Josef Stefan (24.03.1835-7.01.1893) was an Austrian physicist and a member of the Austrian Academy of Sciences (1865). He was born in St Pölten. He studied at the University of Vienna, where he was lecturer from 1858, professor from 1863 and from 1869 director of the university's Institute of Experimental Physics (rector of the university in 1876 — 77).
His work was in the fields of optics, acoustics, electromagnetism, the kinetic theory of gases, hydrodynamics and the theory of thermal radiation. He developed the theory of the diffusion of gases (1871), studied their thermal conductivity, and found the values of the coefficients of thermal conductivity of many gases. In 1879 he established experimentally that the energy radiated by a heated body is proportional to the fourth power of its absolute temperature (the Stefan — Boltzmann law). This law was derived theoretically in 1884 by L. Boltzmann. He trained many Austrian physicists.