THE CONSTITUTION OF NATURE. 31 



tremors of heat to raise a weight, as is done through the 

 intermediation of an elastic fluid in the steam-engine, a 

 certain definite portion of the molecular motion is de- 

 stroyed in raising the weight. In this sense, and this sense 

 only, can the heat be said to be converted into gravity, or, 

 more correctly, into potential energy of gravity. It is not 

 that the destruction of the heat has created any new 

 attraction, but simply that the old attraction has now a 

 power conferred upon it, of exerting a certain definite pull 

 in the interval between the starting-point of the falling 

 weight and its collision with the earth. 



When, therefore, writers on the conservation of energy 

 speak of tensions being " consumed " and " generated," 

 they do not mean thereby that old attractions have been an- 

 nihilated, and new ones brought into existence, but that, 

 in the one case, the power of the attraction to produce 

 motion has been diminished by the shortening of the dis- 

 tance between the attracting bodies, and that in the other 

 case the power of producing motion has been augmented 

 by the increase of the distance. These remarks apply to 

 all bodies, whether they be sensible masses or molecules. 



Of the inner quality that enables matter to attract 

 matter we know nothing; and the law of conservation 

 makes no statement regarding that quality. It takes the 

 facts of attraction as they stand, and affirms only the con- 

 stancy of working-power. That power may exist in the 

 form of MOTION ; or it may exist in the form of FORCE, with 

 distance to act through. The former is dynamic energy, 

 the latter is potential energy, the constancy of the sum of 

 both being affirmed by the law of conservation. The con- 

 vertibility of natural forces consists solely in transforma- 

 tions of dynamic into potential, and of potential into dy- 

 namic energy, which are incessantly going on. In no other 

 sense has the convertibility of force, at present, any scien- 

 tific meaning. 



