i 3 4 CORRELATION OF PHYSICAL FORCES. 



of opinion : we have, in support of it, the general fact that 

 magnets do deteriorate in the course of years ; and we have 

 the further general fact of the instability, or fluxional state, 

 of all nature, when we have an opportunity of fairly investi- 

 gating it at different and remote periods : in many cases, how- 

 ever, the action is so slow that the changes escape human 

 observation, and, until this can be brought to bear over a pro- 

 portionate period of time, the proposition cannot be said to 

 be experimentally or inductively proved, but must be left to 

 the mental conviction of those who examine it by the light of 

 already acknowledged facts. 



All cases of static force present the same difficulty : thus, 

 two springs pressing against each other would be said to be 

 exercising force ; and yet there is no resulting action, no heat, 

 no light, &c. 



So if gas be compressed by a piston, at the time of com- 

 pression heat is given off; but when this is abstracted, 

 although the pressure continues, no further heat is eliminated. 

 Thus, by an equilibrium produced by opposing forces, motion 

 is locked up, or in abeyance, as it were, and may be again de- 

 veloped when the forces are relieved from the tension. But 

 in the first instance, in producing the state of tension, force has 

 to be employed ; and as we have said in treating of mecha- 

 nical force, so with the other forces the original change which 

 disturbs equilibrium produces other changes which go on 

 without end. Thus, by the act of charging a Leyden phial, 

 the cylinder, the rubber, and the adjoining portions of the 

 electrical machine have each and all their states changed, and 

 thence produce changes in surrounding bodies ad infinitum ; 

 when the jar is discharged, converse changes are again produced. 



The term potential energy has of late been much used to 

 signify locked up force, and though I am far from denying its 

 occasional utility, I am a little fearful of its extension ; like 

 other words, it has only a relative application ; a heated globe 

 has a potential energy capable of becoming active when 

 brought within colder surroundings, but none if all around it 

 be of the same temperature as itself. A piece of ice may be 

 said to have a potential energy when brought into warmer 

 surroundings. Oxygen has a potential energy when brought 



