spark.

1542. It is also very interesting and instructive to trace the transition
from spark to glow, through the intermediate condition of stream, between
ends in a vessel containing air more or less rarefied; but I fear to be
prolix.

1543. All the effects show, that the glow is in its nature exactly the same
as the luminous part of a brush or ramification, namely a charging of air;
the only difference being, that the glow has a continuous appearance from
the constant renewal of the same action in the same place, whereas the
ramification is due to a momentary, independent and intermitting action of
the same kind.

* * * * *

_Dark discharge._

1544. I will now notice a very remarkable circumstance in the luminous
discharge accompanied by negative glow, which may, perhaps, be correctly
traced hereafter into discharges of much higher intensity. Two brass rods,
0.3 of an inch in diameter, entering a glass globe on opposite sides, had
their ends brought into contact, and the air about them very much rarefied.
A discharge of electricity from the machine was then made through them, and
whilst that was continued the ends were separated from each other. At the
moment of separation a continuous glow came over the end of the negative
rod, the positive termination remaining quite dark. As the distance was
increased, a purple stream or haze appeared on the end of the positive rod,
and proceeded directly outwards towards the negative rod; elongating as the
interval was enlarged, but never joining the negative glow, there being
always a short dark space between. This space, of about 1/16th or 1/20th of
an inch, was apparently invariable in its extent and its position, relative
to the negative rod; nor did the negative glow vary. Whether the negative
end were inductric or inducteous, the same effect was produced. It was
strange to see the positive purple haze diminish or lengthen as the ends
were separated, and yet this dark space and the negative glow remain
unaltered (fig. 133).

1545. Two balls were then used in a large air-pump receiver, and the air
rarefied. The usual transitions in the character of the discharge took
place; but whenever the luminous stream, which appears after the spark and
the brush have ceased, was itself changed into glow at the balls, the dark
space occurred, and that whether the one or the other ball was made
inductric, or positive, or negative.

1546. Sometimes when