(* January 2, 1822 ; + August 24, 1888) was a German physicist and mathematician who is considered as one of the central founders of the science of thermodynamics. On the basis of Sadi Carnot's principle known as the Carnot cycle, he put the theory of heat on a truer basis. Within his most important paper, On the mechanical theory of heat (1850), he first stated the ideas of the second law of thermodynamics. In 1865 he introduced the concept of entropy. Clausius was born in Köslin in the Province of Pomerania. He started his education at his father´s school. A few years, he entered the Gymnasium in Stettin and later graduated from the University of Berlin in 1844 where he had studied Mathematics and Physics alongside with Heinrich Magnus, Johann Dirichlet and Jakob Steiner and others. He also studied History with Leopold von Ranke. In 1847, he pursued his doctorate from the University of Halle on optical effects in the Earth's atmosphere. He then became professor of physics at the Royal Artillery and Engineering School in Berlin and a private teacher at the Berlin University. In 1855 he became professor at the ETH Zürich, the Swiss Federal Institute of Technology in Zürich, where he remained in office until 1867. In the same year, he moved to Würzburg and two years later, in 1869 to Bonn. In 1870 Clausius organized an ambulance corps in the Franco-Prussian War. He was wounded in battle, leaving him with a lasting disability. He was awarded the Iron Cross for his services. His wife, Adelheid Rimpham, died in childbirth in 1875, leaving him to raise their six children. He continued to teach, but had to cut time on research. He died in Bonn in 1888. Clausius' 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, but regardless, Clausius used a far more mathematical approach than his predecessors. His most famous paper, "Über die bewegende Kraft der Wärme" ("On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom") was published in 1850, and dealt with the mechanical theory of heat. In the paper he stated that there was a contradiction between Carnot's principle and the concept of conservation of energy. Clausius restated the two laws of thermodynamics to overcome this contradiction. The paper marked his big success. During 1857, Clausius contributed to the field of kinetic theory after refining August Krönig's very simple gas-kinetic model to include translational, rotational and vibrational molecular motions. In the same work he presented the concept of 'Mean free path' of a particle. Clausius deduced the Clausius-Clapeyron relation from thermodynamics. This relation, that characterizes the phase transition between two states of matter, e.g. solid and liquid, had originally been developed in 1834 by Émile Clapeyron. Entropy In 1865, Clausius first gave a mathematical version of the concept of entropy, and named it. He used the now abandoned unit 'Clausius' (symbol: Cl) for entropy. Clausius chose the word "entropy" because the meaning, from Greek, en+tropein, is "content transformative" or "transformation content" 1 Cl = 1 cal/°C = 4.1868 joules per kelvin (J/K)