Cesium is a bright silvery metal which is a liquid in a warm room (28oC). Its name is from the Latin caesius which is a description of a sky blue spectral emission by which it was discovered in 1860 by Bunsen and Kirchhoff.
Cesium is so reactive that it will even explode on contact with ice! It has been used as a "getter" in the manufacture of vacuum tubes (i.e., it helps remove trace quantities of remaining gases). An isotope of cesium is used in the atomic clocks.
Stephen R. Marsden (ChemTopics)