evolved at the
moment. But it may also be due to that specific relation of the particles
and the electric forces which I have assumed as the basis of a theory of
induction; the facts do not oppose such a view; and in that view the
variation strengthens the argument for molecular action, as it would seem
to show the influence of the latter in every part of the electrical effect
(1423. 1454.).

1422. The appearances of the sparks in different gases have often been
observed and recorded[A], but I think it not out of place to notice briefly
the following results; they were obtained with balls of brass, (platina
surfaces would have been better,) and at common pressures. In _air_, the
sparks have that intense light and bluish colour which are so well known,
and often have faint or dark parts in their course, when the quantity of
electricity passing is not great. In _nitrogen_, they are very beautiful,
having the same general appearance as in air, but have decidedly more
colour of a bluish or purple character, and I thought were remarkably
sonorous. In _oxygen_, the sparks were whiter than in air or nitrogen, and
I think not so brilliant. In _hydrogen_, they had a very fine crimson
colour, not due to its rarity, for the character passed away as the
atmosphere was rarefied (1459.)[B]. Very little sound was produced in this
gas; but that is a consequence of its physical condition[C]. In _carbonic
acid gas_, the colour was similar to that of the spark in air, but with a
little green in it: the sparks were remarkably irregular in form, more so
than in common air: they could also, under similar circumstances as to size
of ball, &c., be obtained much longer than in air, the gas showing a
singular readiness to cause the discharge in the form of spark. In
_muriatic acid gas_, the spark was nearly white: it was always bright
throughout, never presenting those dark parts which happen in air,
nitrogen, and some other gases. The gas was dry, and during the whole
experiment the surface of the glass globe within remained quite dry and
bright. In _coal gas_, the spark was sometimes green, sometimes red, and
occasionally one part was green and another red: black parts also occur
very suddenly in the line of the spark, i.e. they are not connected by any
dull part with bright portions, but the two seem to join directly one with
the other.

[A] See Van Marum's description of the Teylerian machine, vol. i. p.
112, and vol. ii. p. 196; also E