Odorless, colorless gas which gives off a slightly greenish tinted color under high voltage. m.p. -156.6 ºC; -249.9 ºF b.p. -152.30±0.10 ºC; -242.14±0.18 ºF density 0.003733 g/cc (0 ºC); 0.233044 pound/cubic foot (32 ºF)	memory peg
	1898 Sir William Ramsay & Morris W. Travers, England
	κρυπτος (kryptos) = hidden (Greek)

After his discovery of Argon, Sir William Ramsay (1852-1916), joined by an assistant, Morris W. Travers (1872-1961), continued to search for another member of the expected inert gas family lighter than Argon.

They failed to find such a gas in rare minerals. From a liter of liquid air, they distilled Nitrogen, Oxygen, and Argon had distilled. On 30 May 1898, they observed the spectra of the inert remaining 25 cm³ of the original liter. Unique bright yellow and brilliant green lines suggested a new element which, because it remained hidden in the liquid, they named Krypton, from the Greek κρυπτος [kryptos] = hidden.
After measuring the density they found that Krypton had to be placed between Bromine and Rubidium on the periodic table.

Since this is heavier then Argon, Krypton was not the expected lighter gas lighter and they continued their search. After a few weeks they found the expected lighter gas, Neon.

After the publication by Ramsay and Travers of their findings, Ramsay's friend Marcellin Berthelot (1827-1907) suggested a better name for Krypton: "La forte raie verte 5566.3 du krypton coïncide sensiblement avec la brillante raie n° 4 (5567) de l’aurore boréale. Dès lors on pour-rait peut-être désigner ce gaz sous le nom plus harmonieux d’eosium, nom qu’il prend la liberté de suggerer a Monsieur Ramsay" (The strong green line 5566.3 of krypton coincides appreciably with the brilliant line n° 4 (5567) of the "aurora borealis" (the northern light). Consequently one could perhaps give this gas the more harmonious name of Eosium, I take the freedom of suggesting this name to Mr. Ramsay. Thus, Eosium from Greek εως [eos] = dawn. Obviously, Ramsay did not follow this suggestions, since the name Eosium is never heard of. (Until Marco Fontani found it again, thanks Marco).

Swedish stamp of 1964 in the series of Nobel Prize winners. This stamp shows Ramsay and I.P. Pavlov.

Further reading:

• W. Ramsay, The Gases of the Atmosphere: The History of Their Discovery. London: Macmillan, 1915.
• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 764-765.
• Edelgasse. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 1 (1926).
• Fontani, Marco, Mariagrazia Costa, and Mary Virginia Orna, "Sir William Ramsay; Il piu «nobile» tra i chimici". La Chimica e l’Industria 86, no. 5, Giugno 2004.