firmness vs solidity

firmness

noun
  • physical durability; rigidness (of material) 

  • security; steadfastness; good grip 

  • strictness; mental strength 

solidity

noun
  • The state or quality of being solid. 

  • Moral firmness; validity; truth; certainty. 

  • The solid contents of a body; volume; amount of enclosed space. 

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