 In a conventional lamp, the evaporated tungsten eventually condenses on the inner surface of the glass envelope, darkening it.For bulbs that contain a vacuum, the darkening is uniform across the entire surface of the envelope.When a filling of inert gas is used, the evaporated tungsten is carried in the thermal convection currents of the gas, depositing preferentially on the uppermost part of the envelope and blackening just that portion of the envelope.