fuel vs natural gas

fuel

noun
  • Substance consumed to provide energy through combustion, or through chemical or nuclear reaction. 

  • Substance that provides nourishment for a living organism; food. 

  • Something that stimulates, encourages or maintains an action. 

verb
  • To exacerbate, to cause to grow or become greater. 

  • To provide with fuel. 

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 fuel and natural gas occurred in a corpus of books? (source: Google Ngram Viewer )