mercury
liquid metal; innermost planet
Today this word meansThe metal that is liquid at room temperature — and the planet closest to the Sun.
From Mercurius, the Roman name of Hermes, swiftest messenger of the gods.
The metal rolls away like beads and won't be caught; the planet moves fastest across the sky. Both are "quick and ungraspable."
Mercurial, meaning volatile, comes from the same image.