Halogen light bulbs function similarly to regular lightbulbs by delivering an electrical impulse via a tungsten wire, that dissolves as it lights. The gas is the distinction between the two. Standard lights are loaded with nitrogen-argon, whereas halogen lightbulbs are filled with bromine. When tungsten dissipates in a typical lightbulb, it leaves metallic layers on the interior of the lens, giving antique lightbulbs their black appearance.