naphtha vs propane

naphtha

noun
  • Any of a wide variety of aliphatic or aromatic liquid hydrocarbon mixtures distilled from petroleum or coal tar, especially as used in solvents or petrol. 

propane

noun
  • An aliphatic hydrocarbon, C₃H₈, a constituent of natural gas. 

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