Georg Ernst Stahl. Image provided by Edgar Fahs Smith Collection, University of Pennsylvania Library. Georg Ernst Stahl (1660–1734) Some may think it a dubious honor to go down in history as the founder of a discarded theory, but Georg Ernst Stahl’s development of phlogiston theory was an important milestone in the development of modern science. His theory held that flammable materials contained a substance called phlogiston, which was released when they were burned. Metals also contained phlogiston, which was released when the metal was heated in air to form a calx. The calx could then be recombined with phlogiston to regenerate the metal. Though Antoine-Laurent Lavoisier would later show that combustion was really the chemical combination of a material with oxygen, and calcification the combination of a metal with oxygen, Stahl’s phlogiston theory represented one of the first attempts to develop a formal theory of how a chemical process took place based on observations. The research that overturned this theory constitutes one of the most important chapters in chemical history. Stahl actually devoted more attention to medicine and its philosophy than to chemistry. He became a professor of medicine at the University of Halle when that school was founded in 1693, due to his friendship with Elector Frederick III of Brandenburg, and later moved to Berlin as a court physician. He believed in an early form of vitalism and a form of atomism, and was influenced by the mercurial school of alchemy.