Cæsium, or cesium as it is usually spelled in the United States, is the fifty-fifth element. It was discovered by German scientists Robert Bunsen and Gustav Kirchhoff in 1860. Cæsium is the neuter form of the Latin adjective cæsius, meaning bluish-gray. It is so called because spectral analysis of the element reveals two bright lines in that portion of the spectrum. From an 1861 issue of the London, Edinburgh, and Dublin Philosophical Magazine:
A faint blue line not due to strontium or potassium or to the lately discovered cæsium.1
Cæsium has the chemical symbol Cs.
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