Fire extinguisher, portable appliance for putting out small fires. Extinguishers work either by cooling or by depriving the fire of oxygen (as typified by the simplest, a bucket of water or sand), and most do both. The soda-acid extinguisher contains a sodium bicarbonate solution and a small, stopped bottle of sulfuric acid. Depression of a plunger shatters the bottle, mixing the chemicals so that carbon dioxide (CO2) gas is generated, forcing the water out of a nozzle. Foam extinguishers employ a foaming agent (usually animal protein or detergent) and an aerating agent. They are effective against oil fires, as they float on the surface. Carbon dioxide extinguishers provide a smothering blanket of CO2. Dry chemical extinguishers provide a powder consisting mainly of sodium bicarbonate, from which the fire's heat generates CO2.