(* march 15th 1821; + july 8th 1895) was a notable scientist who performed groundbreaking work in chemistry, physics (thermodynamics, optics, electrodynamics), and crystal forms. Born a son of poor Bohemian farmers in Putschirn, now part of Karlovy Vary, Czech Republic, Loschmidt became professor of physical chemistry at the University of Vienna in 1868. Lohschmidt had two early mentors. One of them was a Bohemian priest called Adalbert Czech, who advised Loschmidt's parents to send young Josef to high school in the Piarist monastery in Schlackenwerth and to later advanced high-school classes in Prague. He spent two years of philosophy and mathematics at Prague's Charles University, where he met his second important mentor, the philosophy professor Franz S. Exner, whose eyesight was failing, and who therefore asked Loschmidt to become his personal reader. Exner was famous for his innovative school reforms, which included promoting mathematics and science as important subjects. He advised Loschmidt to apply mathematics to psychological phenomena. By doing this he became a highly skilled mathematician. His 1861 booklet, Chemische Studien ("chemical studies"), proposed two-dimensional representations for over 300 molecules in a similar style to that used by modern chemists. Among these were aromatic molecules such as benzene (C6H6), and related triazines. Loschmidt illustrated the benzene nucleus as a large circle in order to indicate the yet-undetermined structure of the compound. Some have argued, however, that he intended this as the suggestion of a cyclical structure, four years before that of Kekulé, who is better known and is generally credited with the discovery of benzene's cyclic structure. In 1865, Loschmidt was the first to estimate the size of the molecules that make up the air: his result was only twice the true size, a remarkable feat given the approximations he had to make. His method allowed the size of any gas molecules to be related to measurable phenomena, and to determine how many molecules are present in a given volume of gas. This latter quantity is known as the Loschmidt constant in his honour, and its modern value is 26.9 million molecules per cubic centimetre. Loschmidt and his younger university colleague Ludwig Boltzmann became good friends. His critique of Boltzmann's attempt to derive the second law of thermodynamics from kinetic theory became famous as the "reversibility paradox". It led Boltzmann to his statistical concept of entropy as a logarithmic tally of the number of microscopic states corresponding to a given thermodynamic state. Loschmidt retired from university in 1891 and died in 1895 in Vienna.