from the wire as the impedance of the latter is enormously increased, and merely heating the ends of the wire by reason of their resistance to the passage of the discharge.  But it is not at all necessary that the gas in the tube should he conducting; it might be at an extremely low pressure, still the ends of the wire would be heated—­as, however, is ascertained by experience—­only the two ends would in such, case not be electrically connected through the gaseous medium.  Now what with these frequencies and potentials occurs in an exhausted tube occurs in the lightning discharges at ordinary pressure.  We only need remember one of the facts arrived at in the course of these investigations, namely, that to impulses of very high frequency the gas at ordinary pressure behaves much in the same manner as though it were at moderately low pressure.  I think that in lightning discharges frequently wires or conducting objects are volatilized merely because air is present and that, were the conductor immersed in an insulating liquid, it would be safe, for then the energy would have to spend itself somewhere else.  From the behavior of gases to sudden impulses of high potential I am led to conclude that there can be no surer way of diverting a lightning discharge than by affording it a passage through a volume of gas, if such a thing can be done in a practical manner.

There are two more features upon which I think it necessary to dwell in connection with these experiments—­the “radiant state” and the “non-striking vacuum.”

Any one who has studied Crookes’ work must have received the impression that the “radiant state” is a property of the gas inseparably connected with an extremely high degree of exhaustion.  But it should be remembered that the phenomena observed in an exhausted vessel are limited to the character and capacity of the apparatus which is made use of.  I think that in a bulb a molecule, or atom, does not precisely move in a straight line because it meets no obstacle, but because the velocity imparted to it is sufficient to propel it in a sensibly straight line.  The mean free path is one thing, but the velocity—­the energy associated with the moving body—­is another, and under ordinary circumstances I believe that it is a mere question of potential or speed.  A disruptive discharge coil, when the potential is pushed very far, excites phosphorescence and projects shadows, at comparatively low degrees of exhaustion.  In a lightning discharge, matter moves in straight lines as ordinary pressure when the mean free path is exceedingly small, and frequently images of wires or other metallic objects have been produced by the particles thrown off in straight lines.

[Illustration:  FIG. 31.—­BULB SHOWING RADIANT LIME STREAM AT LOW EXHAUSTION.]