28 FRAGMENTS OF SCIENCE 



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 toward its neighbor 

 with accelerated speed; thus, by attraction, converting its 

 potential into dynamic energy. Its motion in this direc- 

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

 energy is all potential. It once more 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 ex- 

 ample, 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 dy- 

 namic 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 



