serious vs solemn

serious

adj
  • Really intending what is said (or planned, etc); in earnest; not jocular or deceiving 

  • Committed. 

  • Without humor or expression of happiness; grave in manner or disposition 

  • Important; weighty; not insignificant 

adv
  • In a serious manner; seriously. 

solemn

adj
  • Inspiring serious feelings or thoughts; sombrely impressive. 

  • Characterized by or performed with appropriate or great ceremony or formality. 

  • Of or pertaining to religious ceremonies and rites; (generally) religious in nature; sacred. 

  • Deeply serious and sombre; grave. 

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