criterion vs litmus

criterion

noun
  • A standard or test by which individual things or people may be compared and judged. 

litmus

noun
  • A simple test of any attribute; a litmus test. 

  • A dyestuff extracted from certain lichens, that changes color when exposed to pH levels greater than or less than certain critical levels. 

  • A simple test of acidity in a liquid using litmus, usually in the form of litmus paper. 

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