THOMSEN, HANS PETER JöRGEN JULIUS

(b Copenhagen, Denmark, 16 February 1826; d Copenhagen, 13 February 1909)

chemistry.

The son of Thomas Thomsen, a bank auditor, and the former Jensine Friederike Lund, Thomsen left secondary school without graduating. Since he was interested in chemistry, he planned to enter a pharmacy; but before he could do so, the professor of chemistry at the University of Copenhagen gave him the opportunity to work in his laboratory and at the same time to prepare for the entrance examination to the Polytekniske Laereanstalt, practical experience plus the examination being the alternative to the general certificate for admission to that school. Thomsen passed the examination in 1843 and obtained the candidate’s degree (M.Sc.) in applied natural sciences in 1846. From 1847 to 1853 he was assistant in the chemical laboratory and, from 1850 to 1856, instructor in agricultural chemistry, at the Polytekniske Laereanstalt. He took time for a study tour to France and Germany in 1853-1854. He was director of weights and measures at Copenhagen from 1856 to 1859 and then, until 1866, taught physics at the Danish Military College. In addition, Thomsen was instructor (1864-1865), lecturer (1865-1866), and professor (1866-1901) of chemistry at the University of Copenhagen and head of the chemical laboratory, at the same time serving as professor of chemistry at the Polytekniske Laereanstalt.

Thomsen’s first chemical work was elaboration of a method for the fabrication of soda from cryolite. He obtained a monopoly; and after overcoming some technical difficulties, he opened a factory at Copenhagen (1859), followed by others in Germany, Poland, and the United States. The Solvay method for manufacturing soda soon made cryolite commercially uncompetitive for use in soda production; but it proved to be a valuable raw material for making milk glass, enamel, and especially aluminum. Thus Thomsen’s factory could continue separating cryolite from accompanying, less valuable minerals.

This technical achievement was, however, only a minor facet of Thomsen’s work; his main activities were in pure science. In 1852 he had submitted “Bidrag til et Thermochemisk System” to the Royal Danish Academy of Sciences and Letters. For this paper he received the Academy’s silver medal and 50 rigsdaler (at that time equivalent to $28) “to be used for the purchase of precision equipment”. This was the start of a thirty-year program of thermochemical studies, during which Thomsen personally carried out more than 3,500 calorimetric measurements in a room kept at a temperature of 18°C. The results were later collected in the four-volume Thermochemische Untersuchungen, of which the three first volumes give the experimental material and the fourth Thomsen’s theoretical reflections. His fundamental thought was that the evolution of heat accompanying a chemical reaction (which he called varmetoning, equivalent to enthalpy change) is an exact expression of the chemical affinity of the reaction.

Nearly the same theory was advanced a short time later by Berthelot, and a heated discussion between the two scientists took place and continued for several years. Supplementary experiments made by Thomsen (spontaneous reactions accompanied by “negative varmetoning,” enthalpy loss–that is, endothermic reactions that are nevertheless spontaneous) led him to realize that his theory was only an approximation (it was later found that the theory is valid at absolute zero), and he publicly admitted its inexactitude. Berthelot, however, maintained the theory for many years, despite the facts that told against it.

Thermochemical studies were not Thomsen’s only scientific contribution. He determined the heat of neutralization for acids and bases, and used the results to calculate the basicity of polybasic acids; through these and similar measurements he verified Guldberg and Waage’s law of mass action. In his chemical experiments performed in galvanic cells, Thomsen found that the electromotive force can be used to calculate the mechanical work necessary for separating a compound into its elementary particles. In many instances, by measuring the electromotive force Thomsen obtained the same value for the affinity as in previous calorimetric experiments, but in other instances a difference was found. It is now known that the electrochemical measurements are theoretically correct, not the calorimetric ones. Thomsen discussed the constitution of benzene, and in a long series of papers he treated the theory that the atoms of an element are composed of smaller elementary particles; these speculations made him suggest a new way of presenting the periodic system of the elements. Just after the discovery of the first noble gas, he predicted the existence of five more members of this group; indicated their place in the periodic system of elements; and predicted their approximate atomic weights. All five were found in the next few years, and Thomsen’s predictions of their atomic weights were astonishingly near the values found. It is interesting to note that Thomsen’s periodic system was the one used by Niels Bohr when he gave his explanation of the periodic system based upon the number of electrons surrounding the nuclei of the atoms (1922).

The importance of Thomsen’s scientific work was rapidly recognized both in Denmark and abroad. In 1860 he was elected a member of the Royal Danish Academy of Sciences and Letters. Nine years later he was nominated as professor of physical chemistry at the University of Leipzig, but he refused the offer. Many foreign scientists asked to work under his guidance, but he was afraid that the comparability of the results obtained would be endangered when more than one person performed the measurements and therefore refused all such requests. Thus no school was formed around him. Thomsen was a foreign member of various academies and honorary member of learned societies, and held honorary doctorates from several universities (but none in France, because of the conflict with Berthelot).

Thomsen was rector of the University of Copenhagen (1886–1887, 1891–1892), principal of the Polytekniske Laereanstalt (1883–1902), and president of the Royal Danish Academy of Sciences and Letters (1888–1909). He was an efficient administrator not only in science and education: He was a member of the Copenhagen city council (1861–1894), and a member of the commission for the reform of the Danish monetary system and system of weights and measures (1863). The conversion of Denmark’s money to a decimal system was completed in 1874, and the reform of the system of weights and measures in 1910–1911. Upon his retirement from active service in 1902, Thomsen was appointed titular privy councillor in recognition of his public service.

BIBLIOGRAPHY

I. Original Works. Thomsen’s 227 papers in Danish and foreign scientific journals include “Thermochemische Untersuchungen I-XXXII,” in Poggendorffs Annalen der Physik (1859–1873); Journal für praktische Chemie (1873–1880); and Berichte der Deutschen chemischen Gesellschaft (1873–1880); studies on electromotive forces in Wiedemanns Annalen, 11 (1880), 246–269; on the constitution of benzene, in Berichte der Deutschen chemischen Gesellschaft, 13 (1880), 1808–1811, 2166–2168; “Über die mutmassliche Gruppe inaktiver Elemente,” in Zeitschrift für anorganische. . .Chemie, 8 (1895), 283–288; and “Systematische Gruppierung der chemischen Elemente,” ibid., 9 (1895), 190–193. His books include Thermochemische Untersuchungen, 4 vols. (Leipzig, 1882–1886); and Systematische Durchführung thermochemischer Untersuchungen. Zahlenwerte und theoretische Ergebnisse (Stuttgart, 1906).

II. Secondary Literature. Obituaries are in Berichte der Deutschen chemischen Gesellschaft, 42 (1909), 4971–4988; and Oversigt Dan. Vid. Selsk, (1909), 27–31. See also Dansk Biografisk Leksikom, XXIII (1942), 568–575; and Stig Veibel, Kemien i Danmark, I (1939), 202–210; and II (1943), 426–444, with complete bibliography.

Stig Veibel