A force in nature which is recognized in various effects, but
   especially in the phenomena of fusion and evaporation, and which, as
   manifested in fire, the sun's rays, mechanical action, chemical
   combination, etc., becomes directly known to us through the sense of
   feeling. In its nature heat is a mode if motion, being in general a
   form of molecular disturbance or vibration. It was formerly supposed to
   be a subtile, imponderable fluid, to which was given the name caloric.