essay vs treatise

essay

noun
  • A written composition of moderate length, exploring a particular issue or subject. 

  • A proposed design for a postage stamp or a banknote. 

verb
  • To move forth, as into battle. 

treatise

noun
  • A formal, usually lengthy, systematic discourse on some subject. 

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