THE CONSTITUTION OF NATURE 31 



to clasli with a certain definite velocity against tlie earth. 

 Heat is thereby developed, and this is the only sense in 

 which gravity 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 attrac- 

 tion 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 con- 

 sumed. 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 attrac- 

 tion 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 ''generated,'* 

 they do not mean thereby that old attractions have been 

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

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

 motion has been diminished by the shortening of the dis- 



