24 FRAGMENTS OF SCIENCE. 



on 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 dis- 

 tributed 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 together 

 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 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 constituents of the earth's crust. They took 

 their present form in obedience to molecular force ; they 

 turned their potential energy into dynamic, and yielded 

 it as radiant heat to the universe, ages before man ap- 

 peared 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 com- 

 parison 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 

 circumstanced as to be able to produce motion, they are 

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

 a moment 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 the attraction, and in falling can perform work. 

 When it rests at its lowest level it is not a source of 

 power or energy, because it can fall no farther. But 



