chemist whose studies of heat in chemical reactions formed the foundation of thermochemistry.

After practicing medicine for several years in Irkutsk, Russia, Hess became professor of chemistry in 1830 at the Technological Institute, University of St. Petersburg. His early investigations concerned minerals and the natural gas found near Baku. He discovered the oxidation of sugars to yield saccharic acid and in 1834 published a work on chemistry that became a standard text in Russia for many years.

In 1840 Hess announced the law of constant heat summation, also known as Hess's law, which states that the amount of heat involved in producing one chemical from another is always the same, no matter how many stages are taken to obtain the desired product. Hess's law heralded the formulation of the more complete law of conservation of energy and prepared the way for the development of chemical thermodynamics in the late 19th century by the American physicist Josiah Willard Gibbs.