ether which
separates these substances; but this conception, although in tolerable
agreement with the hypothesis that the ether and matter are not of
different essence, involves, on a closer examination, suppositions
hardly satisfactory as to the nature of movements in the ether.

For a long time physicists have admitted that the ether as a whole
must be considered as being immovable and capable of serving, so to
speak, as a support for the axes of Galileo, in relation to which axes
the principle of inertia is applicable,--or better still, as M.
Painleve has shown, they alone allow us to render obedience to the
principle of causality.

But if it were so, we might apparently hope, by experiments in
electromagnetism, to obtain absolute motion, and to place in evidence
the translation of the earth relatively to the ether. But all the
researches attempted by the most ingenious physicists towards this end
have always failed, and this tends towards the idea held by many
geometricians that these negative results are not due to imperfections
in the experiments, but have a deep and general cause. Now Lorentz has
endeavoured to find the conditions in which the electromagnetic theory
proposed by him might agree with the postulate of the complete
impossibility of determining absolute motion. It is necessary, in
order to realise this concord, to imagine that a mobile system
contracts very slightly in the direction of its translation to a
degree proportioned to the square of the ratio of the velocity of
transport to that of light. The electrons themselves do not escape
this contraction, although the observer, since he participates in the
same motion, naturally cannot notice it. Lorentz supposes, besides,
that all forces, whatever their origin, are affected by a translation
in the same way as electromagnetic forces. M. Langevin and M. H.
Poincare have studied this same question and have noted with precision
various delicate consequences of it. The singularity of the hypotheses
which we are thus led to construct in no way constitutes an argument
against the theory of Lorentz; but it has, we must acknowledge,
discouraged some of the more timid partisans of this theory.[48]

[Footnote 48: An objection not here noticed has lately been formulated
with much frankness by Professor Lorentz himself. It is one of the
pillars of his theory that only the negative electrons move when an
electric current passes through a metal, and that the posi