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.
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