ain article: 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. Clausius chose the word because the meaning (from Greek ?ν en "in" and τροπ? trop? "transformation") is "content transformative" or "transformation content" ("Verwandlungsinhalt"). 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. —?Rudolf Clausius, Ueber verschiedene für die Anwendung bequeme Formen der Hauptgleichungen der mechanischen Wärmetheorie He used the now abandoned unit 'Clausius' (symbol: Cl) for entropy. 1 Clausius (Cl) = 1 calorie/degree Celsius (cal/°C) = 4.1868 joules 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: The energy of the universe is constant. The entropy of the universe tends to a maximum. Leon Cooper added that in this way he succeeded in coining a word that meant the sam