digester vs water boiler

digester

noun
  • A strong closed vessel in which bones or other substances may be subjected, usually in water or other liquid, to a temperature above that of boiling, in order to soften them. 

  • One who, or that which, digests. 

water boiler

noun
  • An electric device which heats water up to boiling temperature. 

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