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Phlogiston theorists identified three essences which comprise all matter: sulfur or terra pinguis, the essence of inflammability; mercury or terra mercurialis, the essence of fluidity; and salt or terra lapida, the essence of fixity and inertness. In this respect phlogiston theory is similar to the ancient alchemical notions of earth, air, fire, and water. The terra pinguis was renamed phlogiston. In this view, metals were made of a "calx" (or residue) combined with phlogiston, the fiery principle, which was liberated during combustion, leaving only the calx. Air, according to the theory, was merely the receptacle for phlogiston; all combustible or calcinable substances, in fact, were not elements but compounds containing phlogiston. Rusting iron, for instance, was believed to be losing its phlogiston and thereby returning to its elemental state.
Phlogiston theory was widely supported throughout the eighteenth century, although it came under increasing attack as empirical research pointed up its difficulties. When it was determined that some metals actually gained mass when burnt, partisans explained it by giving phlogiston a negative mass. Even Priestley believed in the theory until his death, convinced that his discovery of oxygen was "dephlogisticated air." It was up to Lavoisier to realize the significance of his discovery.
Lavoisier made a symbolic break with phlogiston theory by burning all textbooks that supported the theory, just as Paracelsus had destroyed his copies of the works of the medieval medical authorities. His theory of oxidation soon replaced phlogiston theory, and remains a part of modern chemistry.