Cesium

Cesium, an alkaline metal, almost always found along with rubidium, was discovered by Bunsen and Kirchhoff in 1860 by spectrum analysis. The metal, isolated for the first time in 1882, is silver white, soft and extensible, and like rubidium is highly analogous to potassium. Its symbol is Cs, and its atomic weight 133. Its melting-point is 26.5° C., and its specific gravity 1.88. It ignites spontaneously in the air, and when thrown on water, behaves like sodium, potassium, and rubidium. The name is derived from cæsius, 'sky-coloured,' from the colour given to the blowpipe flame by cæsium. See RUBIDIUM.