s in speed can be transmitted, they again bring us face to
face, and generally, with the results pointed out by the ordinary
electromagnetic theory. Certain peculiarities, however, are not
absolutely the same. Thus the theory of Lorentz, as well as that of
Maxwell, leads us to foresee that if an insulating mass be caused to
move in a magnetic field normally to its lines of force, a
displacement will be produced in this mass analogous to that of which
Faraday and Maxwell admitted the existence in the dielectric of a
charged condenser. But M.H. Poincare has pointed out that, according
as we adopt one or other of these authors' points of view, so the
value of the displacement differs. This remark is very important, for
it may lead to an experiment which would enable us to make a definite
choice between the two theories.

To obtain the displacement estimated according to Lorentz, we must
multiply the displacement calculated according to Hertz by a factor
representing the relation between the difference of the specific
inductive capacities of the dielectric and of a vacuum, and the first
of these powers. If therefore we take as dielectric the air of which
the specific inductive capacity is perceptibly the same as that of a
vacuum, the displacement, according to the idea of Lorentz, will be
null; while, on the contrary, according to Hertz, it will have a
finite value. M. Blondlot has made the experiment. He sent a current
of air into a condenser placed in a magnetic field, and was never able
to notice the slightest trace of electrification. No displacement,
therefore, is effected in the dielectric. The experiment being a
negative one, is evidently less convincing than one giving a positive
result, but it furnishes a very powerful argument in favour of the
theory of Lorentz.

This theory, therefore, appears very seductive, yet it still raises
objections on the part of those who oppose to it the principles of
ordinary mechanics. If we consider, for instance, a radiation emitted
by an electron belonging to one material body, but absorbed by another
electron in another body, we perceive immediately that, the
propagation not being instantaneous, there can be no compensation
between the action and the reaction, which are not simultaneous; and
the principle of Newton thus seems to be attacked. In order to
preserve its integrity, it has to be admitted that the movements in
the two material substances are compensated by that of the