XIII
247
EXPERIMENTS ON THE CATHODE DISCHARGE
already described, which was used in the preliminary experi-
ment of this section, was exhausted so far that the discharges
of a large induction coil could only just traverse it: under the
action of such discharges there was a brilliant phosphorescence
at the end opposite the cathode. After what we have already
said, there can be no doubt that the current-paths are re-
stricted to the immediate neighbourhood of the electrodes,
which are quite near to one another, and that only the cathode
rays traverse the length of the tube. Now at the phosphor-
escing end of this tube there happened to be a drop of mercury.
When that part of the tube was heated, so as to vapourise the
mercury and produce there a gas of comparatively high density,
the end of the tube became filled with crimson light, which
showed the spectrum of mercury. The green phosphorescence
of the glass then faded away, and ceased entirely when the
stria of mercury vapour attained a certain thickness. By
means of a magnet the cathode rays could be made to follow
a path in which they had not to traverse the vapour; the
luminescence of the latter then ceased, and was replaced by a
green phosphorescence on the glass at the side of the tube,
where the rays now fell In this way one could at will pro-
duce a luminescence of the glass or of the mercury vapour.
By further heating and distilling, a larger portion of the tube
could be filled with the heavy vapour; it was then found that
the luminescence only extended to the 5 or 6 cm. of the
portion which lay nearest to the cathode, the part of the tube
behind it remaining dark. Finally, when the whole tube was
filled with the heavy vapour, the luminescence-in the form of
the ordinary cathode light-filled the space about the cathode
for a distance of a few centimetres. Thus the cathode rays
first excite luminescence when they enter a denser medium
and are themselves absorbed by it. For this absorption an
infinitely thin stria of a solid suffices, but a finite stria of a
gas is requisite. The denser the gas the shorter the distance
through which the cathode rays can penetrate into it. This
is probably one reason why the cathode light in comparatively
dense gases is restricted to the immediate neighbourhood of
the cathode.
3. There can be no doubt that in the preceding experi-
ment the luminescence of the gas, even in the immediate