570 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



can shew that electrified bodies in a dielectric medium will act on one another 

 with forces obeying the same laws as are established by experiment. 



The energy, by the expenditure of which electrical attractions and repul- 

 sions are produced, we suppose to be stored up in the dielectric medium which 

 surrounds the electrified bodies, and not on the surface of those bodies them- 

 selves, which on our theory are merely the bounding surfaces of the air or other 

 dielectric in which the true springs of action are to be sought. 



Note on the Attraction of Gravitation. 



(82) After tracing to the action of the surrounding medium both the 

 magnetic and the electric attractions and repulsions, and finding them to depend 

 on the inverse square of the distance, we are naturally led to inquire whether 

 the attraction of gravitation, which follows the same law of the distance, is 

 not also traceable to the action of a surrounding medium. 



Gravitation differs from magnetism and electricity in this ; that the bodies 

 concerned are all of the same kind, instead of being of opposite signs, like 

 magnetic poles and electrified bodies, and that the force between these bodies 

 is an attraction and not a repulsion, as is the case between like electric and 

 magnetic bodies. 



The lines of gravitating force near two dense bodies are exactly of the 

 same form as the lines of magnetic force near two poles of the same name ; 

 but whereas the poles are repelled, the bodies are attracted. Let E be the 

 intrinsic energy of the field surrounding two gravitating bodies M lt M tl and 

 let E* be the intrinsic energy of the field surrounding two magnetic poles, 

 TO,, m^ equal in numerical value to M H v and let X be the gravitating 

 force acting during the displacement Sx, and X' the magnetic force, 



now X and X' are equal in numerical value, but of opposite signs ; so that 



8E= -8R, 

 or E=C-E' 



where a, ft, y are the components of magnetic intensity. If R be the resultant 



