340 Mr. R. S. Hutton on the 



How through the vacuum-tube by continuing the action of 

 the pump. In this way it was possible to fractionate off the 

 hydrogen. Although the work was carried out with as great 

 care as possible, I was unable to detect any difference in the 

 spectrum of the fractionated gas notwithstanding that the 

 method was varied. The compound line-spectrum was ex- 

 tremely bright all the time, at any rate until a fairly high 

 vacuum had been obtained, and working under pressures 

 between 2 and *1 millim. the second spectrum was always 

 very evident; the colour of the discharge in the capillary was 

 mostly of a greyish-blue colour and never red. 



3. Influence of Presence of Traces of Oxygen upon the 

 Hydrogen Spectrum. 



Special precautions had been taken to prevent carbonaceous 

 contamination, the lubrication of all the Geissler mercury- 

 trapped taps being effected with phosphoric acid. Still I 

 thought it might be possible to find if any hydrocarbon were 

 present or not by introducing a little oxygen into the spectrum- 

 tube containing hydrogen, and then making observations for 

 the carbon spectrum. The introduction of oxygen was effected 

 by warming the permanganate-tube mentioned above ; but 

 although 1 repeated the experiment many times, no carbon 

 spectrum was to be seen. It was most remarkable, however, 

 that so soon as the oxygen reached the tube the colour of the 

 discharge in the capillary changed to a very bright red ; and 

 under these conditions it was possible almost entirely to get 

 rid of the second spectrum, or at any rate it was so very 

 dim in comparison with the principal lines that it could 

 not be detected visually. 



These observations were confirmed in many separate expe- 

 riments, photographs of the spectrum being also taken. It 

 was most noticeable in each case that the sudden change took 

 place ; and although in the photographs, some of which had 

 25 minutes exposure, all the brighter lines of the second 

 spectrum came out dimly, the background of continuous 

 spectrum, which seems generally to accompany this second 

 spectrum, was quite absent. 



I next tried sparking with magnesium electrodes to make 

 sure that the excess of oxygen had been removed, and in this 

 way the red colour of the discharge was not altered; and 

 other experiments, in which several fresh additions of hydrogen 

 were made to the tube without causing the disappearance of 

 the red colour, lead me to think that the amount of oxygen 

 necessary is very small. 



This sudden change from the bluish colour to the red seems 



