Ultimate World — Hugo Gernsback

One was a plan to equip certain cities with huge tanks of chloroform (or similar narcotic gases). Then, when a 10-Ball descended over a house in that city, the special gas was to be diverted into the city's regular gas mains. Citizens about to be kidnapped would quickly open their gas burners, freeing the narcotic gasses. These gases would then go up through the purple tube and render the Xenos unconscious.