Discovery of argon, the first discovery of an inert gas. In 1904 Ramsay was awarded the Nobel Prize in Chemistry "in recognition of his services in the discovery of the inert gaseous elements in air, and his determination of their place in the periodic system."

For the same discovery Lord Rayleigh received the 1904 Nobel Prize in Physics "for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies."

(Thanks to Juan Weiss for this reference and its interpretation.)