er to form brushes, and unlike muriatic acid in its own facility to
continue the spark. These differences add further force, first to the
observations already made respecting the spark in various gases (1422.
1423.), and then, to the proofs deducible from it, of the relation of the
electrical forces to the particles of matter.

1464. The peculiar characters of nitrogen in relation to the electric
discharge (1422. 1458.) must, evidently, have an important influence over
the form and even the occurrence of lightning. Being that gas which most
readily produces coruscations, and, by them, extends discharge to a greater
distance than any other gas tried, it is also that which constitutes
four-fifths of our atmosphere; and as, in atmospheric electrical phenomena,
one, and sometimes both the inductive forces are resident on the particles
of the air, which, though probably affected as to conducting power by the
aqueous particles in it, cannot be considered as a good conductor; so the
peculiar power possessed by nitrogen, to originate and effect discharge in
the form of a brush or of ramifications, has, probably, an important
relation to its electrical service in nature, as it most seriously affects
the character and condition of the discharge when made. The whole subject
of discharge from and through gases is of great interest, and, if only in
reference to atmospheric electricity, deserves extensive and close
experimental investigation.

_Difference of discharge at the positive and negative conducting surfaces._

1465. I have avoided speaking of this well-known phenomenon more than was
quite necessary, that I might bring together here what I have to say on the
subject. When the brush discharge is observed in air at the positive and
negative surfaces, there is a very remarkable difference, the true and full
comprehension of which would, no doubt, be of the utmost importance to the
physics of electricity; it would throw great light on our present subject,
i.e. the molecular action of dielectrics under induction, and its
consequences; and seems very open to, and accessible by, experimental
inquiry.

1466. The difference in question used to be expressed in former times by
saying, that a point charged positively gave brushes into the air, whilst
the same point charged negatively gave a star. This is true only of bad
conductors, or of metallic conductors charged intermittingly, or otherwise
controlled by collateral induction. If me