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Profs. J. J. Thomson and R. Threlfall. [May 6, 



about 8 to 12 per cent. We thought at first that this diminution 

 might be due to the combination of the nitrogen with the sulphuric 

 acid vapour. In order to test this we had a tube made with a mercury 

 gauge of the kind already described, the difference of level between 

 the mercury in the two legs of the gauge was increased by 0*9 mm. of 

 mercury by the sparking : 09 mm. of mercury are equivalent to about 

 6*5 of sulphuric acid, so that the magnitude of the effect is practically 

 the same whether the guage be filled with mercury or sulphuric acid, 

 and the effect therefore cannot be due to any combination of nitrogen 

 with the vapour of sulphuric acid. 



We then thought it might possibly be due to condensation of gas- 

 on the sides of the tube, though it seemed very improbable that this 

 cause could produce such a large effect upon the pressure. To test 

 this, however, we had a discharge-tube made whose diameter was 

 about 3^- times that of the tubes we had previously been using, 

 so that in this case the glass was much further away from the line 

 joining the electrodes, and in fact was so little affected by the glass 

 that the heating was scarcely perceptible. In this case the final 

 result was the same as for the smaller tubes, though it took longer to 

 arrive at a state of equilibrium ; the difference between the levels of 

 the sulphuric acid in the limbs of the gauge was increased by 4*8 mm. 

 Another reason why the diminution in pressure can scarcely be due to 

 this cause is that it depends very much upon the pressure of the gas. 

 We sealed off a tube at a pressure of 16 mm. of mercury, and found 

 that when the discharge had been sent through it until the pressure 

 had reached a steady state, the pressure had diminished only by that 

 due to 2*5 mm. of sulphuric acid ; so that the diminution is only about 

 half the absolute value, and therefore only one- quarter of the relative 

 value of that which takes place at a pressure of 8 mm. of mercury. It 

 would seem to be difficult to explain this result by the hypothesis that 

 it is due to an adhesion of gas to the surface of the glass. The 

 experiment with the large tube shows that it cannot be due to an 

 absorption of gas by the electrodes, for in this case the diminution in 

 pressure would depend upon the ratio of the volume of the electrodes 

 to the volume of the tube, so that if we increased the volume of the 

 tube, keeping that of the electrodes the same, the effect ought to be 

 diminished ; the experiment with the wide tube showed that it is not. 

 We also found that the effect was not diminished by using a very long 

 tube, about three times as long as the ordinary ones. 



We next tried whether the diminution in the pressure depended on 

 the nature of the electrodes by having a tube made with aluminium 

 electrodes, we got, however, with this tube, the same diminution as we 

 had previously obtained with those furnished with platinum electrodes ; 

 the tube, however, was more troublesome to prepare, as the aluminium 

 electrodes seemed to contain more hydrogen than the platinum ones. 



