ncy. Britan., vol. vi., Article
Electricity, pp. 505, 507.

[B] Van Marum says they are about four times as large in hydrogen as
in air. vol. i. p. 122.

[C] Leslie. Cambridge Phil. Transactions, 267.

1423. These varieties of character impress my mind with a feeling, that
they are due to a direct relation of the electric powers to the particles
of the dielectric through which the discharge occurs, and are not the mere
results of a casual ignition or a secondary kind of action of the
electricity, upon the particles which it finds in its course and thrusts
aside in its passage (1454.).

1424. The spark may be obtained in media which are far denser than air, as
in oil of turpentine, olive oil, resin, glass, &c.: it may also be obtained
in bodies which being denser likewise approximate to the condition of
conductors, as spermaceti, water, &c. But in these cases, nothing occurs
which, as far as I can perceive, is at all hostile to the general views I
have endeavoured to advocate.

_The electrical brush._

1425. The _brush_ is the next form of disruptive discharge which I shall
consider. There are many ways of obtaining it, or rather of exalting its
characters; and all these ways illustrate the principles upon which it is
produced. If an insulated conductor, connected with the positive conductor
of an electrical machine, have a metal rod 0.3 of an inch in diameter
projecting from it outwards from the machine, and terminating by a rounded
end or a small ball, it will generally give good brushes; or, if the
machine be not in good action, then many ways of assisting the formation of
the brush can be resorted to; thus, the hand or any _large_ conducting
surface may be approached towards the termination to increase inductive
force (1374.): or the termination may be smaller and of badly conducting
matter, as wood: or sparks may be taken between the prime conductor of the
machine and the secondary conductor to which the termination giving brushes
belongs: or, which gives to the brushes exceedingly fine characters and
great magnitude, the air around the termination may be rarefied more or
less, either by heat or the air-pump; the former favourable circumstances
being also continued.

1426. The brush when obtained by a powerful machine on a ball about 0.7 of
an inch in diameter, at the end of a long brass rod attached to the
positive prime conductor, had the general appearance as to form represented
in fig. 117: a sho