richen vs strengthen

richen

verb
  • To gain richness; become heightened or intensified in brilliancy. 

  • To make or render rich or richer. 

  • To become rich or richer; become superior in quality, condition or effectiveness. 

strengthen

verb
  • To augment; to improve; to intensify. 

  • To make strong or stronger; to add strength to; to increase the strength of; to fortify. 

  • To empower; to give moral strength to; to encourage; to enhearten. 

  • To reinforce, to add to, to support (someone or something) 

  • To grow strong or stronger. 

  • To substantiate; to corroborate (a belief, argument, etc.) 

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