The Austrian physicist Ludwig Eduard Boltzmann, 1844-1906. Boltzmann obtained his doctorate in Vienna in 1866, and held professorships during his career at Graz, Vienna, Munich and Leipzig. He extended the kinetic theory of gases, and used the mechanics and statistics of large numbers of particles to give definitions of heat and entropy (a measure of the disorder of a system). He also worked on electromagnetism and black body radiation. Throughout his life Boltzmann suffered depression. This may have been intensified by attacks on his work by the logical positivist philosophers who opposed atomistic theories of phenomena. He committed suicide at the age of 62.