Lavoisier's precise, quantitative experiments established the law of conservation of mass and demolished the phlogiston theory of combustion, earning him recognition as a founder of modern chemistry. A tax collector by profession, he was guillotined in 1794 during the Terror, a loss the mathematician Lagrange lamented took only a moment while a century might not produce another such head.
Born into wealth in Paris, Antoine-Laurent de Lavoisier trained as a lawyer but found his calling in the laboratory, where he insisted on meticulous measurement at a time when chemistry was still steeped in alchemical vagueness. Using a sealed apparatus and a balance sensitive to fractions of a gram, he demonstrated that matter was neither created nor destroyed in chemical reactions, only rearranged — a principle now taught to every schoolchild as the law of conservation of mass.
He demolished the reigning phlogiston theory, which held that a fire-like element escaped burning substances, by showing that combustion involved a measurable gas he named 'oxygène.' He also helped systematize chemical nomenclature and co-founded the metric system, giving science a shared vocabulary and shared units.
Lavoisier's career intersected fatally with politics. His work as a tax farmer for the crown made him a target during the Revolution, and despite pleas that his scientific work be spared, he was guillotined in May 1794. Legend holds he asked for a delay to finish one last experiment; whether true or not, his death remains one of history's starkest illustrations of genius consumed by revolutionary fury.