acerbate vs sadden

acerbate

verb
  • To make bitter or sour. 

  • To exasperate; to irritate. 

adj
  • Embittered; having a sour disposition or nature. 

sadden

verb
  • To make sad or unhappy. 

  • To darken a color during dyeing. 

  • To become sad or unhappy. 

  • To render heavy or cohesive. 

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