Editing Rudolf Clausius (section)

==Work==
Clausius's PhD thesis concerning the refraction of light proposed that we see a blue sky during the day, and various shades of red at sunrise and sunset (among other phenomena) due to reflection and refraction of light. Later, [[Lord Rayleigh]] would show that it was in fact due to the scattering of light.

His most famous paper, ''Ueber<!--[sic]--> die bewegende Kraft der Wärme'' ("On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom")<ref>{{cite journal  | last=Clausius |first=R.  | title =Ueber die bewegende Kraft der Wärme und die Gesetze, welche sich daraus für die Wärmelehre selbst ableiten lassen |url=http://gallica.bnf.fr/ark:/12148/bpt6k15164w/f518.table  | journal =Annalen der Physik  | volume =79 |issue=4  | pages=368–397, 500–524| year =1850 | doi=10.1002/andp.18501550403|bibcode = 1850AnP...155..500C |hdl=2027/uc1.$b242250 | hdl-access=free}}. See English Translation: [https://archive.org/details/londonedinburghd02lond On the Moving Force of Heat, and the Laws regarding the Nature of Heat itself which are deducible therefrom]. Phil. Mag. (1851), series 4, '''2''', 1–21, 102–119.  Also available on [https://books.google.com/books?id=JbwdWbbM1KgC&pg=RA1-PA1 Google Books].</ref>
was published in 1850, and dealt with the mechanical theory of heat. In this paper, he showed there was a contradiction between [[Nicolas Léonard Sadi Carnot|Carnot]]'s principle and the concept of [[conservation of energy]]. Clausius restated the two [[laws of thermodynamics]] to overcome this contradiction. This paper made him famous among scientists. (The [[third law of thermodynamics|third law]] was developed by [[Walther Nernst]], during the years 1906–1912).

Clausius's most famous statement of the second law of thermodynamics was published in German in 1854,<ref>{{cite journal|last=Clausius|first=R.|title=Ueber eine veränderte Form des zweiten Hauptsatzes der mechanischen Wärmetheoriein|journal=[[Annalen der Physik|Annalen der Physik und Chemie]]|year=1854|volume=93|issue=12|pages=481–506|url=http://gallica.bnf.fr/ark:/12148/bpt6k15178k/f499.image|access-date=25 June 2012|doi=10.1002/andp.18541691202|bibcode=1854AnP...169..481C}}</ref> and in English in 1856.<ref>{{cite journal|last=Clausius|first=R.|title=On a Modified Form of the Second Fundamental Theorem in the Mechanical Theory of Heat|journal=[[Philosophical Magazine|Phil. Mag.]]|date=August 1856|volume=12|series=4|issue=77|pages=81–98|url=https://archive.org/stream/londonedinburghd12lond#page/80/mode/2up|access-date=25 June 2012|doi=10.1080/14786445608642141}}</ref> 
{{blockquote | Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.}}

During 1857, Clausius contributed to the field of [[kinetic theory of gases|kinetic theory]] after refining [[August Krönig]]'s very simple gas-kinetic model to include translational, rotational and vibrational molecular motions. In this same work he introduced the concept of '[[Mean free path]]' of a particle.<ref>{{Citation  | author=Clausius, R.  | title =Über die Art der Bewegung, die wir Wärme nennen   | url =http://gallica.bnf.fr/ark:/12148/bpt6k15185v/f371.table  | journal =Annalen der Physik  | volume =100 | issue =3   | pages =353–379 | year =1857|doi=10.1002/andp.18571760302 |bibcode = 1857AnP...176..353C }}</ref><ref>{{Citation  | author=Clausius, R.  | title =Ueber<!--[sic]--> die Wärmeleitung gasförmiger Körper   | url =http://gallica.bnf.fr/ark:/12148/bpt6k15200x/f13.table  | journal =Annalen der Physik  | volume =115 | issue =1 | pages =1–57 | year =1862 |doi=10.1002/andp.18621910102 |bibcode = 1862AnP...191....1C }}</ref><ref>Clausius, R. (1864), [http://gallica.bnf.fr/notice?N=FRBNF37278069 ''Abhandlungen über die Mechanische Wärmetheorie'']. Electronic manuscript from the [[Bibliothèque nationale de France]].</ref>

Clausius deduced the [[Clausius–Clapeyron relation]] from thermodynamics. This relation, which is a way of characterizing the [[phase transition]] between two states of matter such as [[solid]] and [[liquid]], had originally been developed in 1834 by [[Émile Clapeyron]].<gallery>
File:Clausius-1-1.jpg|1864 edition of Clausius' ''Abhandlungen über die mechanische Wärmetheorie'', volume I
File:Clausius-1-2.jpg|Title page of an 1864 edition of Clausius' ''Abhandlungen über die mechanische Wärmetheorie'', volume I
File:Clausius-2-1.jpg|1879 English translation of Clausius' ''The Mechanical Theory of Heat''
File:Clausius-2-2.jpg|Title page of an 1879 English translation of Clausius' ''The Mechanical Theory of Heat''
</gallery>

===Entropy===
{{Main|History of entropy}}{{See also|History of thermodynamics#Rudolf Clausius}}
In 1865, Clausius gave the first mathematical version of the concept of [[entropy]], and also gave it its name.<ref name="Cropper2004"/>  Clausius chose the word because the meaning (from [[Ancient Greek|Greek]] ἐν ''en'' "in" and τροπή ''tropē'' "transformation") is "''content transformative''" or "''transformation content''" ("''Verwandlungsinhalt''").<ref name="Theory of Heat">{{cite book|last=Clausius|first=R.|title=The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies|year=1867|publisher=John van Voorst|location=London|url=https://archive.org/details/mechanicaltheor04claugoog|quote=editions:PwR_Sbkwa8IC.|access-date=19 June 2012}} Contains English translations of many of his other works.</ref><ref>{{Citation  | author=Clausius, R.  | title =Ueber verschiedene für die Anwendung bequeme Formen der Hauptgleichungen der mechanischen Wärmetheorie | url =http://gallica.bnf.fr/ark:/12148/bpt6k152107/f369.table  | journal =Annalen der Physik  | volume =125 | issue =7 | pages =353–400 | year =1865 |doi=10.1002/andp.18652010702|bibcode = 1865AnP...201..353C }}</ref><ref name="Cooper">{{cite book|last1=Cooper|first1=Leon N.|title=An Introduction to the Meaning and Structure of Physics|date=1968|publisher=Harper|page=331}}</ref>{{Blockquote|text=I prefer going to the ancient languages for the names of important scientific quantities, so that they may mean the same thing in all living tongues. I propose, accordingly, to call S the entropy of a body, after the Greek word 'transformation'. I have designedly coined the word entropy to be similar to 'energy', for these two quantities are so analogous in their physical significance, that an analogy of denomination seemed to me helpful.|author=Rudolf Clausius|title=Ueber verschiedene für die Anwendung bequeme Formen der Hauptgleichungen der mechanischen Wärmetheorie}}He used the now abandoned unit 'Clausius' (symbol: '''Cl''') for entropy.<ref>{{Cite journal|last1=Huang|first1=Mei-Ling|last2=Hung|first2=Yung-Hsiang|last3=Chen|first3=Wei-Yu|date=2010-10-01|title=Neural Network Classifier with Entropy Based Feature Selection on Breast Cancer Diagnosis|journal=Journal of Medical Systems|language=en|volume=34|issue=5|pages=865–873|doi=10.1007/s10916-009-9301-x|pmid=20703622|s2cid=6658005|issn=1573-689X}}</ref>
:1 Clausius (Cl) = 1 [[calorie]]/[[degree Celsius]] (cal/°C) = 4.1868 [[joule]]s per [[kelvin]] (J/K)

The landmark 1865 paper in which he introduced the concept of entropy ends with the following summary of the first and second laws of thermodynamics:<ref name="Theory of Heat" />
{{blockquote |The energy of the universe is constant.<br />
The entropy of the universe tends to a maximum.}}

[[Leon Cooper]]<ref name="Cooper" /> added that in this way he succeeded in coining a word that meant the same thing to everybody: nothing.