firmness vs fixity

firmness

noun
  • strictness; mental strength 

  • security; steadfastness; good grip 

  • physical durability; rigidness (of material) 

fixity

noun
  • The state or condition of being fixed. 

  • The position of an operator relative to its operand (prefix, postfix) or operands (infix). 

  • Something fixed. 

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