Praxis
From Tardis Wiki, the free Doctor Who reference
Praxis gas (or gas of the Praxis range) was a type of gas. Celery would turn purple in Praxis gas. (DW: The Caves of Androzani)
The Doctor was allergic to Praxis gas and so kept a sprig of celery on his lapel. (DW: The Caves of Androzani)
The Already Dead soldiers used Flechette guns which used compressed Praxis gas. (TWN: The Undertaker's Gift)
