Pipe in ceiling behind old light fixture
In the early 1900's many houses used natural gas for lighting. Around pittsburgh, Pa. most old houses were piped for this natural gas lighting. Usually by now all that old piping has been disconnected. Find the source of the connection to the gas lighting, disconnect and cap/plug the piping. The house I lived in until I left for the Navy, had this piping that was removed from the gas supply and made safe.