re
beautifully connected with the mechanical currents (1576.) taking place
within the receiver. These glows in rarefied air are often highly exalted
in beauty by a spark discharge at the conductor (1551. _Note_.).

1530. To obtain a _negative glow_ in air at common pressures is difficult.
I did not procure it on the rod 0.3 of an inch in diameter by my machine,
nor on much smaller rods; and it is questionable as yet, whether, even on
fine points, what is called the negative star is a very reduced and minute,
but still intermitting brush, or a glow similar to that obtained on a
positive point.

1531. In rarefied air the negative glow can easily be obtained. If the
rounded ends of two metal rods, about O.2 of an inch in diameter, are
introduced into a globe or jar (the air within being rarefied), and being
opposite to each other, are about four inches apart, the glow can be
obtained on both rods, covering not only the ends, but an inch or two of
the part behind. On using _balls_ in the air-pump jar, and adjusting the
distance and exhaustion, the negative ball could be covered with glow,
whether it were the inductric or the inducteous surface.

1532. When rods are used it is necessary to be aware that, if placed
concentrically in the jar or globe, the light on one rod is often reflected
by the sides of the vessel on to the other rod, and makes it apparently
luminous, when really it is not so. This effect may be detected by shifting
the eye at the time of observation, or avoided by using blackened rods.

1533. It is curious to observe the relation _of glow, brush_, and _spark_
to each other, as produced by positive or negative surfaces; thus,
beginning with spark discharge, it passes into brush much sooner when the
surface at which the discharge commences (1484.) is negative, than it does
when positive; but proceeding onwards in the order of change, we find that
the positive brush passes into _glow_ long before the negative brush does.
So that, though each presents the three conditions in the same general
order, the series are not precisely the same. It is probable, that, when
these points are minutely examined, as they must be shortly, we shall find
that each different gas or dielectric presents its own peculiar results,
dependent upon the mode in which its particles assume polar electric
condition.

1534. The glow occurs in all gases in which I have looked for it. These are
air, nitrogen, oxygen, hydrogen, coal gas, c