Johann Josef Loschmidt (Carlsbad, March 15, 1821 – Vienna, July 8, 1895), while relatively little-known today, made major contributions to physical chemistry, thermodynamics, electromagnetism and organic chemistry. His work on the size of molecules was sufficiently important that in German-speaking countries, "Loschmidt's number", a term proposed by Ludwig Boltzmann in 1899, is the term for what English-speaking countries call Avogadro's number. 
Loschmidt was born in Carlsbad, then part of the Austrian empire, now Karlovy Vary in the Czech Republic; he died in Vienna, where he was Professor emiritus of Physics at the University of Vienna.
Loschmidt, to historians of science, is a man whose name should have been associated with more basic ideas, at least as coauthor. He conceived the concept of a benzene ring before Friedrich Kekulé, but did not have the chemical bonding quite right. His contributions to thermodynamics led to the work by James Maxwell and his friend Ludwig Boltzmann. Also Heinrich Hertz built on his ideas. 
His 1876 critique of Boltzmann's attempt (1872) to derive the second law of thermodynamics from (mechanical) kinetic theory became famous as the "reversibility paradox".
- ↑ Bader, Alfred; Parker, Leonard (March 2001), Physics Today, http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_54/iss_3/45_1.shtml
- ↑ Lienhard, John H., "No. 1858: Johann Josef Loschmidt", Engines of our Ingenuity, College of Engineering, University of Houston, http://www.uh.edu/engines/epi1858.htm