XII A PHENOMENON ACCOMPANYING THE ELECTRIC DISCHARGE 221
the luminosity to impurities,' did I not feel confident that I
had recognised the spectrum of the emitted light to be a
hydrogen spectrum. However, the vibrations of a plate of
mica placed across the jet are just as lively in moist as in dry
air, in freshly prepared hydrogen as in that which has ceased
to become luminous; so that the visibility of the jet seems to
be only an accidental property.
10. The jets may also be produced in gases at atmospheric
pressure; it is advisable for this purpose to use a discharge
apparatus similar to but smaller than that used before. The
appearance, it is true, is only a few millimetres long and not
very striking, but further experiments may conveniently be
made on it. The heating effect and impact of the jet can be
directly felt. The jet scatters smoke and small flames at a
distance 2 to 3 cm. from the mouth of the glass tube. A strong
current of air bends the jet and drives it to one side. When .
we blow through the opening at which the jet is formed, it
lengthens out; when we suck in air, it shortens.
When we
pass another gas through the opening and invert a test-tube
over it, we get the appearance corresponding to that gas; so
with hydrogen we may obtain a very distinct blue jet, only
a few millimetres long. If coal gas is passed through and
lighted, the flame oscillates violently when the sparks pass;
the apparatus described in § 7 shows that each spark drives out
a small cloud of gas, which burns above the mouth of the tube
and apart from the remaining portion of gas.
11. According to all that has been said, there can hardly
be any doubt that the jet is formed by a luminous portion of
gas escaping from the tube, and it is natural to assume that
the projective impulse is the force of expansion occasioned by
the rise in the temperature of the gaseous content. But if
we place the electrode, which previously was outside the tube,
close to the mouth of the tube inside it, or if we allow sparks
to pass inside a glass tube sealed at both ends and possessing
a lateral opening, in these cases also jets escape from the mouth
of the tube; but they are much weaker than those which would
be produced if the spark also passed through the opening. If
rise of temperature were the cause of the emission, such a
difference could not exist. The above assumption is con-
1 The hydrogen was prepared from pure zinc and dilute sulphuric acid.