A method and apparatus for preventing the formation of leachable mercury in mercury arc vapor discharge lamps is provided which comprises providing in the lamp structure an effective amount of a silver compound, gold compound, or combination thereof. In particular, the slots are disposed in a triangular pattern or around a triangular panel proximate the face. The nose dome can be connected axially to the impeller in order to guide an axial inflow andor outflow of the impeller, and includes a coupling device.