foment vs spark off

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. 

spark off

verb
  • To initiate something by providing the necessary conditions. 

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