Phoshor (not mercury) is the white powder on the fluorescent bulb tubes that allows the bulb to glow white.
How does a CFL or compact fluorescent light bulb work?
Fluorescent light bulbs (including compact fluorescents) are more energy-efficient than regular bulbs because of the different method they use to produce light. Regular bulbs (also known as incandescent bulbs) create light by heating a filament inside the bulb; the heat makes the filament white-hot, producing the light that you see. A lot of the energy used to create the heat that lights an incandescent bulb is wasted.
A fluorescent bulb, on the other hand, contains a gas that produces invisible ultraviolet light (UV) when the gas is excited by electricity. The UV light hits the white coating inside the fluorescent bulb and the coating changes it into light you can see. Because fluorescent bulbs don't use heat to create light, they are far more energy-efficient than regular incandescent bulbs.