504 Mr Horton, The spectrum of the discharge 
The experiments above described indicated that the lines 
observed in the orange and red of the spectrum were caused by 
the presence of mercur}^ vapour in the discharge tube. It was 
therefore expected that they would be more brilliant if a mercury 
anode was used. This was found to be the case. The orange 
lines were seen with a current as small as 2 milli-amperes, and 
the red lines when about 10 milli-amperes were passing through 
the tube. 
Experiments were next made to see whether the gas condensed 
by immersing part of the apparatus in liquid air would give these 
red and orange lines when subjected to the ordinary discharge 
from an induction coil. For this purpose a small vacuum tube 
with aluminium electrodes was joined on to the discharge bulb, 
being separated from it by a mercury barometer cut-off so that 
anything condensed could be confined in a small volume by raising 
the level of the mercury barometer reservoir. It was hoped by this 
means to obtain the gas at a sufficient pressure to enable its 
spectrum to be studied. The apparatus was pumped down to a 
low pressure and then pure hydrogen was let in and pumped out, 
for, as already mentioned, the strange lines were best seen when 
the residual gas in the apparatus was hydrogen. The pressure 
being very low, it was with great difficulty that the discharge 
could be made to pass in the vacuum tube. The spectrum of the 
light, which was very faint, showed the hydrogen led line and the 
mercury yellow and green (the blue and violet regions were not 
examined). 
The vacuum tube was then placed in liquid air and the 
discharge started from the lime cathode to the mercury anode in 
the discharge bulb. After about half an hour the vacuum tube 
was isolated from the discharge bulb by means of the mercury 
cut-off. The liquid air was then removed and the aluminium 
electrodes of the vacuum tube were connected to the terminals of 
an induction coil. By the time the spectroscope had been got 
into position for observing the spectrum the tube had warmed up 
a little, but was covered with solidified moisture from the air. 
The spectrum was the elementary line spectrum of hydrogen and 
was faint, very little luminosity being given by the tube. In a few 
seconds the tube became much brighter and a brilliant carbon 
monoxide spectrum was observed. This, after a few seconds, faded 
away, leaving the secondary spectrum of hydrogen. The tube 
meanwhile was getting much harder and the mercury lines were 
gradually coming into the spectrum. When the mercury spectrum 
had become fairly bright the three orange lines A = 6232, A, = 6121, 
A = 6070 were noticed to stand out brightly among the secondary 
hydrogen lines. These latter got fainter and disappeared and the 
tube became very hard, the mercury lines and those in the orange 
