firmness vs slackness

firmness

noun
  • security; steadfastness; good grip 

  • strictness; mental strength 

  • physical durability; rigidness (of material) 

slackness

noun
  • A subgenre of dancehall music with sexual lyrics. 

  • The state of being slack; the quality of having slack. 

  • Vulgarity in West Indian culture. 

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