Cesium, or caesium, was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff, the first element ever found by spectroscopy, detected through two bright blue lines in the spectrum of a mineral water. They named it from the Latin caesius, sky-blue, after those lines. Cesium is a soft, golden-tinged alkali metal, liquid near room temperature and violently reactive, whose most famous role is in the atomic clock. Besides defining the second, caesium is used in the photocells and drilling fluids of industry and in ion propulsion, and its extreme reactivity and single loosely held electron make it a favourite working material of precision physics.
Caesium's reactivity is so extreme that the pure metal reacts violently even with ice at low temperatures, and it must be shipped and stored in sealed glass ampoules under vacuum or inert gas rather than handled as an ordinary laboratory reagent. The caesium atomic clock's extraordinary precision, accurate to better than a second over tens of millions of years in the best laboratory standards, underlies the exact timing signals that make GPS satellite navigation and international financial and telecommunications networks function, so that a metal almost too dangerous to touch quietly keeps the clocks that keep the modern world synchronised.