in consequence of the common phrase
_direction of a current_) as indicating a breaking forth in different
directions of the original force, rather than a tendency to convergence and
union in one line of passage. But the ordinary case of the brush may be
compared, for its illustration, with that in which, by holding the knuckle
opposite to highly excited glass, a discharge occurs, the ramifications of
a brush then leading from the glass and converging into a spark on the
knuckle. Though a difficult experiment to make, it is possible to obtain
discharge between highly excited shell-lac and the excited glass of a
machine: when the discharge passes, it is, from the nature of the charged
bodies, brush at each end and spark in the middle, beautifully illustrating
that tendency of discharge to facilitate like action, which I have
described in a former page (1418.).

1454. The brush has _specific characters_ in different gases, indicating a
relation to the particles of these bodies even in a stronger degree than
the spark (1422. 1423.). This effect is in strong contrast with the
non-variation caused by the use of different substances as _conductors_
from which the brushes are to originate. Thus, using such bodies as wood,
card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride of
lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of
vitriol, sulphur, sulphuret of antimony, and haematite, no variation in the
character of the brushes was obtained, except that (dependent upon their
effect as better or worse conductors) of causing discharge with more or
less readiness and quickness from the machine[A].

[A] Exception must, of course, be made of those cases where the root
of the brush, becoming a spark, causes a little diffusion or even
decomposition of the matter there, and so gains more or less of a
particular colour at that part.

1455. The following are a few of the effects I observed in different gasses
at the positively charged surfaces, and with atmospheres varying in their
pressure. The general effect of rarefaction was the same for all the gases:
at first, sparks passed; these gradually were converted into brushes, which
became larger and more distinct in their ramifications, until, upon further
rarefaction, the latter began to collapse and draw in upon each other, till
they formed a stream across from conductor to conductor: then a few lateral
streams shot out towards the gl