AND MODERN PHYSICS. 179 



In his introduction to the paper, ho discusses in a 

 general way the various explanations of electric pheno- 

 mena which had been given, and points out that 



" It appears, therefore, that certain phenomena in electricity 

 and magnetism lead to the same conclusion as those of optics, 

 namely, that there is an it- the rial medium iervading all bodies, 

 and modified only in degree by their presence ; that the part* 

 of this medium are capable of being set in motion by electric 

 currents and magnets ; that this motion is communicated from 

 one part of the medium to another by forces arising from the 

 connection of those parts ; that under the action of these 

 forces there is a certain yielding depending on the elasticity of 

 these connections ; and that, therefore, energy in two different 

 forms may exist in the medium, the one form being the actual 

 energy of motion of its parts, and the other being the potential 

 energy stored up in the connection* in virtue of their elasticity. 



"Thus, then, \ve are led to the conception of a complicated 

 mechanism capable of a vast variety of motion, but at the 

 same time so connected that the motion of one part depends, 

 according to definite relations, on the motion of other parts, 

 these motions being communicated by forces arising from the 

 relative displacement of the connected parts, in virtue of their 

 elasticity. Such a mechanism must be subject to the general 

 laws of dynamics, and we ought to be able to work out all the 

 consequences of its motion, provided we know the form of the 

 relation between the motions of the parts." ^ 



These general laws of dynamics, applicable to the 

 motion of any connected system, had been developed 

 by Lagrange, and are expressed in his generalised 

 equations of motion. It is one of Maxwell's chief 

 claims to fame that he saw in the electric field a 

 connected system to which Lagrange's equations could 

 bo applied, and that ho was able to deduce the 

 mechanical and electrical actions which take place by 

 moans of fundamental propositions of dynamics. 



