naphtha vs natural gas

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. 

natural gas

noun
  • A mixture of gaseous hydrocarbons associated with petroleum deposits; mostly methane with smaller amounts of ethane, propane and butane; principally used as a fuel. 

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