THE CONSTITUTION OF NATURE. 19 



can be said to be converted into heat. In no case is the 

 force which produces the motion annihilated or changed 

 into anything else. The mutual attraction of the earth 

 and weight exists when they are in contact, as when they 

 were separate; but the ability of that attraction to employ 

 itself in the production of motion does not exist. 



The transformation, in this case, is easily followed by the 

 mind's eye. First, the weight as a whole is set in motion 

 by the attraction of gravity. This motion of the mass is 

 arrested by collision with the earth, being broken up into 

 molecular tremors, to which we give the name of heat. 



And when we reverse the process, and employ those 

 tremors of heat to raise a weight which is done through 

 the intermediation of an elastic fluid in the steam-engine 

 a certain definite portion of the molecular motion is 

 consumed. In this sense, and in this sense only, can the 

 heat be said to be converted into gravity ; or, more cor- 

 rectly, into potential energy of gravity. Here the destruc- 

 tion of the heat has created no new attraction ; but the old 

 attraction has conferred upon it a power of exerting a cer- 

 tain definite pull, between the starting-point of the falling 

 weight and 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 anni- 

 hilated, 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, while, 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 mat- 

 ter 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 constancy of 

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

 MOTION ; or it may exist in the form of FOKCE, with dis- 

 tance 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 converti- 

 bility of natural forces consists solely in transformations of 

 dynamic into potential, and of potential into dynamic 



