Hydrochloric acid is the solution of hydrogen chloride (HCl) in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid. Historically called muriatic acid or spirits of salt, hydrochloric acid was produced from vitriol and common salt. During the Middle Ages, it was used by alchemists in the quest for the philosopher's stone, and later by European scientists including Glauber, Priestley, and Davy in their scientific research. With major production starting in the Industrial Revolution, hydrochloric acid is used in the chemical industry as a chemical reagent in the large-scale production of vinyl chloride for PVC plastic, and MDI/TDI for polyurethane. It has numerous smaller-scale applications, including household cleaning, production of gelatin and other food additives, descaling, and leather processing. About 20 million metric tonnes of hydrochloric acid are produced annually.