Ludwig Boltzmann. Portrait of Ludwig Eduard Boltz- mann (1844-1906), Austrian physicist. Boltzmann obtained his doctorate in Vienna in 1866, and held several professorships. 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 electro- magnetism and the radiation of energy from a black body. Throughout his life Boltzmann suffered depression. This may have been intensified by attacks on his work by scientists who opposed theories which explained phenomena in terms of atoms. He committed suicide at the age of 62.