"a yellow, choking cloud of brimstone"

 

Brimstone
GNU Free Documentation LicenseBrimstone - Credit: de:User:Man77

Brimstone is sulfur.  English translations of the Bible commonly referred to burning sulfur as brimstone, giving rise to the term 'fire-and-brimstone' sermons, in which listeners are reminded of the fate of eternal damnation that awaits the unbelieving and unrepentant.  Bartimaeus is trying his best to scare the magician who dared summon him.