Germain Henri Hess

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Hess, Germain Henri

(b. Geneva, Switzerland, 8 August 1802; d. St. Petersburg, Russia [now Leningrad, U.S.S.R.], 13 December 1850)


Hess was noted chiefly for his thermochemical investigations which laid the groundwork for later research in chemical thermodynamics. The son of a Swiss artist, he was taken to Russia at the age of three when his father became a tutor in a rich Moscow family. In Russia, where he remained for the rest of his life, he was called German Ivanovich Gess. He took a medical degree at the University of Dorpat (now Tartu, Estonia) in 1825 and then visited the laboratory of Berzelius in Stockholm. Although he stayed only a month, he became a lifelong friend of the Swedish chemist, corresponded constantly with him, and was strongly influenced by him in his scientific career.

In 1826 Hess established a medical practice at Irkutsk, where he carried out a number of studies of Siberian mineral resources that resulted in his election as an adjunct in chemistry of the Imperial Academy of Sciences in St. Petersburg on 11 November 1828. After returning to the capital he became a full academician in 1834 and held teaching posts in most of the city’s institutions of higher education.

Under the influence of Berzelius, almost all of Hess’s early work was concerned with analysis of inorganic and organic substances, but he was also well aware of contemporary theoretical problems. Like most chemists of the period, he accepted Dalton’s atomic theory and the law of definite proportions. He was interested in the question of the nature of affinity but did not accept Berzelius’ electrochemical theory, which was then the most popular among chemists. As early as 1830 he began to think that a solution to the problem of affinity could be found by studying the quantities of heat evolved in chemical reactions.

Hess then took up the calorimetric work of Lavoisier and Laplace. He was at first uncertain whether heat was due to a vibrational motion of particles or to a material substance, caloric, but finally decided that the caloric theory was more realistic, believing that if he could find examples of the combination of caloric with chemical elements in definite proportions, he would obtain a clearer view of the nature of affinity and the inner constitution of chemical compounds. He began serious experimental studies with an ice calorimeter in 1838 and by 1840 was able to formulate his two major thermochemical laws.

His experimental studies of the heat that developed in the formation of various hydrates of sulfuric acid and in a number of neutralization reactions showed that the amount of heat was always the same, whether the reaction proceeded directly or through a number of intermediate steps. This law of the constant summation of heat was obviously a special case of the law of the conservation of energy, which had not yet been formally stated. Hess saw clearly the practical utility of his law in determining heats of reaction that could not be measured directly. His second law, that of thermoneutrality, stated that there was no heat effect when neutral salts underwent double decomposition in water solution. The explanation of this law was not given until Arrhenius published his ionic theory in 1887.

The thermochemical work of Hess was continued extensively in the second half of the nineteenth century through the studies of Thomsen and Berthelot. Both Berthelot’s principle of maximum work and the thermodynamic theories of affinity which came to prevail were clearly foreshadowed in the work of Hess.

In addition to his internationally known research in thermochemistry, Hess was very influential in the development of chemistry in Russia. His text Osnovania chistoy khimii (“Fundamentals of Pure Chemistry”) went through seven editions and did much to establish the chemical nomenclature of the Russian language. He was always interested in technological questions, and many of his students later contributed to Russia’s industrial development.


I. Original Works. Hess’s papers are found chiefly in the various publications of the St. Petersburg Academy of Sciences from 1827 to 1849 and in Annalen der Physik und Chemie (1827–1848). The most important thermochemical papers are reprinted in G. I. Gess. Termokhimicheskie issledovania (“G. I. Hess. Thermochemical Investigations”; Moscow, 1958). Selections from these papers were also published as Thermochemische Untersuchungen von G. Hess, vol. IX of Ostwalds Klassiker der Exacten Wissenschaften (Leipzig, 1890). See also Osnovania chistoy khimii (“Fundamentals of Pure Chemistry”; St. Petersburg, 1831; 7th ed., 1849).

II. Secondary Literature. On Hess and his work see Y. I. Soloviev, German Ivanovich Gess (Moscow, 1962). An account of the students who carried on Hess’s work is Z. I. Sheptunova, “Khimicheskaya shkola G. I. Gessa” (“The Chemical School of G. I. Hess”), in Trudy Instituta istorii estestvoznaniya i tekhniki. Akademiya nauk SSSR, 18 (1958), 75–103. An English account is H. M. Leicester, “Germain Henri Hess and the Foundations of Thermochemistry,” in Journal of Chemical Education, 28 (1951), 581–583.

Henry M. Leicester

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Hess, Germain Henri


Germain Henri Hess is noted today for two fundamental principles of thermochemistry: the law of constant summation of heat (known simply as Hess's law) and the law of thermoneutrality. These discoveries were remarkable in that they were postulated without any supporting theoretical framework and took place in a field of study almost totally neglected by his contemporaries. Hess's law is of immense practical importance and is used to this day to determine heats of reaction when their direct measurements are difficult or impossible.

Hess was born on August 8, 1802, in Geneva, Switzerland. At the age of three, he moved with his family to Russia when his father, an artist, became a tutor to a rich family. Hess began his medical studies in 1822 and obtained an M.D. in 1826. While in school, he became interested in chemistry and geology, and upon graduation traveled to Stockholm, Sweden, to study with Jöns Jakob Berzelius, the most renowned chemist of the period. Although he spent barely a month in Berzelius's laboratories, Hess was strongly influenced by him in his later career, and they remained lifelong friends and correspondents. After practicing medicine in Irkutsk for two years, Hess returned to St. Petersburg, where he remained a member of the academic establishment for the remainder of his life.

Although Hess, like most of his colleagues, was involved with the discovery and analysis of new substances, he became interested in the more theoretical aspects of chemistry. His investigations into the nature of chemical affinitywhy atoms are attracted to each otherled him to study the amounts of heat generated by chemical reactions. His experiments, carried out on the various hydrates of sulfuric acid, showed that the heat evolved in their formation was always the same, whether the reactions proceeded directly or stepwise through intermediates . Although this can be seen in hindsight as a specific example of the law of the conservation of energy, Hess developed it two years before Julius Robert von Mayer elucidated the more general principle in 1842.

Hess was immediately aware of the significance of his discovery and continued his studies in the same realm. In 1842 he proposed the law of thermoneutrality, which stated that no heat is evolved by the exchange reactions of neutral salts in aqueous solutions . He was not able to fully explain these observations, and it was not until forty-five years later that the process of electrolytic dissociation was more completely elucidated by the Swedish physicist and chemist Svante Arrhenius.

Although his research activity diminished after these two major discoveries, Hess remained influential in the development of chemistry in Russia. His textbook Fundamentals of Pure Chemistry saw seven editions and remained the standard Russian text in chemistry until 1861. He remained active in teaching and mentoring younger scientists, until declining health forced his retirement in 1848. He died on December 13, 1850, at the relatively young age of forty-eight.

see also Arrhenius, Svante; Berzelius, JÖns Jakob; Thermochemistry.

Bartow Culp


Leicester, Henry M. (1951). "Germain Henri Hess and the Foundation of Thermo-chemistry." Journal of Chemical Education 28:581583.