appetency vs avidity

appetency

noun
  • Strong desire; craving; powerful instinct. 

avidity

noun
  • Eagerness; intenseness of desire. 

  • Greediness; strong appetite. 

  • The measure of the synergism of the strength of individual interactions between proteins. 

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