H₂O vs light water

H₂O

noun
  • Water as a chemical substance. 

  • Hishammuddin Hussein Onn, Malaysian politician and son of Hussein Onn, third prime minister of Malaysia. 

light water

noun
  • Water in its standard form of H₂O, containing two protium hydrogen atoms, as opposed to heavy water. 

  • Foam formed by water and a fluorocarbon surfactant, used in firefighting because it floats on flammable liquids lighter than water. 

How often have the words H₂O and light water occurred in a corpus of books? (source: Google Ngram Viewer )