empyrean vs the welkin

empyrean

adj
  • Of the sky or the heavens; celestially refined. 

noun
  • The region of pure light and fire; the highest heaven, where the pure element of fire was supposed by the ancients to exist: the same as the ether, the ninth heaven according to ancient astronomy, and Dante's poem. 

the welkin

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