THE CONSTITUTION OF NATURE ^ 29 



the other, constitute a vast store of energy of this kind — 

 vast, but far from infinite. We have, besides our coal- 

 fields, metallic bodies more or less sparsely distributed 

 through the earth's crust. These bodies can be oxi- 

 dized; 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 together and 

 produced the motion of heat, which was taken up by the 

 ether and carried away through stellar space. It is lost 

 forever as far as we are concerned. In those ages the 

 hot conflict of carbon, oxygen, and calcium produced the 

 chalk and limestone hills which are now cold; and from 

 this carbon, oxygen, and calcium no further energy can 

 be derived. So it is with almost all the other constit- 

 uents of the earth's crust. They took their present form 

 in obedience to molecular force; they turned their poten- 

 tial energy into dynamic, and yielded it as radiant heat 

 to the universe, ages before man appeared upon this 

 planet. For him a residue of potential energy remains, 

 vast, truly, in relation to the life and wants of an individ- 

 ual, 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, repulsions, and 

 motions. If the attractions and repulsions be so circum- 

 stanced as to be able to produce motion, they are sources 

 of working-power, but not otherwise. As stated a mo- 

 ment ago, the attraction exerted between the earth and 

 a body at a distance from the earth's surface, is a source 

 of working-power; because the body can be moved by 



