argon (ärˈgŏn) [key] [Gr., = inert], gaseous chemical element; symbol Ar; at. no. 18; at. wt. 39.948; m.p. - 189.2°C; b.p. - 185.7°C; density 1.784 grams per liter at STP; valence 0. Argon is a colorless, odorless, tasteless gas occurring in air (of which it constitutes 0.94% by volume) and in some volcanic gases. It is a member of Group 18 of the periodic table, a group called the noble or inert gases from the mistaken former belief that none of its members could form chemical compounds; in fact, other members of the group, e.g., krypton, xenon, and radon, do form compounds. Argon is prepared by fractional distillation of liquid air. Its extreme inertness has caused it to be substituted for nitrogen in electric light bulbs. It is mixed with neon in so-called neon signs (gas discharge tubes) to produce a green-to-blue glow. It is used as a protective atmosphere in arc welding, in the refining of reactive elements, and in the growing of crystals for use in semiconductor devices. Argon was first obtained by Lord Rayleigh and Sir William Ramsay in 1894. Previously Lord Rayleigh had noticed that a liter of supposedly pure nitrogen drawn from the air weighed more than a liter prepared from a nitrogen compound. This difference in weight led him to conclude that another gas was present in the supposedly pure nitrogen. Actually several unreactive gases were present; the first samples of "argon" also contained helium, neon, krypton, and xenon. Ramsay obtained pure argon later by evaporating it from liquid air.