hard to understand vs hermetic

hard to understand

hermetic

adj
  • Isolated, away from outside influence. 

  • Pertaining to alchemy or occult practices; magical, alchemical. 

  • Pertaining to Hermes Trismegistus or the writings attributed to him. 

  • Hermetically sealed. 

  • One who follows/worships Hermes. 

  • Pertaining ancient Greek Olympian God Hermes 

How often have the words hard to understand and hermetic occurred in a corpus of books? (source: Google Ngram Viewer )