ion, which does not obey the principle of equal angles, and for
which the most various gases are already disturbed media. According to
Crookes it possesses also the singular property of carrying with it
electric charges.

This convection of negative electricity by the cathode rays seems
quite inexplicable on the hypothesis that the rays are ethereal
radiations. Nothing then remained in order to maintain this
hypothesis, except to deny the convection, which, besides, was only
established by indirect experiments. That the reality of this
transport has been placed beyond dispute by means of an extremely
elegant experiment which is all the more convincing that it is so very
simple, is due to M. Perrin. In the interior of a Crookes tube he
collected a pencil of cathode rays in a metal cylinder. According to
the elementary principles of electricity the cylinder must become
charged with the whole charge, if there be one, brought to it by the
rays, and naturally various precautions had to be taken. But the
result was very precise, and doubt could no longer exist--the rays
were electrified.

It might have been, and indeed was, maintained, some time after this
experiment was published, that while the phenomena were complex inside
the tube, outside, things might perhaps occur differently. Lenard
himself, however, with that absence of even involuntary prejudice
common to all great minds, undertook to demonstrate that the opinion
he at first held could no longer be accepted, and succeeded in
repeating the experiment of M. Perrin on cathode rays in the air and
even _in vacuo_.

On the wrecks of the two contradictory hypotheses thus destroyed, and
out of the materials from which they had been built, a theory has been
constructed which co-ordinates all the known facts. This theory is
furthermore closely allied to the theory of ionisation, and, like this
latter, is based on the concept of the electron. Cathode rays are
electrons in rapid motion.

The phenomena produced both inside and outside a Crookes tube are,
however, generally complex. In Lenard's first experiments, and in many
others effected later when this region of physics was still very
little known, a few confusions may be noticed even at the present day.

At the spot where the cathode rays strike the walls of the tube the
essentially different X rays appear. These differ from the cathode
radiations by being neither electrified nor deviated by a magnet. In
their turn th