litany vs periodicity

litany

noun
  • A prolonged or tedious list. 

  • A ritual liturgical prayer in which a series of prayers recited by a leader are alternated with responses from the congregation. 

periodicity

noun
  • The quality of being periodic; tendency to recur at regular intervals. 

  • Recurrence of a woman's periods; menstruation. 

  • The quality of a function with a repeated set of values at regular intervals. 

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