ardor vs fervor

ardor

noun
  • Intense heat. 

  • Great warmth of feeling; fervor; passion. 

  • Spirit; enthusiasm; passion. 

fervor

noun
  • Heat. 

  • An intense, heated emotion; passion, ardor. 

  • A passionate enthusiasm for some cause. 

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