 General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly argon and some nitrogen, or sometimes krypton.While inert gas reduces filament evaporation, it also conducts heat from the filament, thereby cooling the filament and reducing efficiency.At constant pressure and temperature, the thermal conductivity of a gas depends upon the molecular weight of the gas and the cross sectional area of the gas molecules.