Swamp gas

Swamp gas was a type of highly volatile gas that would very easily ignite. Swamp gas was, as its name suggests, found in swamps, in areas were water, plants and decomposing material made explosive fumes that had some sort of scent.

The Fourth Doctor tricked the assassin into firing his rifle while surrounded by a cloud of swamp gas. This subsequently set him on fire. (TV: The Deadly Assassin)

The Third Doctor used his sonic screwdriver to ignite swamp gas. (TV: Carnival of Monsters)