A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light.
Taken right from Wikipedia! http://en.wikipedia.org/wiki/Fluorescent_lamp
End of life
The end of life failure mode for fluorescent lamps varies depending on how they are used and their control gear type. Often the light will turn pink (see "Loss of mercury" section for details) with black burns on the ends of the lamp due to sputtering of emission mix (see below). The lamp may also flicker at a noticeable rate (see "Flicker problems" section for details).
Mercury is slowly absorbed into glass, phosphor, and tube electrodes throughout the lamp life, until it can't function.
The failure symptom cycle starts out with a flicker (run-up time to full light output). When the mercury runs out (absorbed by the glass, phosphor, and tube electrodes) the lamp glows a dim pink and the argon base gas takes over as the primary discharge.
The same effect can be observed with new tubes. Mercury is present in the form of an amalgam and takes some time to be liberated in sufficient amount. New lamps may initially glow pink for several seconds after startup. This period is minimized after about 100 hours of operation.