hot air vs liquefied petroleum gas

hot air

noun
  • Air that has been heated, especially so as to function as the lifting agent of a hot-air balloon. 

  • Empty, confused, or exaggerated talk lacking meaning or substance; bluster. 

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. 

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