THE CONSTITUTION OF NATURE. 23 



against its neighbour, and recoils. The ultimate 

 amplitude of the recoil being attained, the motion of 

 the atom in that direction is checked, and for an 

 instant its energy is all potential. It is then drawn 

 towards its neighbour with accelerated speed ; thus, by 

 attraction, converting its potential into dynamic 

 energy. Its motion in this direction is also finally 

 checked, and again, for an instant, its energy is all 

 potential. It once more retreats, converting, by re- 

 pulsion, its potential into dynamic energy, till the 

 latter attains a maximum, after which it is again 

 changed into potential energy. Thus, what is true of 

 the 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, and rhythm within rhythm. 



When a body is heated, a change of molecular 

 arrangement always occurs, and to produce this change 

 heat is consumed. Hence, a portion only of the heat 

 communicated to the body remains as dynamic energy. 

 Looking back on some of the statements m;ide 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 molecular dynamic energy, developed 

 at the moment of collision, is not the exact equivalent 

 of the sensible dynamic energy destroyed. The true 

 equivalent is this heat, plus the potential energy con- 

 ferred upon the molecules by the placing of greater 

 distances between them. This molecular potential 

 energy 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 



3 



