o 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.]

I have prepared a bulb to illustrate by an experiment the correctness
of these assertions. In a globe L (Fig. 31) I have mounted upon a lamp
filament f a piece of lime l. The lamp filament is connected with a
wire which leads into the bulb, and the general construction of the
latter is as indicated in Fig. 19, before described. The bulb being
suspended from a wire connected to the terminal of the coil, and the
latter being set to work, the lime piece l and the projecting parts of
the filament f are bombarded. The degree of exhaustion is just such
that with the potential the coil is capable of giving phosphorescence
of the glass is produced, but disappears as soon as the vacuum is
impaired. The lime containing moisture, and moisture being given off
as soon as heating occurs, the phosphorescence lasts only for a few
moments. When the lime has been sufficiently heated, enough moisture
has been given off to impair materially the vacuum of the bulb. As the
bombardment goes on, one point of the lime piece is more heated than
other points, and the result is that finally practically all the
discharge passes through that point which is intensely heated, and a
white stream of lime particles (Fig. 31) then breaks forth from that
point. This stream is composed of "radiant" matter, yet the degree of
exhaustion is low. But the particles move in straight lines because
the velocity imparted to them is great, and this is due to three
causes--to the great electric density, the high temperature of the
small point, and the fact that the particles of the lime are easily
torn and thrown off--far more easily than those of carbon. With
frequencies such as we are