Georg Ernst Stahl (1660-1734), historical artwork. Stahl was a German chemist and physician who developed the phlogiston theory of combustion. Phlogiston was thought to be a combustible substance found in flammable objects that was released when they burned. The idea of such a substance was proposed by Johann Becher in 1669. Stahl thought that air was needed to absorb the phlogiston, as combustion only occurs in air. While the theory explained many of the observations associated with combustion, it was overturned by Lavoisier in the 1770s. He showed that combustion occurred because of the oxygen in air.