brushes are produced
simultaneously in relation to each other in air, the former almost always
has a contracted form, as in fig. 125, very much indeed resembling the
figure which the positive brush itself has when influenced by the lateral
vicinity of positive parts acting by induction. Thus a brush issuing from a
point in the re-entering angle of a positive conductor has the same
compressed form (fig. 126.).

1473. The character of the negative brush is not affected by the chemical
nature of the substances of the conductors (1454.), but only by their
possession of the conducting power in a greater or smaller degree.

1474. Rarefaction of common air about a negative ball or blunt point
facilitated the development of the negative brush, the effect being, I
think, greater than on a positive brush, though great on both. Extensive
ramifications could be obtained from a ball or end electrified negatively
to the plate of the air-pump on which the jar containing it stood.

1475. A very important variation of the relative forms and conditions of
the positive and negative brush takes place on varying the dielectric in
which they are produced. The difference is so very great that it points to
a specific relation of this form of discharge to the particular gas in
which it takes place, and opposes the idea that gases are but obstructions
to the discharge, acting one like another and merely in proportion to their
pressure (1377.).

1476. In _air_, the superiority of the positive brush is well known (1467.
1472.). In _nitrogen_, it is as great or even greater than in air (1458.).
In _hydrogen_, the positive brush loses a part of its superiority, not
being so good as in nitrogen or air; whilst the negative brush does not
seem injured (1459.). In _oxygen_, the positive brush is compressed and
poor (1457); whilst the negative did not become less: the two were so alike
that the eye frequently could not tell one from the other, and this
similarity continued when the oxygen was gradually rarefied. In _coal gas_,
the brushes are difficult of production as compared to nitrogen (1460.),
and the positive not much superior to the negative in its character, either
at common or low pressures. In _carbonic acid gas_, this approximation of
character also occurred. In _muriatic acid gas_, the positive brush was
very little better than the negative, and both difficult to produce (1462.)
as compared with the facility in nitrogen or air.

1477. Th