combust vs set afire

combust

verb
  • To burn; to catch fire. 

  • To erupt with enthusiasm or boisterousness. 

adj
  • In close conjunction with the sun (so that its astrological influence is "burnt up"), sometimes specified to be within 8 degrees 30'. 

set afire

verb
  • Synonym of set on fire 

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