A tiny density mismatch in nitrogen revealed a hidden gas

Lord Rayleigh noticed that nitrogen extracted from air was consistently heavier than nitrogen made from chemical compounds. Chasing that stubborn discrepancy with chemist William Ramsay, they isolated a new, chemically inert gas hiding in plain sight in every breath — argon, nearly 1% of the atmosphere.

— Lord Rayleigh & William Ramsay, Argon, a New Constituent of the Atmosphere — Proceedings of the Royal Society, 1895