Today is the birthday of physicist Louis Essen, inventor of the cesium atomic clock. Cesium (also spelled caesium) was discovered using the new method of flame spectroscopy in 1860 by two German scientists, Robert Bunsen (yes, of Bunsen burner fame) and Gustav Kirchhoff. They decided to name the new element after its unusual and unique spectrographic signature, specifically the preponderance of the color sky blue, which you can see in the spectrograph above. The word came from the Latin word caesius meaning blue-gray, often referring to the color of eyes.
Image of pollucite (a common mineral rich in cesium) courtesy Rob Lavinsky. Image of ampule of liquid cesium (although a metal, cesium is liquid at room temperature) by argentoratum. To see the spectrograph of any element, check out the cool site by University of Oregon.