flatulence vs natural gas

flatulence

noun
  • The release of such gas; breaking wind. 

  • The state of having gas, often smelly, trapped (and when released, frequently with noise) in the digestive system of a human and some other animals; wind; and when released, a flatus, a fart. 

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