150 COEEELATION OF PHYSICAL FOKCES. 



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 

 developed 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 

 mechanical 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 infini- 

 tum ; when the jar is discharged, converse changes are again 

 produced. 



As with heat, light, and electricity, the daily accumulating 

 observations tend to show that each change in the phenomena 

 to which these names are given is accompanied by a change 

 either temporary or permanent in the matter affected by them ; 

 so many recent experiments on magnetism have connected 

 magnetic phenomena with a molecular change in the subject 

 matter. Thus M. Wertheim has shown that the elasticity of 

 iron and steel is altered by magnetisation ; the co-efficient of 

 elasticity in iron being temporarily, in steel permanently 

 diminished. 



He has also examined the effects of torsion upon magnet- 

 ised iron, and concludes, from his experiments, that in a bar 



