Praxis

Praxis gas (or gas of the Praxis range) was a type of gas. Celery in Praxis gas would turn purple. (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)