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 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 air.  The coil here used is immersed in oil, and the ends of the secondary reaching out of the oil are covered with an air-tight cover of hard rubber of great thickness.  All cracks, if there are any, should be carefully stopped up, so that the brush discharge cannot form anywhere except on the small spheres or plates which are exposed to the air.  In this case, since there are no large plates or other bodies of capacity attached to the terminals,