fourth estate vs periodical

fourth estate

noun
  • Journalism or journalists considered as a group; the press. 

periodical

noun
  • A regularly issued thematic publication that contains the most current information in its field, often the primary means for communication of original scholarship or creative work at the cutting edge of research in its field. 

  • A publication issued regularly, but less frequently than daily. 

adj
  • Periodic. 

  • Published at regular intervals of more than one day, especially weekly, monthly, or quarterly. 

  • Of, or relating to such a publication. 

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