serenity vs tranquillity

serenity

noun
  • The state of being serene; calmness; peacefulness. 

  • A lack of agitation or disturbance. 

  • A title given to a reigning prince or similar dignitary. 

tranquillity

noun
  • The state of being tranquil; peacefulness, the absence of disturbance or stress; serenity; calm. 

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