4 MOLECULAR MOTION AND ITS ENERGY 1 



* 

 at least potential energy, as it consists of the work spent 



in overcoming cohesion, while sensible heat, which we feel 

 with our hand and measure with the thermometer, is kinetic 

 energy. 



We cannot, therefore, in general use Rumford's 1 ex- 

 pression, and say that heat is motion, but we may assume 

 that sensible heat is a mode of motion, though this motion 

 is invisible and almost unknown to us. As its carrier we 

 take the particles, supposed immeasurably small, of which 

 bodies are composed, and to these ultimate particles we 

 ascribe motions of different kinds, assuming that some may 

 move forward in straight lines, that others may oscillate 

 periodically, that others again may revolve about each other 

 in this small world imitating the planets and that each 

 may further rotate about an axis of its own ; and the sum 

 of the kinetic energies of these motions represents the 

 mechanical energy of the contained heat. 



In the mechanical theory of heat we extend this specu- 

 lation, as a rule, no further, so as not unnecessarily to make 

 our reasonings and conclusions depend on doubtful hypo- 

 theses. In this connection physical investigation has special 

 reason to avoid hypothesis, as the high value and great 

 significance of the mechanical theory of heat rest on the 

 general and unconditional validity of its propositions, 

 whereby we are enabled to measure forces of unknown 

 nature equally with known forces, and to subject them to 

 calculation with equal certainty. 



It would, however, be a censurable restriction of investi- 

 gation to follow out only those laws of nature which have 

 a general application and are free from hypothesis ; for 

 mathematical physics has won most of its successes in 

 the opposite way, namely, by starting with an unproved and 

 unprovable, but probable, hypothesis, analytically following 

 out its consequences in every direction, and determining its 

 value by comparison of these conclusions with the results of 

 experiment. 



For the mechanical theory of heat, too, this method has 

 already borne good fruit. By ascribing special forms to 



1 Phil. Trans. Ixxxviii. 1798, p. 80. 



