346 THE MECHANICAL EQUIVALENT OF HEAT, 



tinction from matter) imponderable objects. (Conf. paper al- 

 ready quoted, pp. 328, 329.) 



It is easy to see that this definition embraces, among other 

 things, the fact that the motion which disappears in mechani- 

 cal processes of different kinds bears a constant relation to 

 the heat thereby produced, or that motion is convertible, as 

 an indestructible magnitude, into heat. Thus heat is, like mo- 

 tion, a force ; and motion, like heat, an imponderable. 



I have characterized the relation which various forces 

 bear to one another by saying (Phil. Mag. S. 4, vol. xxiv. p. 

 252) that they are " different forms under which one and the 

 same object makes its appearance." At the same time I 

 have expressly guarded myself from making the certainly 

 plausible, but unproved, and, as it seems to me, hazardous de- 

 duction that thermal phenomena are to be regarded as merely 

 phenomena of motion. The following is what I said upon 

 this point (loc. cit.) p. 376 : 



" But just as little as the connection between falling-force 

 and motion authorizes the conclusion that the essence of fall- 

 ing-force is motion, can such a conclusion be adopted in the 

 case of heat. We are, on the contrary, rather inclined to 

 infer that before it can become heat, motion whether simple, 

 or vibratory as in the case of light and radiant heat, &c. 

 must cease to exist as motion." 



The relation which, as we have seen, subsists between 

 heat and motion has regard to quantity, not to quality ; for 

 (to borrow the words of Euclid) things which are equal to 

 one another are not therefore similar. Let us beware of leav- 

 ing the solid ground of the objective, if we would not entan- 

 gle ourselves in difficulties of our own making. 



In the mean time it at least results from the foregoing 

 considerations that the phenomena of heat, electricity, and 

 magnetism do not owe their existence to any particular fluids ; 

 and the immateriality of heat, asserted half a century ago by 



