improve vs richen

improve

verb
  • To make (something) better; to increase the value or productivity (of something). 

  • To become better. 

richen

verb
  • To make or render rich or richer. 

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

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

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