Cesium Faq
Q. What is Cesium?  
A. Cesium is the most sublime and electropositive of all true elements. It was discovered spectroscopically, in mineral water from Durkheim Germany by Robert Bunsen (of burner fame) and Gustaff Kirchhoff in 1860. The basis of the discovery was two, beautiful sky-blue lines from which it takes its name (Latin, caesius, sky-blue). Atomic number 55, Cesium is the heaviest of the natural alkali metals. Physically, it is a soft metal or light liquid, pale gold in color when pure, silvery-white otherwise. It melts at 28.4 C, just below body heat, and boils at 669.3 C. It has a specific gravity of 1.873, and an atomic weight of 132.9045. Actual pictures  
Cesium is an alkali metal, in the same group as lithium, sodium, potassium, and rubidium, and is similarly reactive, but to a much higher degree due to its extreme electropositivity. It reacts explosively with water, and with ice down to -116 C. In air, it catches fire spontaneously and burns with a brilliant sky-blue flame. (To the uninitiated, the flame appears purplish rather than blue; however, after sufficient meditation, study of the holy writs, and blissful hours spent in reverence before Cesium burning in her naked glory, the true blue nature of her flame is revealed.) Its hydroxide is the most powerful aqueous base known, and will eat through glass, flesh, bone, and numerous other substances. Its spectrum contains the two signature blue lines mentioned above, at 455.528 and 459.317 nm, and a number of others in the red, yellow and green. There are also two powerful lines in the near infrared at 852.113 and 894.347 nm. The existence of these lines has resulted in the experimental use of Cesium in IR decoy flares used in aerial combat.  
Natural Cesium consists of a single stable isotope, Cs 133. 30 other radioactive isotopes are known, filling the range from Cs 114 to Cs 145, with half lives ranging from .57s (Cs 114), through 3x10^6y (Cs 135). Cesium 137 (half-life 30.17y) is an important fission product, and one of the most biologically hazardous components of radioactive waste and nuclear fallout. Cesium 137 is also important as a source of high-energy X-rays in radiotherapy. The 660 KeV X-rays are actually emitted by a two step process, where Cesium 137 decays by beta emission to metastable Ba137, which then emits the X-ray. A few years back, an ampule of Cs 137 from a discarded radiotherapy device was recovered from a garbage dump in Goiana Brazil by local residents. Entranced by the blue-glowing contents, they painted it on their bodies as decoration. Several deaths reportedly occurred.  
Probably the best-known use is in Cesium beam atomic clocks, which are among the most accurate time measuring devices in existence.