ing produced at the machine[A], each termination may be made to show
luminous phenomena, whilst more or less of the interval is quite dark. The
discharge will pass as suddenly as a retarded spark (295. 334.), i.e. in an
interval of time almost inappreciably small, and in such a case, I think it
must have passed across the dark part as true disruptive discharge, and not
by convection.

[A] By spark current I mean one passing in a series of spark between
the conductor of the machine and the apparatus: by a continuous
current one that passes through metallic conductors, and in that
respect without interruption at the same place.

1552. Hence I conclude that dark disruptive discharge may occur (1547.
1550.); and also, that, in the luminous brush, the visible ramifications
may not show the full extent of the disruptive discharge (1444. 1452.), but
that each may have a dark outside, enveloping, as it were, every part
through which the discharge extends. It is probable, even, that there are
such things as dark discharges analogous in form to the brush and the
spark, but not luminous in any part (1445.).

1553. The occurrence of dark discharge in any case shows at how low a
tension disruptive discharge may occur (1548,), and indicates that the
light of the ultimate brush or spark is in no relation to the intensity
required (1368. 1370.). So to speak, the discharge begins in darkness, and
the light is a mere consequence of the quantity which, after discharge has
commenced, flows to that spot and there finds its most facile passage
(1418. 1435.). As an illustration of the growth generally of discharge, I
may remark that, in the experiments on the transition in oxygen of the
discharge from spark to brush (1518.), every spark was immediately preceded
by a short brush.

1554. The phenomena relative to dark discharge in other gases, though
differing in certain characters from those in air, confirm the conclusions
drawn above. The two rounded terminations (1544.) (fig. 133.), were placed
in _muriatic acid gas_ (1445. 1463.) at the pressure of 6.5 inches of
mercury, and a continuous machine current of electricity sent through the
apparatus: bright sparks occurred until the interval was about or above an
inch, when they were replaced by squat brushy intermitting glows upon both
terminations, with a dark part between. When the current at the machine was
in spark, then each spark caused a discharge across the muriatic acid gas,