Flourescent tubes contain a near vacuum with a little mercury vapor.
If you could find a flourescent tube made from clear glass with no white coating, you'd be able to see some tiny droplets of mercury rolling around inside. When high voltage is applied across the ends of the tube, a big, silent, fuzzy spark jumps through the low pressure gas inside, and this makes the gas glow. The gas gives off various frequencies of light, and humans usually see these as colors. Other gasses give off other frequencies. Mercury gas gives off green, blue, and UV light.