406 Dr. Richardson: Effect of a Luminous Diseharge 
measurements of this quantity were made at a temperature 
so high that the fraction of the imparted emissibility which 
was not driven off was negligible in comparison with the 
total. To make sure that Ce was no further emission, 
readings were taken every minute for three or four minutes 
o 
after heating the wire. 
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TIME IN MINUTES 
The preceding experiments were all made by passing the 
discharge in air. Oxygen and nitrogen were next tried, and 
found to give about as great an effect as air. Purification 
of the nitrogen by heating sodium in it by Warburg’s method 
did not appear to influence the result. On letting hydrogen 
into the apparatus, a small imparted emissibility was obtained 
at first which gradually diminished each time the experiment 
was repeated. Hven with fresh hydrogen the effect was 
small compared with that in air, so that the discharge in pure 
hydrogen does not seem to produce this effect. ‘The effect 
was obtained in mixtures of hydrogen and air, but the amount 
appeared to be less than would be given by air at the same 
pressure if it were not diluted with hydrogen. It was also 
noticed that when the apparatus had been filled with hydrogen, 
the effect of the discharge in air was small at first, “but 
eradually increased during the first three or four times the 
discharge was passed until it reached a comparatively steady 
value. 
The influence of the pressure e of the gas in the discharge- 
tube was also examined. The discharge-current was kept 
