THE NEW DOCTRINE OF FORCES. Xlll 



of motion in ordinary matter ; forms of energy which are capable 

 of mutual conversion. Heat is a mode of energy manifested by 

 certain effects. It may be transformed into electricity, which is 

 another form of force producing different effects. Or the process 

 may be reversed ; the electricity disappearing and the heat reap- 

 pearing. Again, mechanical motion, which is a motion of masses, 

 may be transformed into heat or electricity, which is held to be a 

 motion of the atoms of matter, while, by a reverse process, the mo- 

 tion of atoms, that is, heat or electricity, may be turned back again 

 into mechanical motion. Thus a portion of the heat generated in 

 a locomotive is converted into the motion of the train, while by 

 the application of the brakes the motion of the train is changed 

 back again into the heat of friction. 



These mutations are rigidly subject to the laws of quantity. A 

 given amount of one force produces a definite quantity of another . 

 so that power or energy, like matter, can neither be created nor 

 destroyed : though ever changing form, its total quantity in the uni- 

 verse remains constant and unalterable. Every manifestation of 

 force must have come from a preexisting equivalent force, and must 

 give rise to a subsequent and equal amount of some other force. 

 When, therefore, a force or effect appears, we are not at liberty to 

 assume that it was self-originated, or came from nothing ; when it 

 disappears we are forbidden to conclude that it is annihilated : we 

 must search and find whence it came and whither it has gone ; that 

 is, what produced it and what effect it has itself produced. These 

 relations among the modes of energy are currently known by the 

 phrases Correlation and Conservation of Force. 



The present condition of the philosophy of forces is perfectly 

 paralleled by that of the philosophy of matter toward the close of 

 the last century. So long as it was admitted that matter in its 

 various changes may be created or destroyed, chemical progress 

 was impossible. If, in his processes, a portion of the material dis- 

 appeared, the chemist had a ready explanation the matter was 

 destroyed; his analysis was therefore worthless. But when he 



