24 FEAGMENTS OF SCIENCE. 



ergy is afterwards, on the cooling of the body, con- 

 verted into heat. 



Wherever two atoms capable of uniting together by 

 their mutual attractions exist separately, they form a 

 store of potential energy. Thus our woods, forests, 

 and coal-fields on the one hand, and our atmospheric 

 oxygen on the other, constitute a vast store of energy of 

 this kind vast, but far from infinite. We have, be- 

 sides our coal-fields, metallic bodies more or less sparsely 

 distributed through the earth's crust. These bodies 

 can be oxydised; and hence they are, so far as they go, 

 stores of energy. But the attractions of the great mass 

 of the earth's crust are already satisfied, and from them 

 no further energy can possibly be obtained. Ages ago 

 the elementary constituents of our rocks clashed to- 

 gether and produced the motion of heat, which was 

 taken up by the ether and carried away through stellar 

 space. It is lost for ever as far as we are concerned. 

 In those ages the hot conflict of carbon, oxygen, and cal- 

 cium produced the chalk and limestone hills which are 

 now cold; and from this carbon, oxygen, and cal- 

 cium no further energy can be derived. So it is with 

 almost all the other constituents of the earth's crust. 

 They took their present f orm in obedience to molecular 

 force; they turned their potential energy into dynamic, 

 and yielded it as radiant heat to the universe, ages be- 

 fore man appeared upon this planet. For him a residue 

 of potential energy remains, vast, truly, in relation to 

 the life and wants of an individual, but exceedingly 

 minute in comparison with the earth's primitive store. 



To sum up. The whole stock of energy or working- 

 power in the world consists of attractions, repiilsions, 

 and motions. If the attractions and repulsions be so 

 circumstanced as to be able to produce motion, they are 

 sources of working-power, but not otherwise. As stated 



