Praxis

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 guns which used compressed Praxis gas. (TWN: The Undertaker's Gift)