tallic points project _freely_
into the air, the positive and negative light upon them differ very little
in appearance, and the difference can be observed only upon close
examination.

1467. The effect varies exceedingly under different circumstances, but, as
we must set out from some position, may perhaps be stated thus: if a
metallic wire with a rounded termination in free air be used to produce the
brushy discharge, then the brushes obtained when the wire is charged
negatively are very poor and small, by comparison with those produced when
the charge is positive. Or if a large metal ball connected with the
electrical machine be charged _positively_, and a fine uninsulated point be
gradually brought towards it, a star appears on the point when at a
considerable distance, which, though it becomes brighter, does not change
its form of a star until it is close up to the ball: whereas, if the ball
be charged negatively, the point at a considerable distance has a star on
it as before; but when brought nearer, (in my case to the distance of 1-1/2
inch,) a brush formed on it, extending to the negative ball; and when still
nearer, (at 1/8 of an inch distance,) the brush ceased, and bright sparks
passed. These variations, I believe, include the whole series of
differences, and they seem to show at once, that the negative surface tends
to retain its discharging character unchanged, whilst the positive surface,
under similar circumstances, permits of great variation.

1468. There are several points in the character of the negative discharge
to air which it is important to observe. A metal rod, 0.3 of an inch in
diameter, with a rounded end projecting into the air, was charged
negatively, and gave a short noisy brush (fig. 122.). It was ascertained
both by sight (1427. 1433.) and sound (1431.), that the successive
discharges were very rapid in their recurrence, being seven or eight times
more numerous in the same period, than those produced when the rod was
charged positively to an equal degree. When the rod was positive, it was
easy, by working the machine a little quicker, to replace the brush by a
glow (1405. 1463.), but when it was negative no efforts could produce this
change. Even by bringing the hand opposite the wire, the only effect was to
increase the number of brush discharges in a given period, raising at the
same time the sound to a higher pitch.

1469. A point opposite the negative brush exhibited a star, and as i