ich tend to facilitate the charge of the air by the
excited conductor, and also those which tend to keep the tension at the
same degree notwithstanding the discharge, assist in producing the glow;
whereas those which tend to resist the charge of the air or other
dielectric, and those which favour the accumulation of electric force prior
to discharge, which, sinking by that act, has to be exalted before the
tension can again acquire the requisite degree, favour intermitting
discharge, and, therefore, the production of brush or spark. Thus,
rarefaction of the air, the removal of large conducting surfaces from the
neighbourhood of the glowing termination, the presentation of a sharp point
towards it, help to sustain or produce the glow: but the condensation of
the air, the presentation of the hand or other large surface, the gradual
approximation of a discharging ball, tend to convert the glow into brush or
even spark. All these circumstances may be traced and reduced, in a manner
easily comprehensible, to their relative power of assisting to produce,
either a _continuous_ discharge to the air, which gives the glow; or an
_interrupted_ one, which produces the brush, and, in a more exalted
condition, the spark.

1540. The rounded end of a brass rod, 0.3 of an inch in diameter, was
covered with a positive glow by the working of an electrical machine: on
stopping the machine, so that the charge of the connected conductor should
fall, the glow changed for a moment into brushes just before the discharge
ceased altogether, illustrating the necessity for a certain high continuous
charge, for a certain sized termination. Working the machine so that the
intensity should be just low enough to give continual brushes from the end
in free air, the approach of a fine point changed these brushes into a
glow. Working the machine so that the termination presented a continual
glow in free air, the gradual approach of the hand caused the glow to
contract at the very end of the wire, then to throw out a luminous point,
which, becoming a foot stalk (1426.), finally produced brushes with large
ramifications. All these results are in accordance with what is stated
above (1539.).

1541. Greasing the end of a rounded wire will immediately make it produce
brushes instead of glow. A ball having a blunt point which can be made to
project more or less beyond its surface, at pleasure, can be made to
produce every gradation from glow, through brush, to