744 Ionic Mobilities in Hydrogen. 



However, in the corresponding number of the Bericht. d. 

 D. Phys. Gesellschaft (xii. p. 291, 1910), Franck published 

 his work on the mobility of ions in pure nitrogen and argon. 

 The similarity between his results and ours was so striking 

 that it justified the assumption that we had greatly over- 

 estimated the part (if any) played by back-discharge, and 

 that, as in nitrogen and argon, we were in reality dealing 

 with true changes in mobility. This was pointed out by one 

 of us in 'Nature' (vol. lxxxiv. p. 530, 1910), and it was 

 later shown (Phil. Mag. xxi. p. 585, 1911) that other effects 

 in point-discharge in pure and impure hydrogen could be 

 readily explained by the existence of electronic negative ions. 



Though Franck recorded a negative result in hydrogen, 

 Haines has now confirmed the existence of electrons in the 

 pure gas ; but he also appears to have obtained, in addition, 

 evidence of other types of negative ion. The wind-pressure 

 method merely gives the average mobility of all ions present, 

 but the proportion of normal or intermediate ions to electrons 

 in the pure hydrogen WQ used must have been small. Possibly 

 this may be attributed to the stronger fields in which the 

 ions travel in point-discharge ; or possibly to the efficacy with 

 which the walls of the vessel and the surfaces of the elec- 

 trodes may be "cleaned" by a preliminary point-discharge. 

 In negative point-discharge the rate of combination per 

 coulomb between hydrogen and oxygen is abnormally high 

 when oxygen is present in minute quantities. [The rate of 

 combination is probably a function of the number of electrons 

 transferred, loc. cit.~\ As oxygen and air were the only 

 impurities used, it was comparatively a simple matter to wash 

 the vessel out with pure gas, carry out a preliminary heavy 

 negative discharge, and then perform the mobility measure- 

 ments before the diffusion of oxygen from the deeper layers 

 in the surfaces had time to make itself felt. The efficacy of 

 this procedure was shown by the fact that the effect on the 

 mobility of the negative ion of adding 0'0002 per cent, of 

 oxygen to pure hydrogen was readily observed, and its rapid 

 disappearance with continued discharge traced. 



Haines observed that hydrogen was neutral when added 

 as an impurity to nitrogen. We found that nitrogen was 

 neutral when added as an impurity to nitrogen ; thus the 

 effect on the mobility of adding oxygen was identical with 

 that of adding air containing the same percentage of oxygen. 

 This naturally follows from the view that nitrogen and 

 hydrogen in the pure state at atmospheric pressure both, 

 yield electronic negative ions. 



University of Bristol, Yours £ait ^ fu i 1 /' 



October 16, 1915. A. M. TVNDALL. 



