ether the energy whose evolution
is the cause of the actions manifested, as, for example, in the
discharge of a condenser.

Consider the barrel of a pump placed in a vacuum and closed by a
piston at each end, and let us introduce between these a certain mass
of air. The two pistons, through the elastic force of the gas, repel
each other with a force which, according to the law of Mariotte,
varies in inverse ratio to the distance. The method favoured by Ampere
would first of all allow this law of repulsion between the two pistons
to be discovered, even if the existence of a gas enclosed in the
barrel of the pump were unsuspected; and it would then be natural to
localize the potential energy of the system on the surface of the two
pistons. But if the phenomenon is more carefully examined, we shall
discover the presence of the air, and we shall understand that every
part of the volume of this air could, if it were drawn off into a
recipient of equal volume, carry away with it a fraction of the energy
of the system, and that consequently this energy belongs really to the
air and not to the pistons, which are there solely for the purpose of
enabling this energy to manifest its existence.

Faraday made, in some sort, an equivalent discovery when he perceived
that the electrical energy belongs, not to the coatings of the
condenser, but to the dielectric which separates them. His audacious
views revealed to him a new world, but to explore this world a surer
and more patient method was needed.

Maxwell succeeded in stating with precision certain points of
Faraday's ideas, and he gave them the mathematical form which, often
wrongly, impresses physicists, but which when it exactly encloses a
theory, is a certain proof that this theory is at least coherent and
logical.[23]

[Footnote 23: It will no doubt be a shock to those whom Professor
Henry Armstrong has lately called the "mathematically-minded" to find
a member of the Poincare family speaking disrespectfully of the
science they have done so much to illustrate. One may perhaps compare
the expression in the text with M. Henri Poincare's remark in his last
allocution to the Academie des Sciences, that "Mathematics are
sometimes a nuisance, and even a danger, when they induce us to affirm
more than we know" (_Comptes-rendus_, 17th December 1906).]

The work of Maxwell is over-elaborated, complex, difficult to read,
and often ill-understood, even at the present day. Maxwell i