In chemistry, Julius Thomsen (1826-1909) was a Danish chemist noted for his 1854 theory, similar to French chemist Marcellin Berthelot (1864), that the heat evolved in a chemical reaction is the measure of the chemical affinity, on the grounds that the heat represented the work done by chemical forces. [1] Thomsen and Berthelot similarly defined affinity as the force that unites the component parts of a chemical compound and that must be overcome by an equivalent force, the quantity of which can be measured by the heat evolved. [2] Thomsen appears to have been the first, in 1853, to have applied the first law version of the conservation of energy to the field of chemistry. [3] That same year, Thomsen did work with Danish engineer Ludwig Colding, an early formulator of the conservation of energy.

References
1. (a) Thims, Libb. (2007). Human Chemistry (Volume Two) (Julius Thomsen, pgs. 434, 661, 750) (preview) (Google books). Morrisville, NC: LuLu.
(b) Nye, Mary J. (1993). From Chemical Philosophy to Theoretical Chemistry: Dynamics of Matter and Dynamics of Disciplines: 1800-1950 (section: From Chemical Affinity to Chemical Thermodynamics, pgs. 116-20). University of California Press.
2. Marcellin Berthelot, Essai de Mecanique Chimique fondee sur la Thermochime, and Julius Thomsen, “Ueber die Berthollet’sche Affinitatstheorie”, Ann. Phys. 138 (1869): 65-102, discussed in Helge Kragh, “Julius Thomsen and Classical Thermochemistry”, BJHS 17 (1984): 255-72. Also see: J. G. Kim, “Practice and Representation”, 136.
3. Look, Dwight C., Sauer, Harry J., and Alexander, Graham I. (1988). Engineering Thermodynamics (pg. 764). Van Nostrand Reinhold.

External links
● Hans Peter Jørgen Julius Thomsen – Wikipedia.
● Thomsen-Berthelot principle – Wikipedia.