apropos vs in the air

apropos

prep
  • Regarding or concerning. 

adv
  • To the purpose; appropriately. 

  • By the way. 

  • Timely; at a good time. 

adj
  • Of an appropriate or pertinent nature. 

  • by the way, incidental 

in the air

prep
  • In the surrounding ambience. 

  • Unsupported and liable to be turned or taken in flank. 

  • Not in a fixed or stable position; unsettled; up in the air. 

  • Prevalent without traceable origin or authority. 

  • In the generally prevailing feeling, idea, atmosphere, or mood. 

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