Mercury

From TARDIS Index File, the free Doctor Who reference.

For the planet of the same name, see Mercury.

Mercury is a chemical element required for the proper functioning of the TARDIS' fluid links. The Doctor falsely claimed that a fluid link had run out of mercury as a ruse to explore the Dalek City. (DW: The Daleks)

The Doctor visited the Panjistri homeworld to stock up on mercury. (NA: Timewyrm: Apocalypse)

Later, the mercury in the fluid links evaporated, forcing the Doctor and Jamie McCrimmon to evacuate the TARDIS. (DW: The Wheel in Space)

The Doctor acquired some needed mercury in 1931 Chicago, unknowingly defeating the Master's plans by doing so. (VD: The Duke of Dominoes)

The planet Vulcan had pools of fuming mercury on its surface. (DW: The Power of the Daleks)

Skaro had geysers of mercury, where the Dalek Emperor attempted to kill Zeg. (DC: Duel of the Daleks)