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)