[2] A friend and colleague of Ludwig A. Colding, who was one of the early advocates of the principle of conservation of energy, Thomsen did much to found the field of thermochemistry.

In particular, between 1869 and 1882, he carried out a great number of determinations of the heat evolved or absorbed in chemical reactions, such as the formation of salts, oxidation and reduction, and the combustion of organic compounds.

In 1857 he established in Copenhagen a process for manufacturing soda from cryolite, obtained from the west coast of Greenland.

[5] Katherine Alice Burke translated his book on systematic research in thermochemistry into English.

[6] His brother, Carl August Thomsen (1834–1894), was lecturer on technical chemistry at the Copenhagen Polytechnic, and a second brother, Thomas Gottfried Thomsen (1841–1901), was assistant in the chemical laboratory at the university until 1884, when he abandoned science for theology, subsequently becoming minister at Norup and Randers.