28 FRAGMENTS OF SCIENCE. 



potential. It is then drawn toward 'its neighbor with 

 accelerated speed, thus, by attraction, converting its poten- 

 tial into dynamic energy. Its motion in this direction is 

 also finally checked, and, for an instant, again its energy is 

 all potential. It again retreats, converting, by repulsion, 

 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 arrange- 

 ment always occurs, and to produce this change heat is 

 consumed. Hence, a portion only of the heat communi- 

 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 collision, 

 is not the equivalent of the sensible dynamic energy de- 

 stroyed. 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, 

 bodies in the metallic condition more or less sparsely dis- 

 tributed in the earth's crust. These bodies can be oxidized, 

 and hence are, so far as they go, stores of potential energy. 



