placidity vs serenity

placidity

noun
  • The state of being placid; peacefulness. 

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. 

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