pilot light vs soot

pilot light

noun
  • A small, persistent flame used to ignite gas. 

  • A small telltale lamp on a dynamo or battery circuit whose brightness indicates the voltage of the current. 

soot

noun
  • Fine black or dull brown particles of amorphous carbon and tar, produced by the incomplete combustion of coal, oil etc. 

verb
  • To cover or dress with soot. 

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