apacities of the metal and of
the insulating medium in which it is plunged.

Electrons, however, which, on arriving at the surface of the metal,
possessed a kinetic energy superior to this work, might be shot forth
and would be disengaged as a vapour escapes from a liquid. Now, the
number of these rapid electrons, at first very slight, increases,
according to the kinetic theory, when the temperature rises, and
therefore we must reckon that a wire, on being heated, gives out
electrons, that is to say, loses negative electricity and sends into
the surrounding media electrified centres capable of producing the
phenomena of ionisation. Edison, in 1884, showed that from the
filament of an incandescent lamp there escaped negative electric
charges. Since then, Richardson and J.J. Thomson have examined
analogous phenomena. This emission is a very general phenomenon which,
no doubt, plays a considerable part in cosmic physics. Professor
Arrhenius explains, for instance, the polar auroras by the action of
similar corpuscules emitted by the sun.

In other phenomena we seem indeed to be confronted by an emission, not
of negative electrons, but of positive ions. Thus, when a wire is
heated, not _in vacuo_, but in a gas, this wire begins to electrify
neighbouring bodies positively. J.J. Thomson has measured the mass of
these positive ions and finds it considerable, i.e. about 150 times
that of an atom of hydrogen. Some are even larger, and constitute
almost a real grain of dust. We here doubtless meet with the phenomena
of disaggregation undergone by metals at a red heat.

CHAPTER IX

CATHODE RAYS AND RADIOACTIVE BODIES

Sec. 1. THE CATHODE RAYS

A wire traversed by an electric current is, as has just been
explained, the seat of a movement of electrons. If we cut this wire, a
flood of electrons, like a current of water which, at the point where
a pipe bursts, flows out in abundance, will appear to spring out
between the two ends of the break.

If the energy of the electrons is sufficient, these electrons will in
fact rush forth and be propagated in the air or in the insulating
medium interposed; but the phenomena of the discharge will in general
be very complex. We shall here only examine a particularly simple
case, viz., that of the cathode rays; and without entering into
details, we shall only note the results relating to these rays which
furnish valuable arguments in favour of the electronic hypothesis and
supply