Dmitri Mendeleev worked in Heidelberg from 1859 to 1861 on capillarity and the spectroscope.
Mendeleev went to Western Europe during an important formative period in his scientific career. In Heidelberg, he investigated the behavior of liquids and studied spectroscopy, the analysis of light emitted or absorbed by substances. These subjects helped broaden his experimental experience before his later work on chemical classification.
His Heidelberg years also placed him in contact with the rapidly developing European chemistry community. He established a private laboratory there and attended the 1860 International Chemical Congress at Karlsruhe, where debates over atomic weights and molecular formulas influenced his thinking.
The city is sometimes confused with Leipzig, Munich, or Berlin because Mendeleev studied and worked in several European academic settings. Heidelberg, however, is the specific German city associated with this 1859–1861 research period. He returned to Russia in 1861 and soon wrote his influential textbook Organic Chemistry.