Argon (element)

Argon was a gas.

The Tenth Doctor described fresh air as "Nitrogen, oxygen, a pinch of argon, a soupçon of carbon dioxide and a squeeze of H₂O" after filtering the ammonia from Chelsea 426 to remove Rutan spores that infected humans. (PROSE: The Taking of Chelsea 426) The Eighth Doctor could detect trace amounts of argon in Earth's atomosphere.

The Fifth Doctor kept argon discharge globe lights in the TARDIS. (TV: Frontios)

Thea Ransome performed a potassium-argon test in the dating of "Eustace". (TV: Image of the Fendahl)

Some car headlights and lamps were argon based. (PROSE: Heart of TARDIS, Tragedy Day)