SIDELIGHTS ON RELATIVITY

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experiments with beta-rays and rapid kathode rays.

This dualism still confronts us in unextenuated form in the theory
of Hertz, where matter appears not only as the bearer of velocities,
kinetic energy, and mechanical pressures, but also as the bearer of
electromagnetic fields. Since such fields also occur _in vacuo_--i.e.
in free ether--the ether also appears as bearer of electromagnetic
fields. The ether appears indistinguishable in its functions from
ordinary matter. Within matter it takes part in the motion of matter
and in empty space it has everywhere a velocity; so that the ether
has a definitely assigned velocity throughout the whole of space.
There is no fundamental difference between Hertz's ether and
ponderable matter (which in part subsists in the ether).

The Hertz theory suffered not only from the defect of ascribing
to matter and ether, on the one hand mechanical states, and on the
other hand electrical states, which do not stand in any conceivable