In The Discarded Image, his otherwise marvelous introduction to the medieval model of the world, C. S. Lewis tells us:
Mercury produces quicksilver. Dante gives his sphere to beneficent men of action. Isidore, on the other hand, says this planet is called Mercurius because he is the patron of profit (mercibus praeest). Gower says that the man born under Mercury will be 'studious' and ' in writinge curious',
bot yit with somdel besinesse
his hert is set upon richesse.
(Confessio, vn, 765.)
The Wife of Bath associates him especially with clerks (D 706). In Martianus Capella's De Nuptiis he is the bridegroom of Philologia - who is Learning or even Literature rather than what we call 'philology'. And I am pretty sure that 'the Words of Mercury' contrasted with 'the Songs of Apollo' at the end of Love's Labour's Lost are 'picked', or rhetorical prose. It is difficult to see the unity in all these characteristics. ' Skilled eagerness' or 'bright alacrity' is the best I can do. But it is better just to take some real mercury in a saucer and play with it for a few minutes. That is what 'Mercurial' means.
The Discarded Image, 107-08.Better to play with mercury?