the negative ball was large, the machine in powerful
action, and the rarefaction high, the ball would be covered over half its
surface with glow, and then, upon a hasty observation, would seem to
exhibit no dark space: but this was a deception, arising from the
overlapping of the convex termination of the negative glow and the concave
termination of the positive stream. More careful observation and experiment
have convinced me, that when the negative glow occurs, it never visibly
touches the luminous part of the positive discharge, but that the dark
space is always there.

1547. This singular separation of the positive and negative discharge, as
far as concerns their luminous character, under circumstances which one
would have thought very favourable to their coalescence, is probably
connected with their differences when in the form of brush, and is perhaps
even dependent on the same cause. Further, there is every likelihood that
the dark parts which occur in feeble sparks are also connected with these
phenomena[A]. To understand them would be very important, for it is quite
clear that in many of the experiments, indeed in all that I have quoted,
discharge is taking place across the dark part of the dielectric to an
extent quite equal to what occurs in the luminous part. This difference in
the result would seem to imply a distinction in the modes by which the two
electric forces are brought into equilibrium in the respective parts; and
looking upon all the phenomena as giving additional proofs, that it is to
the condition of the particles of the dielectric we must refer for the
principles of induction and discharge, so it would be of great importance
if we could know accurately in what the difference of action in the dark
and the luminous parts consisted.

[A] See Professor Johnson's experiments. Silliman's Journal, xxv. p. 57.

1548. The dark discharge through air (1552.), which in the case mentioned
is very evident (1544.), leads to the inquiry, whether the particles of air
are generally capable of effecting discharge from one to another without
becoming luminous; and the inquiry is important, because it is connected
with that degree of tension which is necessary to originate discharge
(1368. 1370.). Discharge between _air and conductors_ without luminous
appearances are very common; and non-luminous discharges by carrying
currents of air and other fluids (1562. 1595.) are also common enough: but
these are not cas