Antoine Lavoisier

Named oxygen, killed phlogiston, wrote chemistry's first modern textbook — and was guillotined five years later for tax-collecting.

Traité Élémentaire de Chimie (1789) gave chemistry a conservation law ("nothing is created, nothing is lost"), a systematic nomenclature still in use, and a table of 33 elements that drew the line alchemy had been trying not to draw: substances that cannot be broken down further by chemical means. Phlogiston theory died on contact. Transmutation died with it — not by argument but by accounting, because Lavoisier's balance showed that mass in equals mass out. Five years after publication the Revolution executed him as a former tax farmer. "It took them only an instant to cut off that head, and a hundred years may not produce another like it." Alchemy did not survive the balance scale.