Cardium

Cardium was a substance which radiated high levels of energy. The planet Go-Ray was rich in cardium, and the Go-Rays used it to power their civilisation. When the Doctor, John and Gillian arrived on the planet, the energy from the cardium gave them the ability to float above the ground. When a cardium reactor in the Go-Ray city malfunctioned, they acquired some mercury from a cave to replace the cardium and repair the reactor. Although the loss of power from the cardium increased the weight of the objects in the reactor room, the Doctor succeeded in getting the mercury into the reactor, repairing it. (TVC: Enter: The Go-Ray)