heroism vs mettle

heroism

noun
  • The qualities characteristic of a hero, such as courage, bravery, fortitude, selflessness, etc.; the display of such qualities. 

mettle

noun
  • A quality of endurance and courage. 

  • Good temperament and character. 

adj
  • Spirited, vigorous, stout-hearted. 

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