erratic vs mercurial

erratic

adj
  • Unsteady, random; prone to unexpected changes; not consistent. 

  • Deviating from normal opinions or actions; eccentric; odd. 

noun
  • A rock moved from one location to another, usually by a glacier. 

  • Anything that has erratic characteristics. 

mercurial

adj
  • Having a lively or volatile character; animated, changeable, quick-witted. 

  • Of or pertaining to the element mercury or quicksilver; containing mercury. 

  • Caused by the action of mercury or a mercury compound. 

  • Pertaining to the astrological influence of the planet Mercury; having the characteristics of a person under such influence (see adjective sense 1). 

  • Pertaining to Mercury, the Roman god of, among other things, commerce, financial gain, communication, and thieves and trickery; hence (comparable), money-making; crafty. 

  • Pertaining to the planet Mercury. 

noun
  • A person born under the influence of the planet Mercury; hence, a person having an animated, lively, quick-witted or volatile character. 

  • A chemical compound containing mercury. 

  • A preparation of mercury, especially as a treatment for syphilis. 

How often have the words erratic and mercurial occurred in a corpus of books? (source: Google Ngram Viewer )