es in point, for they are not discharges between
insulating particles.

1549. An arrangement was made for discharge between two balls (1485.) (fig.
129.) but, in place of connecting the inducteous ball directly with the
discharging train, it was put in communication with the inside coating of a
Leyden jar, and the discharging train with the outside coating. Then
working the machine, it was found that whenever sonorous and luminous
discharge occurred at the balls A B, the jar became charged; but that when
these did not occur, the jar acquired no charge: and such was the case when
small rounded terminations were used in place of the balls, and also in
whatever manner they were arranged. Under these circumstances, therefore,
discharge even between the air and conductors was always luminous.

1550. But in other cases, the phenomena are such as to make it almost
certain, that dark discharge can take place across air. If the rounded end
of a metal rod, 0.15 of an inch in diameter, be made to give a good
negative brush, the approach of a smaller end or a blunt point opposite to
it will, at a certain distance, cause a diminution of the brush, and a glow
will appear on the positive inducteous wire, accompanied by a current of
air passing from it. Now, as the air is being charged both at the positive
and negative surfaces, it seems a reasonable conclusion, that the charged
portions meet somewhere in the interval, and there discharge to each other,
without producing any luminous phenomena. It is possible, however, that the
air electrified positively at the glowing end may travel on towards the
negative surface, and actually form that atmosphere into which the visible
negative brushes dart, in which case dark discharge need not, of necessity,
occur. But I incline to the former opinion, and think, that the diminution
in size of the negative brush, as the positive glow comes on to the end of
the opposed wire, is in favour of that view.

1551. Using rarefied air as the dielectric, it is very easy to obtain
luminous phenomena as brushes, or glow, upon both conducting balls or
terminations, whilst the interval is dark, and that, when the action is so
momentary that I think we cannot consider currents as effecting discharge
across the dark part. Thus if two balls, about an inch in diameter, and 4
or more inches apart, have the air rarefied about them, and are then
interposed in the course of discharge, an interrupted or spark current
be