Gas

Gas was a state of matter.

Earth at an early stage of its development was an enormous mass of superhot gases surrounding a core of molten rock. Over millions of years, the sun "eroded away" the gases, causing the planet to solidify. (AUDIO: Exploration Earth)

The Twelfth Doctor claimed to know of a race made of sentient gas who threw fireballs as a "friendly wave". (TV: Flatline)

The Doctor was susceptible to gases in the praxis range. (TV: The Caves of Androzani, et al.)

The Empty Child plague caused gas masks to be fused to the skin of people's faces. (TV: The Empty Child)

The Zircon planned to wipe out the population of Earth using a galactic cyclone created by the destruction of a dying planet. By the time it approached Earth, it was a cloud of poison gas. (COMIC: Doomcloud) By 2007, Deffry Vale High School had a sign in a classroom reading "GAS CONTROL". (TV: School Reunion )

In the early 21st century, caesofine gas was released into the Earth's atmosphere by ATMOS. (TV: The Sontaran Stratagem / The Poison Sky)

The Gelth were made of gas. When a human body decomposed, they produced gas, and the Gelth used this fact to use the corpses of Charles Dickens' driver and his coach as vehicles. (TV: The Unquiet Dead)