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 Hess's investigation of minerals included the analysis of silver telluride (Ag2Te), which was named hessite in his honour. He also discovered that the oxidation of sugars yielded saccharic acid. Hess died prematurely in 1850, at the age of 48, in St. Petersburg. He was buried at Smolenskoe Lutheran cemetery. Further reading References 1802 births 1850 deaths Russian chemists Saint Petersburg State Institute of Technology alumni Full members of the Saint Petersburg Academy of Sciences Burials at Smolensky Lutheran Cemetery