Use the noun quicksilver when you need a more poetic way to talk about the element known as mercury. If you look closely at an old-fashioned thermometer, you can see the quicksilver inside it.
It's more common to refer to this metal — the only one that is liquid at room temperature — as mercury, but it's also correct to call it quicksilver, which describes the properties of the element very well. Quicksilver is liquid, silver colored, and is fascinating to look at. The word comes from the sense of quick that means "alive;" the Latin root is argentum vivum, which is literally "living silver."