insensible vs iron-hearted

insensible

adj
  • Incapable of emotional feeling; callous; apathetic. 

  • Incapable or deprived of physical sensation. 

  • Unable to be perceived by the senses. 

  • Incapable of mental feeling; indifferent. 

  • Not sensible or reasonable; meaningless. 

  • Unable to be understood; unintelligible. 

iron-hearted

adj
  • pitiless; inflexibly unsympathetic 

How often have the words insensible and iron-hearted occurred in a corpus of books? (source: Google Ngram Viewer )