Praxis
From Tardis Wiki, the free Doctor Who reference
Praxis gas was a type of gas. Celery would turn purple in Praxis gas. The Fifth Doctor was allergic to gas in the Praxis range 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)
