FUNDAMENTAL FORMULATIONS OF ELECTRODYNAMICS. '235 



indisputable, and it seems difficult to realise a state of affairs where the equation 

 without the last term would be generally true. 



The whole question of mechanical forces on polarised media is ultimately bound up 

 with the question of the variation of the intrinsic energy of those media, and the 

 expression 



for the .x-component of the forcive per unit volume implies that the internal energy of 

 those media change in a small displacement by 



(H fl) 



per unit volume. But when the media are in motion the expression for the change 8]. 

 used in this expression involves a part due to the convection of the polarisation which 

 is more properly concerned with the mechanical forces than with the intrinsic elastic 

 or motional ones, as it would exist if the internal constitution of the media was 

 maintained rigidly constant. It is therefore suggested that the result derived above. 

 that the expression for the rate of change of the intrinsic energy is practically 



per unit volume, is the more legitimate form of this expression, as allowance is made 

 in it for the convection, and if this is granted, then the equivalent expression for the 

 mechanical forcive, viz., 



must be regarded as the only adequate form. 



Moreover these two expressions essentially involve the particular form of equation 

 adopted for defining dR/dt, and are the only ones which are capable of fitting in with 

 a general relativity theory. 



The results here derived also emphasise the difficulties involved in treating the 

 currents due to the convection of polarised media as effectively equivalent to a 

 polarisation of the opposite kind. If, for example, we had treated the convection 



current 



Curl [> m ] 



as equivalent to a distribution of magnetic polarity of intensity 



at each place from the outset we should have been led to an entirely erroneous 

 expression for the forcive on the polarised media, the reason. being that the inclusion 



