FUNnA>rEXTAL FORMULATIONS OF ELECTRODYNAMICS. 
235 
indispiitcibl6, and it se6ins difficult to realise a state of affairs where the eejuation 
without the last term would he generally true. 
The whole question of mechanical forces on polarised media is ultimately liound up 
with the question of the variation of the intrinsic energy of those media, and the 
expression 
for the ;r-component of the forcive per unit volume implies that the internal energy of 
those media change in a small displacement hy 
(H^I) 
per unit volume. But when the media are in motion the expression for the change SI 
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 
C / 
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 dJlfclt, 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 
as equivalent to a distribution of magnetic polarity of intensity 
[P-v] 
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 
