kerosene vs natural gas

kerosene

noun
  • A thin colorless to straw-colored petroleum-based fuel heavier than gasoline/petrol or naptha but lighter than diesel, used primarily as jet fuel but also for heating and lighting in some remote or impoverished areas. 

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