ass of the vessel from the conductors; these
became thick and soft in appearance, and were succeeded by the full
constant glow which covered the discharging wire. The phenomena varied with
the size of the vessel (1477.), the degree of rarefaction, and the
discharge of electricity from the machine. When the latter was in
successive sparks, they were most beautiful, the effect of a spark from a
small machine being equal to, and often surpassing, that produced by the
_constant_ discharge of a far more powerful one.

1456. _Air._--Fine positive brushes are easily obtained in air at common
pressures, and possess the well-known purplish light. When the air is
rarefied, the ramifications are very long, filling the globe (1477.); the
light is greatly increased, and is of a beautiful purple colour, with an
occasional rose tint in it.

1457. _Oxygen._--At common pressures, the brush is very close and
compressed, and of a dull whitish colour. In rarefied oxygen, the form and
appearance are better, the colour somewhat purplish, but all the characters
very poor compared to those in air.

1458. _Nitrogen_ gives brushes with great facility at the positive surface,
far beyond any other gas I have tried: they are almost always fine in form,
light, and colour, and in rarefied nitrogen, are magnificent. They surpass
the discharges in any other gas as to the quantity of light evolved.

1459. _Hydrogen_, at common pressures, gave a better brush than oxygen, but
did not equal nitrogen; the colour was greenish gray. In rarefied hydrogen,
the ramifications were very fine in form and distinctness, but pale in
colour, with a soft and velvety appearance, and not at all equal to those
in nitrogen. In the rarest state of the gas, the colour of the light was a
pale gray green.

1460. _Coal gas._--The brushes were rather difficult to produce, the
contrast with nitrogen being great in this respect. They were short and
strong, generally of a greenish colour, and possessing much of the spark
character: for, occurring on both the positive and negative terminations,
often when there was a dark interval of some length between the two
brushes, still the quick, sharp sound of the spark was produced, as if the
discharge had been sudden through this gas, and partaking, in that respect,
of the character of a spark. In rare coal gas, the brush forms were better,
but the light very poor and the colour gray.

1461. _Carbonic acid gas_ produces a very poor bru