Hittorf, Johann Wilhelm

(b Bonn, Germany, 27 March 1824; d Münster, Westphalia, Germany, 28 Novermeber 1914)

Chemistry, Physics.

Hittorf is remembered primarily for his experimental work in the transport of charge by ions in electrolytic solutions and the study of electrical conduction through gases. His father was a merchant in Bonn, where Hittorf was educated and where, after a short period at the University of Berlin, he received the doctorate in 1846, having studied under Julius Plücker. A year later he became a member of the staff at the University of Bonn and at the same time he received a call, which he accepted, to become Privatdozent at the Royal Academy of Münster. When the school became a university, he was appointed professor of chemistry and physics (1852–1876). In 1876 a reorganization of the university permitted him to drop chemistry and he became professor of physics. Ill health made it necessary for him to retire in 1890 and he was named professor emeritus.

Hittorf had a reserved nature and professional advancement was slow. Recognition came to him only late in life. He was a corresponding member of the scientific societies of Göttingen, Berlin, and Munich and foreign member of the Royal Danish Academy of Science and Letters, as well as other foreign societies. In 1897 he received the Prussian order Pour le mérite for science and arts. In 1898 he was elected honorary president of the Deutsche Elektrochemische Gesellschaaft. Hittorf never married and lived with his younger, unmarried sister who kept house for him.

Hittorf’s experimental contributions to physical science included early researches on the allotropic forms of selenium and phosphorus (1851–1865); investigations of the variations in the concentrations of electrolytes during electrolysis (1853–1859); the discovery, with Plücker, of the presence of both band and bright line spectra in the discharges of electricity through a gas at low pressures (1865); the examination and description of gaseous discharges and cathode-ray phenomena (1869–1884); and investigations of the passivity of metals (1900).

After Faraday’s experimental investigations in 1834, it was accepted that the electricity passing through an electrolytic cell was carried by the movement of charged ions produced from the decomposition of the compounds making up the solution. Daniell had extended these ideas in 1839 and showed that salts were compounds not of acid anhydrides and metallic oxides as had been thought, but of metallic cations and elemental or compound acid anions. Believing that the conductivity of solutions was due to these ions, he bagan a study of their transference.

In 1853 Hittorf took up the problem. He extended the ideas of Daniell by reasoning in the following manner: Cations and anions exist in solutions and migrate under the influence of current through the solution. The migration of the cation toward the cathode and away from the anode, and the deposition of the anion on the positive electrode, together result in a decrease of the salt in the neighborhood of the anode. A similar analysis shows that there is also a decrease in the concentration of the salt in the neighborhood of the cathode. If the motion of the two dissimilar ions were the same, the decrease in the concentration of the salt would be the same at the two electrodes.

Hittorf developed an experimental technique which allowed him to measure the changes in concentration at the two electrodes and found that they were not the same. He concluded that the speeds of migration of the cation and the anion were different and he characterized this fact by defining “transport numbers,” which specified the portion of the transport of electricity carried by each ion. He formulated the following ratios:

Hittorf called the rations on the right transport numbers. He based his analysis on the theory of the structure of a solution proposed by Rudolph Clausius in 1857. Clausius postulated that a small number of ions are always present in a solution as the result of random thermal collisions between molecules that are occasionally energetic enough to cause a breaking of the compound into its positive and negative ions. These ions provide the transport of electric charge across the solution in the manner described by Hittorf.

Hittorf’s experimental results and his interpretation of them were not immediately accepted by chemists. But Friedrich Kohlrausch used his analysis as the basis for further work in measuring the conductivity of solutions (1874), and Hittorf’s research was untimately very influential in the advances made by Arrhenius in proposing the electrolytic dissociation theory (1887).

Although the conduction of electricity through gases as well as through solutions was first studied by Faraday (1838), little other than descriptive information was available until better vacuum pumps were built. In 1855 Geissler, a glassblower and instrument maker in Bonn, built a mercury pump with which he was able to devise the first Geissler tube. In 1858 Plücker reported the results of a study at lower pressures using Geissler tubes. He found that the cathode glow would follow the “lines of force” of a magnetic field and that the glass walls of the tube fluoresced near the cathode, this fluorescence also moving in response to a magnetic field.

Following Plücker’s studies, in 1869 Hittorf began a series of investigations of the discharge phenomena. He verified the effect of the magnetic field on the glow discharge and the fluorescence of the glass tube itself. He found that any solid or fluid body, whether an insulator or a coductor, when placed in front of the cathode cut off the glow. By constructing an L-shaped tube with the electrodes at the two ends, Hittorf was able to establish that the glow was confined to the arm in which the cathode was located. He concluded that the glow was generated from a point cathode and traveled in straight lines. “We will therefore speak of rectilinear paths or rays of glow, and consider any point of the cathode as the source of a cone of rays” (“Ueber die Elektricitätsleitung der Gase,” in Annalen der Physik und Chemie, vol. 136 , pt. 1 [1869]). These results led to the brilliant researches on gaseous conduction by Crookes ten years later (1879) and the eventual identification of the cathode rays as electrons by J. J. Thomson (1897).

BIBLIOGRAPHY

I. Original Works. No complete collection of Hittorf’s works has been made. The series of four articles on ions, “Ueber die Wanderugen der lonen Während des Elektrolyse” (1853–1859), were reprinted in nos. 21 and 23 of Ostwald’s Klassiker der Exakten Wissenschaften and the first was republished, with memoirs by Faraday and F. Koklrausch, as “On the Mrgration of lons During Electrolysis,” in Harry Manly Goodwin, The Fundamental Laws of Electrolytic Conduction ( 1899).

II. Seondary Literature. Discussions of Hittorf’s work with solutions are in Wilhelm Ostwald, Elektrochemie, ihre Geschichte und Lehre (1896); and Harry C. Jones, The Theory of Electrolytic Dissociation and Some of Its Applications (1900). For a discussion of his contributions to the conduction of electricity through gases, see David L. Anderson, The Discovery of the Electron (Princetion, 1964), pp. 23, 54. A biography is G. C. Schmidt, Wilhelm Hittorf (Münster, 1924).

Ollin J. Drennan