An Etymological Dictionary of Astronomy and Astrophysics

A soft ductile chemical element of the → alkali metal group;
symbol Cs. It is found in several → silicate minerals, including pollucite. The metal oxidizes in air and reacts violently with water. → Atomic number 55; → atomic weight 132.9054; → melting point 28.4°C; → boiling point 669.3°C; → specific gravity 1.873 at 20°C; and → valence +1. Cesium has several radioactive isotopes, among which ¹³⁴Cs
with a half-life of 2.07 years and ¹³⁷Cs with a half-life of 30.3 years. Cesium was discovered spectroscopically in 1860 by W. Bunsen and G. Kirchhoff in mineral water from Durkheim.

See also: From L. caesius “bluish gray,” which was the color of the cesium line in the spectroscope, + → -ium.