empty space vs free space

empty space

noun
  • Vacuum; a space containing nothing at all 

  • An unoccupied space where something may be stored 

  • A feeling of longing for someone or something that is gone; a sense of a thing's absence. 

  • An unoccupied area or volume. 

free space

noun
  • vacuum; a space free of matter. 

  • Used other than figuratively or idiomatically: see free, space. 

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