e air
at ordinary pressures, we have probably often occasion to witness in
Nature. I have no doubt that if, as many believe, the aurora borealis
is produced by sudden cosmic disturbances, such as eruptions at the
sun's surface, which set the electrostatic charge of the earth in an
extremely rapid vibration, the red glow observed is not confined to
the upper rarefied strata of the air, but the discharge traverses, by
reason of its very high frequency, also the dense atmosphere in the
form of a _glow_, such as we ordinarily produce in a slightly
exhausted tube. If the frequency were very low, or even more so, if
the charge were not at all vibrating, the dense air would break down
as in a lightning discharge. Indications of such breaking down of the
lower dense strata of the air have been repeatedly observed at the
occurrence of this marvelous phenomenon; but if it does occur, it can
only be attributed to the fundamental disturbances, which are few in
number, for the vibration produced by them would be far too rapid to
allow a disruptive break. It is the original and irregular impulses
which affect the instruments; the superimposed vibrations probably
pass unnoticed.

When an ordinary low frequency discharge is passed through moderately
rarefied air, the air assumes a purplish hue. If by some means or
other we increase the intensity of the molecular, or atomic,
vibration, the gas changes to a white color. A similar change occurs
at ordinary pressures with electric impulses of very high frequency.
If the molecules of the air around a wire are moderately agitated, the
brush formed is reddish or violet; if the vibration is rendered
sufficiently intense, the streams become white. We may accomplish this
in various ways. In the experiment before shown with the two wires
across the room, I have endeavored to secure the result by pushing to
a high value both the frequency and potential: in the experiment with
the thin wires glued on the rubber plate I have concentrated the
action upon a very small surface--in other words, I have worked with a
great electric density.

A most curious form of discharge is observed with such a coil when the
frequency and potential are pushed to the extreme limit. To perform
the experiment, every part of the coil should be heavily insulated,
and only two small spheres--or, better still, two sharp-edged metal
discs (dd, Fig. 11) of no more than a few centimetres in
diameter--should be exposed to the