Mercury (element)

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

The Second 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. The Doctor acquired more mercury on Space Station W3. (DW: The Wheel in Space)

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

On Go-Ray, the First Doctor, John and Gillian acquired mercury to repair a cardium reactor. (TVC: Enter: The Go-Ray)

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

Skaro had geysers of mercury. The Dalek Emperor attempted to kill Zeg near one of them. (TV21: Duel of the Daleks)

Mercury poisoning was a common complaint among sailors. Amy Ivans, a passenger of the Lankester, was being poisoned with small doses of mercury, administered by De Requin. (BFA: Cryptobiosis)

A cannister of mercury was used to corrode and disable a Gold Assassin who was on board a Roman sailing vessel. (DWA: Golden Slumbers)