242 
ME. G. H. LIVENS ON THE 
change of momentum, so that we are rather forced to regard these outstanding terms 
as pointing to the failure of the ideas from which we set out. This conclusion does 
not, of course, invalidate the results derived in the simpler electron theory, as the 
concept of momentum will remain under the simplest conditions as a convenient 
mathematical expression for the actual result, whatever be its ultimate physical basis. 
The present formulation possesses another disadvantage which is apparently not 
inherent in the simplest presentations of the momentum idea. In the electron theory, 
as usually developed, the momentum remains as a fundamental quantity and is 
distributed over the field with the density 
1 
Ittc 
[EB] 
at each point; this gives it a purely sethereal constitution as the vectors E and B are 
those which define the conditions in the aethereal field. In the present formulation 
the vector B is replaced by the vector H which is essentially an auxiliary mechanical 
vector in the theory ; the fimdamental nature of the momentum vector is therefore 
entirely lost. We can, of course, assume that some of the momentum is in reality 
attached to the matter, and such an assumption has certain points in its favour. 
The force of electromagnetic origin on the dielectric media for example has an 
X component which per unit volume is 
0E 
“dx 
[P..,,.], - [P C«rl El + 1 
— B 
dt/ 
and this may be written in the form 
The first two terms appear as those appropriate to the energy function in the 
statical theory which would be 
-(PE)-i([p,a B), 
so that the third might be regarded as a kinetic reaction to a rate of change of 
momentum, which would be distributed throughout the medium with a density 
i[PB] 
c 
at each point. 
A similar analysis and analogous results hold for the magnetic media. 
There is, too, a relation satisfied by the momentum vector which appears in the 
simplest form of the theory and to which a fundamental significance is attached by 
