MATTER, MOTION, AND FORCE 



ables gravity to reveal itself by causing down- 

 ward motion. So when nitrous oxide is decom- 

 posed into nitrogen and oxygen, a considerable 

 amount of heat is evolved ; but when all this 

 thermal undulation is restored under appro- 

 priate conditions, and the compound is again 

 formed, it is not that the thermal undulation 

 gives rise to the chemism which draws the atoms 

 of nitrogen and oxygen together ; it is only 

 that the thermal undulation results in such a 

 redistribution of the atoms that their progress 

 toward each other is unimpeded, and thus the 

 latent force of chemism is revealed. 



Now the law of the correlation of forces, which 

 perhaps ought rather to be called the law of the 

 transformation of motion, is simply the obverse 

 of that corollary from the persistence of force, 

 .which affirms that whatever energy has been 

 expended in doing work must reappear as en- 

 ergy. The energy of molar motion which dis- 

 appears when an arrow sticks in its target is 

 really transformed into the energy of molec- 

 ular motion which is recognized partly as heat 

 and partly as electricity. That the different 

 modes of motion are transformable into each 

 other is now one of the commonplaces of phy- 

 sical science, and needs but little illustration 

 here. What is called the arrest of motion by 

 friction is now known to be the change of molar 

 motion into heat, when the rubbing substances 

 J 55 



