Incandescent light
bulbs are often filled with Argon, an inert gas.
Incandescent light bulbs include a filament that glows to create light and an inert gas to keep the filament from burning up too fast. One of the cheapest gases to use is Argon, often found in incandescents. Since the bulbs emit a lot of heat, which equates to a lot of energy, they're now being replaced by more energy-efficient bulbs like compact fluorescents, which provide less heat. Still, incandescents do have some uses that might allow them to stick around.
Warmth for Livestock
Since incandescent bulbs, whether they contain argon or another type of gas, give off 95 percent of their power as heat, they can easily warm a small space, such as the nesting place for chickens, geese or other fowl. If the bulb is warming the area too much, you'll know it because your poultry will not go near the area. In that case, try a smaller bulb with a lower wattage.
Easy Bake Oven
If you have a vintage easy bake oven made before 2006, then you may be in need of an argon-containing incandescent bulb. The children's toy ovens worked their magic with the use of a standard heat-emitting, 100 watt light bulb. After 2006, Hasbro began making the ovens with a heating element, though they were recalled due to safety hazards.
Household Uses
If you have incandescents in your home or office, chances are they are filled with argon. While you could replace them with compact fluorescents or even LED bulbs, as long as they're in proper working order and you're not concerned with pinching pennies on your electric bill, you can leave them where they are. When they need replacing, incandescents are still much cheaper than newer incarnations of the light bulb, though you will pay more in electricity costs.
