add fuel to fire vs foment

add fuel to fire

foment

verb
  • To incite or cause troublesome acts; to encourage; to instigate. 

  • To apply a poultice to; to bathe with a cloth or sponge. 

noun
  • Fomentation. 

How often have the words add fuel to fire and foment occurred in a corpus of books? (source: Google Ngram Viewer )