Rudolf Julius Emanuel Clausius (1822-1888), German physicist and mathematician. Clausius was the first to propose the electrical dissociation of molecules in solution into ions, an idea not fully accepted until decades later. His fundamental contributions were in thermodynamics, especially the concept of entropy. He stated that within a closed system, the ratio of the heat content to the absolute temperature may increase, but will never decrease. The system may become more disordered, but never more ordered. This is now known as the second law of thermodynamics. Clausius was awarded the Royal Society's Copley Medal in 1879. Photographed after 1867.