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Cohen, Ernst Julius

Cohen, Ernst Julius

(b. Amsterdam, Netherlands, 7 March 1869; d. Auschwitz [Oświęcim], Poland, ca. 5 March 1944),

physical chemistry, history of chemistry.

Cohen was a son of the German chemist Jacques Cohen, a pupil of Liebig and Bunsen, who later became a Dutch citizen. After leaving secondary school Cohen studied Latin and Greek and in 1888 passed the state university entrance examination. He then studied chemistry at the University of Amsterdam. In 1890 he passed the “candidate” examination and went, on the advice of his tutor, J. H. van’t Hoff, to Paris, where he worked in the laboratory of Henri Moissan. In 1892 Cohen passed the doctoral examination and in 1893 defended his dissertation for the doctorate in chemistry, which concerned the electrical method of determining transition points and the electromotive force of chemical reactions. Cohen was a devoted pupil and disciple of van’t Hoff, who sponsored him.

Choen was appointed assistant in van’t Hoff’s laboratory. In 1896 he became lecturer, in 1901 extraordinary professor of chemistry in Amsterdam, and a year later ordinary professor of physical chemistry and director of the chemical laboratory at the University of Utrecht, which position he held until his retirement in 1939. As a Jew, Cohen was a victim of the Nazi regime. In 1944 he was arrested, sent to Westerbork, and then transported to Auschwitz, where he died in a gas chamber on or about 5 March. Cohen was well-known inside and outside his native country, especially through his numerous speaking engagements. He published his impressions of the United States, formed during a visit in 1926, in the book Impressions of the Land of Benjamin Franklin (1928).

Cohen was a prolific author, publishing more than 400 papers. His earliest investigations were concerned with photography, in which he was very interested: he studied the action of hydrogen on silver bromide gelatin plates, the solubility of silver halides in various solutions, and the supposed influence of gelatin on the double decomposition of salts (1897). With Georg Bredig and van’t Hoff he worked on transition elements. In 1897 Cohen published a report on his detailed research on the dissociation of dissolved compounds in mixtures of alcohol and water. However, Cohen was known primarily for his famous series of investigations on physical isomerism.

Throughout his life Cohen studied the allotropy of tin. It was known that white tin undergoes a partial transformation into a gray powder, which causes the crumbling of organ pipes and other tin objects (“tin pest” or “tin disease”). Cohen and C. van Eyk showed that the transformation occurred because tin has two allotropes, white tin and gray tin, with very different properties. White tin is stable above 13.2°C., gray tin below that transition point (Cohen first found the value 18°C). The difference in specific volume of white and gray tin is about 25 percent, with the greater value for the gray tin. Thus even a small transformation is easily seen by the formation of “warts” on the white tin. Cohen and van Eyk studied the reversible reaction between white and gray tin by measuring the electromotive force of the galvanic combination of a solution of a tin salt and two electrodes, one of white tin and one of gray tin. The electromotive force changes its sign on passing the transition point. Another way to determine the transition point is to use a dilatometer at a fixed temperature: a positive change of volume means the formation of gray tin from white tin, and vice versa. Cohen and van Eyk found that under ordinary conditions the change does not occur, the white tin being then metastable. The change may, however, be greatly accelerated by lowering the temperature, by the presence of certain solvents, by the “inoculation” of gray tin, and so on.

In a long series of investigations Cohen studied what he called cases of physical isomerism: enantiotropy, with a transition point, as in the case of tin, and monotropy, in which one modification is always stable and the other always metastable (e.g., antimony). The substances investigated by Cohen and his students included phosphorus, tellurium, cadmium, bismuth, zinc, copper, lead, silver, potassium, sodium, antimony iodide, cadmium iodide, silver iodide, thallous picrate, and ammonium nitrate. Attention was paid especially to the existence of metastable phases and to their usual formation under conditions where thermodynamically the stable phase would be expected. Cohen came to the conclusion that enantiotropy and monotropy were exceedingly common among both elements and compounds and that most metals in common use were, if not entirely metastable, at least mixtures of stable and metastable phases.

Cohen also studied intensively the electrochemistry of the galvanic cells, such as the standard Clark and Weston cells, the Daniell cell, and the calomel cell. He investigated the reactions occurring in such cells, the condition of stability or metastability of all phases present, and the values of the cells as standard galvanic elements. He also investigated thermodynamic relations, measurement of affinity in chemical reactions, and the use of transition cells for determining the temperature at which a transition occurs—for example, between different hydrates of the same salt. Cohen and his co-workers in Utrecht also did work in piezochemistry. Their investigations concerned the effect of pressures up to 1,500 atmospheres on the reactions and equilibria occurring in condensed (liquid and solid) systems. They studied the effect of high pressure on the velocity of chemical reactions, on the transition temperatures of solid phases, on the electromotive force of galvanic cells, on the solubilities of solids, on Faraday’s first law, on the velocity of diffusion in liquid systems, on the viscosity of liquids, and on the electrical conductivity of solutions. In 1919 a survey of this work was published by Cohen and W. Schut as Piezochemie kondensierter Systeme and augmented in his Physico-chemical Metamorphosis and Some Problems in Piezochemistry (1928), which contains Cohen’s Baker Lectures, given at Cornell University in 1925–1926.

Besides his numerous investigations in physical chemistry, which clearly showed him to be an experimenter first and foremost, Cohen was deeply interested in the history of chemistry. He published a great many articles in the field, most of them biographical. He was not interested so much in the historical development of chemical theories as in the lives of chemists and in the discoverers of chemical operations and experiments. In 1907 Cohen published a book on the history of laughing gas, and in 1912 his still-important biography of van’t Hoff appeared. Cohen published on such chemists as Gerardus Mulder, professor of chemistry at Utrecht; the German physiological chemist Hartog Jakob Hamburger; the chemist Cornelis Adriaan Lobry de Bruyn; Hermann Boerhaave; and Daniel Fahrenheit. Most of Cohen’s historical articles were originally published in Dutch and later translated into German.

Cohen also published an enlarged edition of van’t Hoff’s famous book Études de dynamique chimique under the title Studien zur chemischen Dynamik (1896). In Abegg’s Handbuch der anorganischen Chemie he published an article on tin (1909), and he also wrote two textbooks for medical students.


I. Original Works. A complete survey of Cohen’s publications is given in Chemisch weekblad15 (1918), 1452–1470; 24 (1927) 489–492; 36 (1939), 519–522; and 41 (1945), 128–129. cohen’s dissertation was Hat bapalen van overgangsunpten langs electrischen was en de electromotorische kracht bij scheikundige omzetting (Amsterdam, 1893). An excerpt appeared as “Die Bestimmug von Umwandlungspunkten auf elektrischem Wege and die elektromotorische Kraft bei chemischer Zersetzung,” in Zeitschrift für physikalische chemie14 (1894), 53–92.

His scientific works include” Das Umwandklungselement and eine neue Art seiner Anwendung,” in Zeitschrifi für physikalische chemie14 (1894), 535–547, written with G. Bredig; “Zur Theorie des Umwandlungselements ohne metastabile Phase,” ibid16 (1895), 453–457, written with J. H. van’t Hoff and G. Bredig; Studien zur chemischen Dynamik. Nach J. H. van’t Hoff’s Études de dynamique chimique (Amsterdam, 1896); Experimentaluntersuchung über die Dissociation gelöster Körper in Alkohol-Wassergemischen (Rotterdam, 1897), “Physikalisch-chemische Studien am Zinn,” in Zeitschrift für physikalische Chemie, 30 (1899), 601–622, written with C. van Eyk; 33 (1900), 57–62; 35 (1900), 588–597; 36 (1901), 513–523; 48 (1904), 243–245; 50 (1904), 225–237, written with E. Goldschmidt; 63 (1908), 625–634; 68 (1909), 214–231; 127 (1927), 178–182. written with K. Douwes Dekker; and 173A (1935) 32–34, written with A. K. W. A. van Lieshout; “Zinn,” in Richard Abgg, Handbuch der anorganischen Chemie, III, pt. 2 (Leipzig, 1909) 531–610; Piezochemie kondensierter Systeme (Leipzig, 1919) written with W. Schut; “The Influence of Pressure on Chemical Transformations,” in Contemporary Developments in Chemistry (New York, 1927); and Physico-chemical Metamorphosis and Some Problems in Piezochemistry (New York 1928).

Among Cohen’s historical works are Das Lachgas. Eine chemisch-kulturhistirische Studue (Leipzig, 1907); Jacobus Henricus van’t Hoff. Sein Laben and Wirken (Leipzing, 1912), Herman Boerhaave en zijne beteekenis voor de chemie (n.p., 1918); “Herman Boerhaave and seine Bedeutung für die Chemie,” Janus23 (1918), 223–290; and “Wat leeren ons de archieven omtrent Gerrit Jan Mulder,” in Verhandelingen der Koninklijke akademie van wetenschappen, afdeeling natuurkunde19 , no. 2 (1948), 1–73.

II. Secindary Literature. On Cohen or his work, see the following, listed chronologically; H. R. Kruyt, “In Memoriam Ernst Cohen, “in Chemisch weekblad41 (1945), 126–129; F. G. Donnan, “Ernst Julius Cohen (1869–1944),” in Obituary Notices of Fellows of the Royal Society of London5 (1948), 667–687; C. A. Brown, “Dr. Ernst Cohen as Historain of Science,” in Journal of Chemical education25 (1948), 302–307; and A. L. T. Moesveld, “The Scientific Work of Ernst Cohen,” ibid 308–314.

H. A. M. Snelders

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