always vs periodically

always

adv
  • Constantly during a certain period, or regularly at stated intervals (opposed to sometimes or occasionally). 

  • At all times; throughout all time; since the beginning. 

  • In any event. 

periodically

adv
  • intermittently or recurrently 

  • In a manner that is regular or periodic. 

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