ntity of electricity.

The conductivity produced is not governed by Ohm's law. The intensity
is not proportional to the electromotive force, and it increases at
first as the electromotive force augments; but it approaches
asymptotically to a maximum value which corresponds to the number of
ions liberated, and can therefore serve as a measure of the power of
the excitement. It is this current which is termed the _current of
saturation_.

M. Righi has ably demonstrated that ionised gas does not obey the law
of Ohm by an experiment very paradoxical in appearance. He found that,
the greater the distance of the two electrode plates from each, the
greater may be, within certain limits, the intensity of the current.
The fact is very clearly interpreted by the theory of ionisation,
since the greater the length of the gaseous column the greater must be
the number of ions liberated.

One of the most striking characteristics of ionised gases is that of
discharging electrified conductors. This phenomenon is not produced by
the departure of the charge that these conductors may possess, but by
the advent of opposite charges brought to them by ions which obey the
electrostatic attraction and abandon their own electrification when
they come in contact with these conductors.

This mode of regarding the phenomena is extremely convenient and
eminently suggestive. It may, no doubt, be thought that the image of
the ions is not identical with objective reality, but we are compelled
to acknowledge that it represents with absolute faithfulness all the
details of the phenomena.

Other facts, moreover, will give to this hypothesis a still greater
value; we shall even be able, so to speak, to grasp these ions
individually, to count them, and to measure their charge.

Sec. 2. THE CONDENSATION OF WATER-VAPOUR BY IONS

If the pressure of a vapour--that of water, for instance--in the
atmosphere reaches the value of the maximum pressure corresponding to
the temperature of the experiment, the elementary theory teaches us
that the slightest decrease in temperature will induce a condensation;
that small drops will form, and the mist will turn into rain.

In reality, matters do not occur in so simple a manner. A more or
less considerable delay may take place, and the vapour will remain
supersaturated. We easily discover that this phenomenon is due
to the intervention of capillary action. On a drop of liquid a
surface-tension takes effect which