Combustion Occurs When Oxygen Combines with a Substance's Fundamental Essence

The transition from phlogiston to oxygen theory was conceptually messy. Early oxygen-based explanations sometimes retained vestigial metaphysical language: combustion was described as oxygen combining with a substance's 'essence' or 'nature,' language borrowed from ancient philosophy. Britannica entries during the 1770s–1790s reflected this transitional state, acknowledging that oxygen was involved in combustion while sometimes using imprecise language about mechanisms. As chemical theory matured and quantitative measurements (stoichiometry) became standard, Britannica shifted toward describing combustion in terms of chemical reactions: molecules, bonds, and energy release. By the 1820s–1830s, Britannica entries presented combustion as a well-understood chemical process involving specific reactants, products, and measurable heat release. The language evolved from mystical (essence, nature) to materialist (molecules, bonds, energy). This linguistic shift reflected deeper conceptual maturation: from chemistry as transmutation-seeking toward chemistry as the study of material composition and reaction dynamics. The episode shows how scientific revolutions involve not just new facts but new conceptual frameworks and vocabularies.