natural gas vs 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. 

petrol

noun
  • petroleum, a fluid consisting of a mixture of refined petroleum hydrocarbons, primarily consisting of octane, commonly used as a motor fuel. 

  • A motor vehicle powered by petrol (as opposed to diesel). 

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