26 FRAGMENTS OF SCIENCE. 



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 

 correctly, into potential energy of gravity. Here the 

 destruction of the heat has created no new attraction ; 

 but the old attraction has conferred upon it a power of 

 exerting a certain 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 ' gene- 

 rated,' they do not mean thereby that old attractions 

 have been annihilated, and new ones brought into exist- 

 ence, but that, in the one case, the power of the attrac- 

 tion to produce motion has been diminished by the 

 shortening of the distance 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 sen- 

 sible 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 



