the gas above a certain point.
Gases drawn from the neighbourhood of flames, electric arcs and sparks,
or glowing pieces of metal or carbon are conductors, as are also gases
through which Rontgen or cathode rays or rays of positive electricity
are passing; the rays from the radioactive metals, radium, thorium,
polonium and actinium, produce the same effect, as does also
ultra-violet light of exceedingly short wave-length. The gas, after
being made a conductor of electricity by any of these means, is found to
possess certain properties; thus it retains its conductivity for some
little time after the agent which made it a conductor has ceased to act,
though the conductivity diminishes very rapidly and finally gets too
small to be appreciable.

[Illustration: FIG. 5.]

This and several other properties of conducting gas may readily be
proved by the aid of the apparatus represented in fig. 5. V is a testing
vessel in which an electroscope is placed. Two tubes A and C are fitted
into the vessel, A being connected with a water pump, while the far end
of C is in the region where the gas is exposed to the agent which makes
it a conductor of electricity. Let us suppose that the gas is made
conducting by Rontgen rays produced by a vacuum tube which is placed in
a box, covered except for a window at B with lead so as to protect the
electroscope from the direct action of the rays. If a slow current of
air is drawn by the water pump through the testing vessel, the charge on
the electroscope will gradually leak away. The leak, however, ceases
when the current of air is stopped. This result shows that the gas
retains its conductivity during the time taken by it to pass from one
end to the other of the tube C.

The gas loses its conductivity when filtered through a plug of
glass-wool, or when it is made to bubble through water. This can readily
be proved by inserting in the tube C a plug of glass-wool or a water
trap; then if by working the pump a little harder the same current of
air is produced as before, it will be found that the electroscope will
now retain its charge, showing that the conductivity can, as it were, be
filtered out of the gas. The conductivity can also be removed from the
gas by making the gas traverse a strong electric field. We can show this
by replacing the tube C by a metal tube with an insulated wire passing
down the axis of the tube. If there is no potential difference between
the wire and the tube then th