inflammable vs knife-edge

inflammable

noun
  • Any inflammable substance. 

adj
  • Incapable of burning; not easily set on fire. 

  • Capable of burning; easily set on fire. 

  • Easily excited; set off by the slightest excuse; easily enraged or inflamed. 

knife-edge

noun
  • Used other than figuratively or idiomatically: the edge of a knife. 

  • A piece of steel sharpened to an acute edge or angle, and resting on a smooth surface, serving as the axis of motion of a pendulum, scale beam, or other piece required to oscillate with the least possible friction. 

  • A precarious balance that could be upset by a very small force in either direction. 

How often have the words inflammable and knife-edge occurred in a corpus of books? (source: Google Ngram Viewer )