The gas in the air that makes a filament in an incandescent light bulb glow is nitrogen.

Here's why:

* Incandescent light bulbs: These bulbs work by heating a thin wire filament to a high temperature, causing it to emit light.

* Vacuum: To prevent the filament from reacting with oxygen and burning, the bulb is filled with an inert gas.

* Nitrogen: While other gases like argon are also used, nitrogen is the most common gas used in incandescent bulbs. Nitrogen is inert, meaning it doesn't react with the hot filament, and it helps to reduce the rate of evaporation of the filament material.

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