The French chemist who helped transform chemistry into a quantitative science by weighing everything before and after a reaction — establishing the law of conservation of mass, naming oxygen and overthrowing the phlogiston theory of combustion, and co-creating modern chemical nomenclature — before being guillotined in 1794 as a former royal tax collector during the Terror of the French Revolution.
Antoine Lavoisier weighed everything. He sealed substances in glass, burned them, rusted them, and put the vessel on a balance before and after. The masses always matched. From that stubborn fact he built a law: in any reaction, matter is rearranged but never created or destroyed. He named oxygen, demolished the reigning theory of fire, and wrote what many regard as the first modern chemistry textbook. In 1794 the French Revolution sent him to the guillotine — not for his science, but because he had once collected taxes for the king. The mathematician Lagrange reportedly said it took an instant to cut off that head, and a hundred years might not produce another like it.