Julius Thomsen (1826-1909) was a prominent Danish chemist who acquired fame for his systematic and meticulous measurements of the thermal effects associated with chemical processes. For this work he received numerous honours, among them the Davy Medal from the Royal Society. Apart from his position as one of the founders of classical thermodynamics, he also has a place in the history of chemical technology due to his invention of a method of producing soda from the Greenlandic mineral cryolite. This invention resulted in a major industrial enterprise which was of great importance to Danish economy. Keenly interested in the electrical sciences, Thomsen constructed a new type of battery which for a while attracted international attention. Last but not least, he made significant contributions to the understanding of the periodic system in terms of the hypothesis of composite atoms. He ended his career by questioning if argon was really a chemical element.