Portrait of Rudolf Julius Emmanuel Clausius (1822- 1888), German physicist. Clausius was primarily a theoretical physicist, using mathematics rather than experiments. He was the first to propose the electrical dissociation of molecules in solution into ions, an idea not fully accepted for 100 years. His most important theory was that of entropy. In this, Clausius suggested that within a closed system, the ratio of the heat content to the absolute temperature may increase, but will never decrease. Put in other words, the system may become more disordered, but never more ordered. This idea has become known as the second law of thermodynamics.