Definitions of mercurial:

• adjective:   relating to or containing or caused by mercury

  Example: "Mercurial preparations"

• adjective:   relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury

  Example: "More than Mercurial thievishness"

• adjective:   relating to or under the (astrological) influence of the planet Mercury

  Example: "The Mercurial canals"

• adjective:   liable to sudden unpredictable change

  Example: "Mercurial twists of temperament"