sh at common pressures,
as regards either size, light, or colour; and this is probably connected
with the tendency which this gas has to discharge the electricity as a
spark (1422.). In rarefied carbonic acid, the brush is better in form, but
weak as to light, being of a dull greenish or purplish line, varying with
the pressure and other circumstances.

1462. _Muriatic acid gas._--It is very difficult to obtain the brush in
this gas at common pressures. On gradually increasing the distance of the
rounded ends, the sparks suddenly ceased when the interval was about an
inch, and the discharge, which was still through the gas in the globe, was
silent and dark. Occasionally a very short brush could for a few moments be
obtained, but it quickly disappeared. Even when the intermitting spark
current (1455.) from the machine was used, still I could only with
difficulty obtain a brush, and that very short, though I used rods with
rounded terminations (about 0.25 of an inch in diameter) which had before
given them most freely in air and nitrogen. During the time of this
difficulty with the muriatic gas, magnificent brushes were passing off from
different parts of the machine into the surrounding air. On rarefying the
gas, the formation of the brush was facilitated, but it was generally of a
low squat form, very poor in light, and very similar on both the positive
and negative surfaces. On rarefying the gas still more, a few large
ramifications were obtained of a pale bluish colour, utterly unlike those
in nitrogen.

* * * * *

1463. In all the gases, the different forms of disruptive discharge may be
linked together and gradually traced from one extreme to the other, i.e.
from the spark to the glow (1405. 1526.), or, it may be, to a still further
condition to be called dark discharge (1544-1560.); but it is,
nevertheless, very surprising to see what a specific character each keeps
whilst under the predominance of the general law. Thus, in muriatic acid,
the brush is very difficult to obtain, and there comes in its place almost
a dark discharge, partaking of the readiness of the spark action. Moreover,
in muriatic acid, I have _never_ observed the spark with any dark interval
in it. In nitrogen, the spark readily changes its character into that of
brush. In carbonic acid gas, there seems to be a facility to occasion spark
discharge, whilst yet that gas is unlike nitrogen in the facility of the
latt