insane vs mentally ill

insane

adj
  • Exhibiting unsoundness or disorder of mind; not sane; utterly mad. 

  • Used by or relating to insane people. 

  • Causing insanity or madness. 

  • Extremely good; incredibly amazing. 

  • Characterized by insanity or the utmost folly; ridiculous; impractical. 

mentally ill

adj
  • Having a psychological disorder. 

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