All 3 Uses of

mercurial
in
Ulysses, by James Joyce

Conscious that the human organism, normally capable of sustaining an atmospheric pressure of 19 tons, when elevated to a considerable altitude in the terrestrial atmosphere suffered with arithmetical progression of intensity, according as the line of demarcation between troposphere and stratosphere was approximated from nasal hemorrhage, impeded respiration and vertigo, when proposing this problem for solution, he had conjectured as a working hypothesis which could not be proved impossible that a more adaptable and differently anatomically constructed race of beings might subsist otherwise under Martian, Mercurial, Veneral, Jovian, Saturnian, Neptunian or Uranian sufficient and equivalent co†
Chpt 17

Definitions:

(1) (mercurial as in: mercurial twists of temperament)
tending to change quickly, frequently, and unpredictably
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, mercurial can mean "related to mercury," as in "a mercurial thermometer." When capitalized, Mercurial refers to the planet Mercury or to the mythological Roman god.