Clausius, Rudolf Julius Emmanuel 1822-88
German physicist

Born in K�slin, Prussia, he studied at the University of Berlin (1840-44), where his predominant interest changed from history to science. After receiving a PhD (Halle, 1847), he taught physics at the Royal Artillery and Engineering School, Berlin (1850), the Zurich Polytechnicum (1855), W�rzburg University (1867), and Bonn University (1869). In 1850 he postulated that heat cannot of itself pass from a colder body to a hotter one (the second law of thermodynamics) in order to validate Sadi Carnot's theorem of perfect engines whilst rejecting the caloric theory. After considering the dissipation of energy which Lord Kelvin had suggested in 1852, he introduced the term 'entropy' (1865) in such a way that dissipation was equivalent to entropy increase, thus enabling the two laws of thermodynamics to be stated succinctly. He studied electrolysis, calculated the mean speed of gas molecules, ignoring collisions (1857), and introduced the concepts of mean free path and effective radius (1858).