The phlogiston theory was a scientific belief from the 1600s that stated that flammable materials contain a substance called phlogiston that gets released when the material is burned.  It was later discovered that flammable materials actually require an outside substance (an oxidizer, e.g. oxygen in the form of atmospheric air) in order to burn.  This is one of those examples where a scientific "fact" was later proven to be completely wrong.  (via Steven Johnson's The Invention of Air)