liquefied petroleum gas vs naphtha

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. 

naphtha

noun
  • Any of a wide variety of aliphatic or aromatic liquid hydrocarbon mixtures distilled from petroleum or coal tar, especially as used in solvents or petrol. 

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