e electroscope will leak when a current of
air is drawn through the vessel, but the leak will stop if a
considerable difference of potential is maintained between the wire and
the tube: this shows that a strong electric field removes the
conductivity from the gas.

The fact that the conductivity of the gas is removed by filtering shows
that it is due to something mixed with the gas which is removed from it
by filtration, and since the conductivity is also removed by an electric
field, the cause of the conductivity must be charged with electricity so
as to be driven to the sides of the tube by the electric force. Since
the gas as a whole is not electrified either positively or negatively,
there must be both negative and positive charges in the gas, the amount
of electricity of one sign being equal to that of the other. We are thus
led to the conclusion that the conductivity of the gas is due to
electrified particles being mixed up with the gas, some of these
particles having charges of positive electricity, others of negative.
These electrified particles are called _ions_, and the process by which
the gas is made a conductor is called the ionization of the gas. We
shall show later that the charges and masses of the ions can be
determined, and that the gaseous ions are not identical with those met
with in the electrolysis of solutions.

[Illustration: FIG. 6.]

One very characteristic property of conduction of electricity through a
gas is the relation between the current through the gas and the electric
force which gave rise to it. This relation is not in general that
expressed by Ohm's law, which always, as far as our present knowledge
extends, expresses the relation for conduction through metals and
electrolytes. With gases, on the other hand, it is only when the current
is very small that Ohm's law is true. If we represent graphically by
means of a curve the relation between the current passing between two
parallel metal plates separated by ionized gas and the difference of
potential between the plates, the curve is of the character shown in
fig. 6 when the ordinates represent the current and the abscissae the
difference of potential between the plates. We see that when the
potential difference is very small, i.e. close to the origin, the curve
is approximately straight, but that soon the current increases much less
rapidly than the potential difference, and that a stage is reached when
no appreciable increase of cu