Rudolf Julius Emmanuel Clausius (1822-1888), German mathematical physicist who made seminal contributions to thermodynamics and the kinetic theory of gases. His paper Über die bewegende Kraft der Wärme (The Mechanical Theory of Heat) read to the Berlin Academy in 1850, marks the foundation of modern thermodynamics. In his paper he first states the basic idea of the Second Law of Thermodynamics: for a 'Carnot cycle', that transmits heat between two heat reservoirs at different temperatures and at the same time converts heat into work, the maximum work obtained from a given amount of heat depends solely upon the temperatures of the heat reservoirs and not upon the nature of the working substance. This disproved the prevailing theory of heat at the time that had been developed by Laplace, Poisson, Sadi Carnot, and Clapeyron, i.e., that the heat in the universe is conserved and that heat in a substance is a function of the state of the substance.
In his 1865 paper, Clausius stated the First and Second Laws of Thermodynamics in the following form: (1) The energy of the universe is constant; (2) The entropy of the universe tends to a maximum. The First and Second Laws are governing principles in the field still today and are important in areas such as determining the maximum efficiency possible for energy conversion systems (such as the use of a steam cycle to generate electricity).
With Maxwell, he also developed the kinetic theory of gases, showing that molecules are very small relative to the distance between molecules, and that molecules are in constant, random motion and frequently collide with each other and with the walls of any container.
Clausius also applied his research of heat, electricity, and molecular physics to the development of a theory of electrolysis wherein he states that electric forces are merely directing agents in the interchange of ions. This view found little favor until 1887, when it was taken up by Svente Arrhenius who who made it the basis of the theory of electrolytic dissociation.
Clausius was a German patriot and, although he was nearing 50 years of age, in 1870 he offered his services to his country in the Franco-German War. He undertook the leadership of an ambulance corps, which he formed of Bonn students, that played a vital role in the great battles of Vionville and Gravelotte. Clausius received the Iron Cross in 1871 for his services to the German campaign.
He was elected a Fellow of the Royal Society of London in 1868 and received its Copley Medal in 1879. He also received the Huygens Medal in 1870, the Poncelet Prize in 1883, and an honorary doctorate from the University of Würzburg in 1882.