220 A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE XII
very faint jets; the colour was nearest to a dark red. In
hydrogen the jets are best developed at about 100 mm. pres-
sure with the help of red Leyden jar sparks: the tint is a
fine blue indigo; the brightness is not great. But the size is
much larger than in air, so that even in a glass receiver 20
cm. high the jet cannot fully develop itself, but spreads out
along the top. In the vapour of turpentine, and of ether, and
in coal gas, the jets are greenish-white, short, sharply defined.
The spectrum of the light is in air and oxygen continuous,
especially bright in the red, yellow, and green; in vapours
containing carbon it is a band spectrum, which could with
certainty be recognised as one of carbon; in hydrogen it was
difficult to observe, owing to the faint light, yet at various
times I recognised several bands with certainty, of which the
most conspicuous was at any rate very close to the greenish-
blue hydrogen line, the others being situated more towards
the violet; in nitrogen a spectrum could not be obtained.
9. In the gases mentioned it is always possible to detect
the presence of a jet by its mechanical effects, but the jet is
by no means clearly visible under all conditions, and its visi-
bility seems to depend on very curious conditions. The air
of a room when moist gives a very much weaker appearance
than when it has been dried. When we place a dish contain-
ing sulphuric acid or phosphorus pentoxide or calcium chloride
under the receiver of the air pump, we see the appearance
become more distinct as the air becomes drier. The behaviour
of hydrogen is still more incomprehensible. When the receiver
was filled with this gas the discharges of the Ruhmkorff coil
did not at once produce the appearance,-Leyden jar sparks
were necessary; but when the jet had once been rendered
visible, it could be maintained without using the Leyden jar.
But it lasted only a few minutes and then went out, without
my being able to reproduce it. I have not succeeded in finding
out the conditions necessary for visibility. Greater or less
humidity seemed without effect; equally without effect was the
presence of a small quantity of oxygen. When the hydrogen
was kept for several hours under the air pump without being
used, it did not lose its power of becoming luminous; but
when this power had once been destroyed by the discharges,
it was not restored even after hours of rest. I should attribute