Another solution was to employ a cluster of solid irons that were heated from a single source: As the iron currently in use cooled down, it could be quickly replaced by a hot one. In the late nineteenth and early twentieth centuries, there were many irons in use that were heated by fuels such as kerosene, ethanol, whale oil, natural gas, carbide gas (acetylene, as with carbide lamps), or even gasoline. Some houses were equipped with a system of pipes for distributing natural gas or carbide gas to different rooms in order to operate appliances such as irons, in addition to lights. Despite the risk of fire, liquid-fuel irons were sold in U.S. rural areas up through World War II.