clag vs liquefied petroleum gas

clag

noun
  • Unburned carbon (smoke) from a steam or diesel locomotive, or multiple unit. 

  • Bits of rubber which are shed from tires during a race and collect off the racing line, especially on the outside of corners (cf. marbles). 

  • A glue or paste made from starch. 

  • Low cloud, fog or smog. 

verb
  • To stick, like boots in mud 

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 clag and liquefied petroleum gas occurred in a corpus of books? (source: Google Ngram Viewer )