vibrations of molecules, causing and caused by vibrations of
     the ether, constitute heat, is discussed. See vol. ii. p.
     587, 2nd ed. In the _Philosophy of the Inductive Sciences_,
     2nd ed., 1847, p. 239, Whewell remarks, _à propos_ of
     Bacon's definition of heat, 'that it is an expansive,
     restrained motion, modified in certain ways, and exerted in
     the smaller particles of the body;' that 'although the exact
     nature of heat is still an obscure and controverted matter,
     the science of heat now consists of many important truths;
     and that to none of these truths is there any approximation
     in Bacon's essay.' In point of fact, Bacon's statement,
     however much open to criticism, does contain a distinct
     approximation to the most important of all the truths
     respecting heat which had been discovered when Whewell
     wrote.

     [J] Perhaps I ought rather to say Button's axiom. For that
     great naturalist and writer embodied the principles of sound
     geology in a pithy phrase of the _Théoris de la Terre_:
     'Pour juger de ce qui est arrivé, et même de ce qui
     arrivera, nous n'avons qu'à examiner ce qui arrive.'

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