1440.  Brush discharge does not essentially require any current of the medium in which the brush appears:  the current almost always occurs, but is a consequence of the brush, and will be considered hereafter (1562-1610.).  On holding a blunt point positively charged towards uninsulated water, a star or glow appeared on the point, a current of air passed from it, and the surface of the water was depressed; but on bringing the point so near that sonorous brushes passed, then the current of air instantly ceased, and the surface of the water became level.

1441.  The discharge by a brush is not to all the particles of air that are near the electrified conductor from which the brush issues; only those parts where the ramifications pass are electrified:  the air in the central dark parts between them receives no charge, and, in fact, at the time of discharge, has its electric and inductive tension considerably lowered.  For consider fig. 128 to represent a single positive brush;—­the induction before the discharge is from the end of the rod outwards, in diverging lines towards the distant conductors, as the walls of the room, &c., and a particle at a has polarity of a certain degree of tension, and tends with a certain force to become charged; but at the moment of discharge, the air in the ramifications b and d, acquiring also a positive state, opposes its influence to that of the positive conductor on a, and the tension of the particle at a is therefore diminished rather than increased.  The charged particles at b and d are now inductive bodies, but their lines of inductive action are still outwards towards the walls of the room; the direction of the polarity and the tendency of other particles to charge from these, being governed by, or in conformity with, these lines of force.

1442.  The particles that are charged are probably very highly charged, but, the medium being a non-conductor, they cannot communicate that state to their neighbours.  They travel, therefore, under the influence of the repulsive and attractive forces, from the charged conductor towards the nearest uninsulated conductor, or the nearest body in a different state to themselves, just as charged particles of dust would travel, and are then discharged; each particle acting, in its course, as a centre of inductive force upon any bodies near which it may come.  The travelling of these charged particles when they are numerous, causes wind and currents, but these will come into consideration under carrying discharge (1319. 1562. &c.).

1443.  When air is said to be electrified, and it frequently assumes this state near electrical machines, it consists, according to my view, of a mixture of electrified and unelectrified particles, the latter being in very large proportion to the former.  When we gather electricity from air, by a flame or by wires, it is either by the actual discharge of these particles, or by effects dependent on their inductive