May 10, 1860 - Robert Bunsen and Gustav Kirchhoff, discover caesium

In 1859, after discovering with his lighter that certain substances exposed to heat gave off different shades, Robert Bunsen and Gustav Kirchhoff decided to devote themselves to the study of spectroscopy (interaction between matter and radiated energy), developing the introduction of the prism to display the spectrum and developing the first spectroscope in 1860. In 1861, together with Kirchhoff, he put forward the theory of spectral analysis, thanks to which they discovered caesium (1860) and rubidium (1861). After numerous attempts, they did not manage to isolate caesium, which was obtained by electrolysis of molten salts by Carl Setterberg.

Bunsen's and Kirchhoff's spectroscopic work facilitated the subsequent discovery of other elements such as thallium, gallium, indium, scandium, and helium.