THE CONSTITUTION OF NATVltK. 17 



earth, as she swings to and fro in her yearly journey round 

 the sun, is also true of her minutest atom. We have wheels 

 within wheels, arid rhythm within rhythm. 



When a body is heated, a change of molecular arrange- 

 ment always occurs, and to produce this change heat is 

 consumed. Hence, a portion only of the heat c'ommuni- 

 cated to the body remains as dynamic energy. Looking 

 back on some of the statements made at the beginning of 

 this article, now that our knowledge is more extensive, we 

 see the necessity of qualifying them. When, for example, 

 two bodies clash, heat is generated; but the heat, or molec- 

 ular dynamic energy, developed at the moment of colli- 

 sion, is not the exact equivalent of the sensible dynamic 

 energy destroyed. The true equivalent is this heat, plus 

 the potential energy conferred upon the molecules by the 

 placing of greater distances between them. This molecular 

 potential energy is afterward, on the cooling of the body, 

 converted 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, besides our coal- 

 fields, metallic bodies more or less sparsely distributed 

 through the earth's crust. These bodies can be oxydized; 

 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 constitu- 

 ents of our rocks clashed together and produced the 

 motion of heat, which was taken up by the ether and car- 

 ried 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 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 appeared upon this planet. For him a residue of 

 potential energy remains, vast, truly, in relation to the life 



