Josef Loschmidt

Johann Josef Loschmidt was born in 1821 in Putschirn, Bohemia, into a poor peasant family; his talent was recognised by a local priest who arranged his scholarship to school. After studies in Prague and Vienna and years of struggle in failed chemical ventures, he became a schoolteacher and only later, in his forties, secured an academic position at the University of Vienna, where he became professor of physical chemistry.

Loschmidt's most celebrated achievement came in 1865, when he combined kinetic theory with data on gases and liquids to make the first credible estimate of the size of molecules and the number of molecules in a given volume of gas. The number density of an ideal gas at standard conditions is still called the Loschmidt constant in his honour; his work was an essential step toward establishing the reality and scale of atoms.

He also made lasting contributions to chemical structure, drawing in his 1861 booklet 'Chemische Studien' hundreds of structural formulae — including a representation of benzene as a ring — years ahead of their general acceptance.

In statistical physics Loschmidt is remembered above all for the reversibility objection he raised to his friend Ludwig Boltzmann in 1876: if the microscopic laws are time-symmetric, no strictly one-way second law can be derived from them. Loschmidt's paradox forced a deeper, statistical understanding of entropy and the arrow of time, and the debate between the two men was conducted in mutual respect throughout.