When current passes through the filament of the incandescent bulb, it becomes white-hot and the temperature reaches as high as 25000C or more. If air is present in the bulb,the filament will oxidize and burn out quickly. So, for durability, the hot filament must not come in contact with the air. It is placed in an evacuated glass bulb. Now a days the glass bulb is filled with a mixture of nitrogen and argon gases at very low pressure.