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portable or movable apparatus used to put out a small fire by directing onto it a substance that cools the burning material, deprives the flame of oxygen, or interferes with the chemical reactions occurring in the flame. Water performs two of these functions: its conversion to steam absorbs heat, and the steam displaces the air from the vicinity of the flame. Many simple fire extinguishers, therefore, are small tanks equipped with hand pumps or sources of compressed gas to propel water through a nozzle. The water may contain a wetting agent to make it more effective against fires in upholstery, an additive to produce a stable foam that acts as a barrier against oxygen, or an antifreeze. Carbon dioxide is a common propellant, brought into play by removing the locking pin of the cylinder valve containing the liquefied gas; this method has superseded the process, used in the soda-acid fire extinguisher, of generating carbon dioxide by mixing sulfuric acid with a solution of sodium bicarbonate.