In 1697, George Ernst Stahl proposed the phlogiston theory of combustion. Based on ideas of Johann Becher, phlogiston theory stated that when a substance burns, it loses phlogiston, an invisible material. Air served a mechanical purpose by absorbing phlogiston. Thus when a burning candle is covered with an inverted glass jar, the flame slowly dies and goes out as the air becomes saturated with phlogiston. A major problem with phlogiston was the observed gain in weight of metals as they oxidize. Despite this problem, phlogiston theory dominated chemistry for over 80 years.