a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures
"The alchemist carefully poured quicksilver into the flask to begin his experiment."
The metal mercury.
"The old alchemist warned me never to touch the quicksilver in the vial, as it would stain my skin for life."
To overlay with quicksilver.
"The alchemist carefully poured molten quicksilver over the heated coin to seal its surface."
liable to sudden unpredictable change
"erratic behavior"
"fickle weather"
"mercurial twists of temperament"
"a quicksilver character, cool and willful at one moment, utterly fragile the next"
Unpredictable, erratic or fickle; mercurial.
"The team's star player was quicksilver in his decision-making, often changing his mind before a single play began."