instability vs soundness

instability

noun
  • The quality of being unstable. 

  • A state that is not in equilibrium, or in which a small change has a large irreversible effect. 

soundness

noun
  • The result or product of being sound. 

  • The property (of an argument) of not only being valid, but also of having true premises. 

  • The property of a logical theory that whenever a wff is a theorem then it must also be valid. Symbolically, letting T represent a theory within logic L, this can be represented as the property that whenever T⊢𝜙 is true, then T vDash 𝜙 must also be true, for any wff φ of logic L. 

  • The state or quality of being sound. 

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