liquefied petroleum gas vs mephitis

liquefied petroleum gas

noun
  • A mixture of propane and butane with traces of some other gases, which is kept in a liquid state at high pressure in metal bottles, and used as a source of gas for cooking, heating and as a fuel in internal combustion engines in vehicles. 

mephitis

noun
  • A poisonous or foul-smelling gas, especially as emitted from the earth; an unpleasant smell. 

  • A dilution of fluids derived from skunks or polecats. 

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