photoelectric cells   Caesium or cesium is the chemical element
with the symbol Cs and atomic number 55. It is a soft, silvery-
gold alkali metal with a melting point of , which makes it one
of only five metals that are liquid at or near room temperature.
Caesium is most notably used in atomic clocks.  Caesium is the
international spelling standardized by the IUPAC, but in North
American English, the spelling cesium is more common.   The
emission spectrum of caesium has two bright lines in the blue
area of the spectrum along with several other lines in the red,
yellow, and green areas. This metal is silvery gold in color and
is both soft and ductile. Caesium is the second most
electropositive and alkaline of the chemical elements and has
the second lowest ionization potential (after francium). Caesium
is the least abundant of the five non-radioactive alkali metals
(francium is the least common alkali metal, but since it is
highly radioactive with an estimated 30 grams in the entire
Earth's crust at one time, i