Robert Bunsen and German physicist Gustav Kirchhoff…

Robert Bunsen and German physicist Gustav Kirchhoff suggest the presence of a new alkali element, and call it caesium, or cesium, derived from the Latin caesius, meaning “blue-gray”, used to designate the blue of the sky.

Investigating the mineral waters in the Palatinate in 1860, they obtain a filtrate that is characterized by two bright lines in the blue region of its spectrum (the light emitted when the sample is inserted into a flame).