In 1830, Hess took up chemistry full-time, researching and teaching, and later became an adjunct professor of Chemistry at the
St. Petersburg Academy of Sciences. His most famous paper, outlining his law on
thermochemistry, was published there in 1840. His principle, a progenitor for the
first law of thermodynamics, came to be called
Hess' law. It states that in a series of chemical reactions, the total energy gained or lost depends only on the initial and final states, regardless of the number or path of the steps. This is also known as the law of constant heat summation. Like most of his colleagues, Hess was primarily an experimental chemist interested in the discovery and analysis of new substances. However, he also developed a strong interest for theoretical investigations. In particular, he wondered how
chemical affinity relates to heat in chemical reactions. His experiments on various
hydrates of
sulfuric acid showed that the heat released when they formed was always the same, whether the reactions proceeded directly or through
intermediates (1840). Hess thus formulated a special case of the
conservation of energy two years before
Julius Robert von Mayer stated a more general principle, in 1842. Hess was fully aware of the importance of his own contribution. In 1842, Hess proposed the law of
thermoneutrality, which states that no heat is evolved in the
exchange reactions of neutral
salts in
aqueous solution. A full explanation would only be given 45 years later, in terms of
electrolytic dissociation, by the Swedish chemist
Svante Arrhenius. After these two major discoveries, Hess was influential in the development of chemistry in Russia. His book
Osnovania Chistoy Khimii (
Fundamentals of Pure Chemistry) went through seven editions and remained the standard Russian textbook for undergraduate chemistry until 1861. Hess was active as a teacher and mentor of young scientists, until his poor health forced him to retire, in 1848. ==Later research and final days==