reflux vs slackening

reflux

noun
  • The backwards flow of any fluid. 

  • A technique, using a reflux condenser, allowing one to boil the contents of a vessel over an extended period. 

  • The leaking of stomach acid up into the oesophagus. 

verb
  • To boil a liquid in a vessel having a reflux condenser 

  • To flow back or return. 

slackening

noun
  • The act by which something slackens; loss of speed, tautness, etc. 

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