Caesium was the first element discovered using a spectroscope.
German chemist Robert Bunsen and physicist Gustav Kirchhoff identified caesium in 1860 through flame spectroscopy, only about a year after the spectroscope’s development. They examined mineral water from Dürkheim and noticed two previously unknown bright blue lines in its emission spectrum. Those distinctive lines revealed the presence of a new element, which they named caesium from the Latin caesius, meaning “bluish grey.”
The discovery showed that spectroscopy could identify elements by their characteristic light emissions, even when only tiny quantities were present. Bunsen and Kirchhoff later found rubidium in 1861 by the same general method. Caesium itself was not isolated as a pure metal until 1882, when Carl Setterberg electrolysed caesium cyanide.
Caesium has the symbol Cs and atomic number 55. It is a very soft, highly reactive alkali metal that melts at about 28.5°C. Its isotope caesium-133 is especially important: since 1967, the SI second has been defined using its microwave transition. The spelling “cesium” is common in American English, while “caesium” is the IUPAC form.