might not be _electrical_ in its nature.

As the production of heat and light is here due to the impact of the
molecules, or atoms of air, or something else besides, and, as we can
augment the energy simply by raising the potential, we might, even
with frequencies obtained from a dynamo machine, intensify the action
to such a degree as to bring the terminal to melting heat. But with
such low frequencies we would have to deal always with something of
the nature of an electric current. If I approach a conducting object
to the brush, a thin little spark passes, yet, even with the
frequencies used this evening, the tendency to spark is not very
great. So, for instance, if I hold a metallic sphere at some distance
above the terminal you may see the whole space between the terminal
and sphere illuminated by the streams without the spark passing; and
with the much higher frequencies obtainable by the disruptive
discharge of a condenser, were it not for the sudden impulses, which
are comparatively few in number, sparking would not occur even at very
small distances. However, with incomparably higher frequencies, which
we may yet find means to produce efficiently, and provided that
electric impulses of such high frequencies could be transmitted
through a conductor, the electrical characteristics of the brush
discharge would completely vanish--no spark would pass, no shock would
be felt--yet we would still have to deal with an _electric_
phenomenon, but in the broad, modern interpretation of the word. In my
first paper before referred to I have pointed out the curious
properties of the brush, and described the best manner of producing
it, but I have thought it worth while to endeavor to express myself
more clearly in regard to this phenomenon, because of its absorbing
interest.