Denmark.

Both Joule's and Mayer's work suffered from resistance and neglect but it was Joule's that eventually drew the wider recognition.

{{For|the dispute between Joule and Mayer over priority|Mechanical equivalent of heat: Priority}}

In 1844, the Welsh scientist [[William Robert Grove]] postulated a relationship between mechanics, heat, [[light]], [[electricity]], and [[magnetism]] by treating them all as manifestations of a single "force" (''energy'' in modern terms). In 1846, Grove published his theories in his book ''The Correlation of Physical Forces''.<ref>{{cite book | author=Grove, W. R. | title=The Correlation of Physical Forces | url=https://archive.org/details/correlationphys06grovgoog | location=London | publisher=Longmans, Green | year=1874 | edition=6th }}</ref> In 1847, drawing on the earlier work of Joule, [[Nicolas Léonard Sadi Carnot|Sadi Carnot]], and [[Émile Clapeyron]], [[Hermann von Helmholtz]] arrived at conclusions similar to Grove's and published his theories in his book ''Über die Erhaltung der Kraft'' (''On the Conservation of Force'', 1847).<ref>{{cite web|title= On the Conservation of Force|url=http://www.bartleby.com/30/125.html|publisher=Bartleby|access-date= 6 April 2014}}</ref> The general modern acceptance of the principle stems from this publication.

In 1850, the Scottish mathematician [[William Rankine]] first used the phrase ''the law of the conservation of energy'' for the principle.<ref>William John Macquorn Rankine (1853) "On the General Law of the Transformation of Energy," ''Proceedings of the Philosophical Society of Glasgow'', vol. 3, no. 5, pages 276-280; reprinted in: (1) ''Philosophical Magazine'', series 4, vol. 5, no. 30, [https://books.google.com/books?id=3Ov22-gFMnEC&pg=PA106 pages 106-117] (February 1853); and (2) W. J. Millar, ed., ''Miscellaneous Scientific Papers: by W. J. Macquorn Rankine'', ... (London, England: Charles Griffin and Co., 1881), part II, [https://archive.org/stream/miscellaneoussci00rank#page/203/mode/1up pages 203-208]: "The law of the ''Conservation of Energy'' is already known—viz. that the sum of all the energies of the universe, actual and potential, is unchangeable."</ref>

In 1877, [[Peter Guthrie Tait]] claimed that the principle originated with Sir Isaac Newton, based on a creative reading of propositions 40 and 41 of the ''[[Philosophiae Naturalis Principia Mathematica]]''. This is now regarded as an example of [[Whig history]].<ref>{{cite book
|title=On the shoulders of merchants: exchange and the mathematical conception of nature in early modern Europe
|first1=Richard W. |last1=Hadden |publisher=SUNY Press
|year=1994 |isbn=978-0-7914-2011-9 |page=13
|url=https://books.google.com/books?id=7IxtC4Jw1YoC}}, [https://books.google.com/books?id=7IxtC4Jw1YoC&pg=PA13 Chapter&nbsp;1, p.&nbsp;13]
</ref>