Bunsen and Kirchhoff discover caesium and rubidium

Background. Spectroscopy measures electromagnetic spectra interacting with matter, and each element has a unique light spectrum. Kirchhoff was called to the University of Heidelberg in 1854, where he collaborated with Bunsen in spectroscopic work. In the summer of 1859, Kirchhoff suggested Bunsen try forming prismatic spectra of colored flames. Together they improved on Joseph von Fraunhofer's 1814 spectroscope, enabling the identification of elements by their spectra.

Caesium Discovery. Caesium was the first element discovered with a spectroscope, which Bunsen and Kirchhoff had invented only a year previously. In 1860, they discovered caesium by the newly developed method of flame spectroscopy, finding it in mineral water from Dürkheim, Germany. After careful distillation of forty tons of this water, Bunsen isolated 17 grams of the new element in the spring of 1860.

Rubidium Discovery. Rubidium was the second element discovered by spectroscopy, shortly after caesium. Bunsen and Kirchhoff discovered rubidium in 1861 in Heidelberg, Germany, using flame spectroscopy. They isolated it from the mineral lepidolite. Their large-scale isolation efforts yielded 9.2 grams of rubidium chloride from 44,000 litres of mineral water.