26 FRAGMENTS OF SCIENCE. 



exertion of a certain pull through a certain space, a, 

 body is caused to clash with a certain definite velocity 

 against the earth. Heat is thereby developed, and this 

 is the only sense in which gravity can be said to be con- 

 verted into heat. In no case is the force which pro- 

 duces 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 molecu- 

 lar 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 at- 

 traction; 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 en- 

 ergy speak of tensions being ' consumed ' and ' gene- 

 rated/ they do not mean thereby that old attractions 

 have been annihilated, and new ones brought into ex- 

 istence, but that, in the one case, the power of the at- 

 traction 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 



