THE CONSTITUTION OF NATURE. 29 



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 constitu- 

 ents 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. And so it 

 is with almost all the other constituents of the earth's 

 crust. They took their present form in obedience to mo- 

 lecular force ; they turned their potential energy into dy- 

 namic, and gave it 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 individual, but exceedingly minute in comparison 

 with the earth's primitive store. 



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

 power in the world consists of attractions, repulsions, and 

 motions. If the attractions and repulsions are so circum- 

 stanced 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 to the earth can perform work. When it 

 rests upon the earth's surface it is not a source of power or 

 energy, because it can fall no farther. But though it has 

 ceased to be a source of energy, the attraction of gravity still 

 acts as & force, which holds the earth and weight together. 



The same remarks apply to attracting atoms and mole- 

 cules. As long as distance separates them, they can move 

 across it in obedience to the attraction, and the motion 

 thus produced may, by proper appliances, be caused to 



