Praxis gas was a type of gas in the praxis range.
Celery would turn purple in praxis gas. The Fifth Doctor was allergic to praxis gas and so kept a sprig of celery on his lapel to identify its presence. (TV: The Caves of Androzani)
The Already Dead soldiers used Flechette guns which used compressed praxis gas. (PROSE: The Undertaker's Gift)
In 2154, IMC trooper Jason Dommer had his throat scarred by praxis gas, making his voice scratchy and grating. (PROSE: Lucifer Rising)
An alien bomb in ancient Greece, with some Sontaran interference, which found its way into the British Museum, was booby trapped with praxis gas, which poisoned the Tenth Doctor when he deactivated it in 1860/1861. Elizabeth Garrett Anderson saved him with natural Quercetin in food and tea. (COMIC: Nurse Who?)