Definition of "Mercurial"

He firſt ſubſiſts in Jupiter, the artificer of the world; next, among the ſupermundane gods; in the third place, among the liberated gods; fourthly, in the planet Mercury; fifthly, in the Mercurial order of demons; […] Likewiſe in this city a certain man once flouriſhed, full of the Mercurial power, becauſe his ſoul formerly exiſted in the heavens of the Mercurial order. […] But afterwards a demon, becauſe from the god Mercury, through a Mercurial demon, gifts of this kind are tranſmitted to a Mercurial ſoul.

1792, The Phædrus of Plato; a Dialogue Concerning Beauty and Love, London: […] Edward Jeffery, […]; R. Faulder, […]; J. Cook, […]; and T. and J. Merrill, […], page 126

The alchemists too spoke of the return of the Mercurial spirit as a ‘celestial rain’, or ‘dew’ (Norton’s stilla roris madidi, Dee’s ros caeli), a ‘divine water’ falling from the ‘heaven’ of the vessel to reanimate the blackened earth of the Stuff.

1980, Charles Nicholl, “The Transmutation of King Lear”, in The Chemical Theatre, London, Boston, Mass., Henley-on-Thames: Routledge & Kegan Paul, section “The dew”, page 205

In addition, the magnetometer on MESSENGER will characterize Mercury’s magnetic field in detail from orbit over four Mercurial years.

2007, Pamela Elizabeth Clark, Dynamic Planet: Mercury in the Context of Its Environment, Springer Science+Business Media, page 198

Then he is called a daemon in order to make it obvious that such gifts are handed down from Mercury himself via a Mercurial daemon to a rational Mercurial soul.

2008, Marsilio Ficino, translated by Michael J. B. Allen, Commentaries on Plato: Phaedrus and Ion, Cambridge, Mass., London: Harvard University Press, page 187

However, despite these talents, despite the fact that he saw Mercurial as well as Jovial strains in his paternity, and despite his opinion that Mercury shared with Jupiter a special need of rehabilitation, Lewis claimed no special relationship with the planet of the second sphere. In fact, he confesses that the Mercurial essence is almost beyond his grasp: […]

2008, Michael Ward, Planet Narnia: The Seven Heavens in the Imagination of C. S. Lewis, Oxford University Press

Looking at it in the apocalypse of the Mercurial dawn, it’s impossible to believe it’s not alive.

2012, Kim Stanley Robinson, “Prologue”, in 2312, Orbit, page 3

We concluded at the end of Chapter 3 that the Mercurial or Herculean French king ruled over the fickle multitude by eloquence, but this woodcut might have suggested that not only “virtuous” eloquence but also “virtùous” eloquence with cunning and deceit, whose significance Machiavelli explored in his works, was needed to govern the people.

2013 October, Takashi Nishi, The Representations of Hercules and Hydra in Shakespeare’s Coriolanus, page 196