Doctor Who’s first script editor was a man named David Whitaker and was probably the bridge between the Drama Department and the BBC’s intent for Doctor Who as a partially educational series for children. In comparison to the Dennis Spooner script edited season 2, the dialogue leans to a more mature level of speech, if a bit formal. In terms of story there is a sense of trying to get reality grounded into the fantastical elements and Whitaker plays to an audience of children who are probably more aware of historical events then those today.
There is of course an element of Victorian and Edwardian science in his writing, not able the inclusion of mercury as a vital component of the running of the TARDIS. How it is supposed to work is anyone’s guess, but in the (Whitaker edited) first Dalek story, the TARDIS is rendered immobile when the fluid link is sabotaged by the Doctor.
Mercury is later featured as a component part in the set up of Maxtible’s laboratory and later in the Wheel in Space, the Doctor is forced to abandoned the TARDIS thanks to the Mercury evaporating in the console room.
The Power of the Daleks features a Mercury swamp as the location point at which the TARDIS arrives and it is implied that the colonists are collecting Mercury from the swamp for technical use.
It seems David Whitaker was fascinated by Mercury.
Fandango’s One Word Challenge: Fluid