Johann Josef Loschmidt (15.03.1821—8.07.1895) was an Austrian physicist and chemist and a member of the Austrian Academy of Sciences (1870). He was born in Putschirn. He studied at the University of Prague. From 1865 he worked at the University of Vienna (from 1868 as professor).
His main work was in the field of the kinetic theory of gases, crystallography and stereochemistry. He was the first to calculate (1865) the diameter of a molecule and to determine the number of molecules of a gas contained in 1 cm3 under normal conditions (the Loschmidt number).