Lava lamp

A lava lamp was a type of lamp that contained an oil-based wax compound surrounded by a semi-viscous liquid. When the wax was heated by the bulb at the base, the wax lowered in density, therefore rising above the denser liquid. When it got farther away from the heat source, it cooled and became denser, therefore falling closer to the heat source. This repeated until the bulb either burned out or the lamp was turned off.

Rain Robinson had a lava lamp in her office at the Griffith Observatory in 1996. 

Lava lamps were present in the San Francisco bar Doctor Leonard McCoy visited in 2285.