364 On tlie Kathode Fall of Potential in Gases. 



accuracy. The earlier experiments on nitrogen showed that a very 

 slight trace of oxygen was sufficient to raise the kathode fall from 

 its true value 232 to 355. In fact, the values for oxygen, nitric 

 oxide, air, and nitrogen with a trace of oxygen, are all nearly the 

 same, which makes it not improbable that in each case the oxygen 

 alone acts as the carrier of the current. 



We have, then, finally the following values for the kathode fall 



Hydrogen 298 



Nitrogen 232 



Oxygen 369 



Water vapour 469 



Ammouia 582 



(Nitric oxide 373) 



The last is enclosed in brackets in consequence of the doubts as to 

 its accuracy. If we leave the result for this last gas out of account, 

 it appears that the kathode fall is approximately an additive quan- 

 tity. Ascribing the values 149, 116, and 184 respectively to the 

 atoms of hydrogen, nitrogen, and oxygen, we get, by addition, 482 

 for water vapour and 563 for ammonia. As each of these depends 

 on three measurements, they may be taken as agreeing with the 

 observed values within the limits of experimental error. Hence, so 

 far as the evidence of these experiments goes, the kathode fall is a 

 property of the atoms rather than of the molecule. 



As the kathode fall is constant for all pressures and currents 

 whilst the potential gradient along the rest of the tube is variable, 

 we may infer that no potential difference less than the kathode fall 

 is capable of causing a discharge through the gas. This conclusion 

 is consistent with the experiments of Mr. Peace,* who found that the 

 minimum difference of potential that gives a discharge in air is 

 something over 300 volts. 



Assuming that the conduction is electrolytic, it seems likely, from 

 the analogy of the electrolysis of liquids, that the kathode fall may 

 prove to be a measure of the energy required to dissociate the gas 

 into the ions that carry the electricity, and the present experiments 

 were undertaken in the hope of finding some confirmation of this 

 hypothesis. They have not, however, provided the kind of evidence 

 that was anticipated. The results can only be reconciled with the 

 hypothesis if further assumptions are made that would put the con- 

 duction in gases on a very different footing from the electrolytic 

 conduction of liquids. The additive nature of the kathode fall 

 might, for instance, be taken as an indication that the carriers of the 

 current are provided by the disintegration of the atoms into much 



* ' Eoy. Soc. Proc./ vol. 52, p. 99. 



