THOMSEN, HANS PETER JORGEN JULIUS (1826 1909), Danish chemist, was born in Copenhagen on Feb. 16, 1826, and spent his life in that city. From 1847 to 1856 he was engaged in teaching chemistry at the Polytechnique, of which from 1883 to 1892 he acted as director, and from 1856 to 1866 he was on the staff of the military high school. In 1866 he was appointed professor of chemistry at the university, and retained that chair until 1891. He was awarded the Davy medal in 1883 and elected a foreign member of the Royal Society in 1902. He died on Feb. 13, 1909.
His name is famous for his researches in thermochemistry, a subject to which he was attracted as early as 1852, as his first published paper, entitled "Contributions to a thermochemical system," shows. But his comprehensive work in this subject was not started until some years later; the results first appeared in 1869, and were published from time to time until 1882. They were collected in Thermochemische Untersuchungen (Leipzig, 1882-88; 4 vols.), and a summary in Danish, entitled Thermokemisker Resultater appeared in 1905; the latter was translated into Eng lish as Thermochemistry by Miss K. A. Burke.
In the course of his thermochemical work Thomsen made about 3,50o calorimetric measurements and he determined the heat evolved or absorbed in a very large number of chemical reactions. Incidentally he verified Kirchoff's equation connecting the change of heat of a reaction with temperature and the specific heats of the reactants and resultants, and also used his measurements to confirm Guldberg and Waage's theory of mass action (1867). Thomsen introduced the term "avidity" to indicate the tendency of an acid to unite with a base and used his results to draw up the first table of relative strengths of acids. He also worked on selenic acid, platinum compounds and hydrogen peroxide, and made accurate determinations of the atomic weights of oxygen and aluminium. In 1853 Thomsen had devised a process for manu facturing soda from cryolite, and this was first worked on a large scale in 1857; the process proved quite successful.
See obituary notice, Proceedings of the Royal Society (1910-II).