out oil, the oil reaching about an inch above the tubes. The free ends
of the secondary are lifted out of the oil and placed parallel to each
other at a distance of about 10 centimetres. The ends which are
scraped should be dipped in the oil. Two four-pint jars joined in
series may be used to discharge through the primary. When the
necessary adjustments in the length and distance of the wires above
the oil and in the arc of discharge are made, a luminous sheet is
produced between the wires which is perfectly smooth and textureless,
like the ordinary discharge through a moderately exhausted tube.
I have purposely dwelt upon this apparently insignificant experiment.
In trials of this kind the experimenter arrives at the startling
conclusion that, to pass ordinary luminous discharges through gases,
no particular degree of exhaustion is needed, but that the gas may be
at ordinary or even greater pressure. To accomplish this, a very high
frequency is essential; a high potential is likewise required, but
this is a merely incidental necessity. These experiments teach us
that, in endeavoring to discover novel methods of producing light by
the agitation of atoms, or molecules, of a gas, we need not limit our
research to the vacuum tube, but may look forward quite seriously to
the possibility of obtaining the light effects without the use of any
vessel whatever, with air at ordinary pressure.
Such discharges of very high frequency, which render luminous the 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
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