24 SECTIONAL ADDRESSES 
communicating that motion to gross matter so as to heat:it and affect it 
in various ways. 
‘ We may therefore receive, as a datum derived from a branch of science 
independent of that with which we have to deal, the existence of a per- 
vading medium, of small but real density, capable of being set in motion, 
and of transmitting motion from one part to another with great, but not 
infinite, velocity. 
‘ Hence the parts of this medium must be so connected that the motion 
of one part depends in some way on the motions of the rest ; and at the 
same time these connexions must be capable of a certain kind of elastic 
yielding, since the communication of motion is not instantaneous, but 
occupies time. 
‘The medium is therefore capable of receiving and storing up two 
kinds of energy, the “actual” energy depending on the motions of its 
parts, and “ potential” energy, consisting of the work which the 
medium will do in recovering from displacement in virtue of its 
elasticity.’ 
Maxwell postulates further that the all-pervading medium possesses 
physical characteristics of the same kind as a homogeneous isotropic 
dielectric, that the effect of the action of an electric force on it is 
the production of what he terms ‘ electric displacement,’ which is ‘ a kind 
of elastic yielding to the action of the force similar to that which takes 
place in structures and machines owing to the want of perfect rigidity 
of the connexions.’ 
He shows that the application of the general equations of electro- 
dynamics, derived from the Ampere-Faraday laws, to the case of a magnetic 
disturbance propagated through a non-conducting field gives the result 
that the only disturbances which can be so propagated are those which 
are transverse to the direction of propagation, and that the velocity of 
propagation is the velocity v, which expresses the number of electro- 
static units of electricity which are contained in one electromagnetic 
unit. 
The all-pervading medium which Maxwell postulates is a medium 
which possesses to some extent the physical characteristics of an elastic 
solid, and it is probable that his replacement of the expression for the 
electrokinetic energy which is obtained from Faraday’s laws by an ex- 
pression which gives the energy in terms of the magnetic force, was 
effected to make it similar to the expression for the kinetic energy function 
of an elastic solid. This replacement is effected by an integration by 
parts and neglecting the surface integral on the ground that at an in- 
definitely great distance the surface integral tends to zero, but this over- 
looks the fact that the law of variation of magnetic force with distance is 
not the same when the magnetic field is varying as it is when the magnetic 
field is steady. This does not affect Maxwell’s investigation of the 
propagation of a magnetic disturbance, as this expression for the electre- 
kinetic energy is not used in that investigation. 
As has been seen, Faraday’s view, as set forth in his 1845 paper, is 
different, and he explains his views in greater detail in a letter which 
