Ionic Mobilities in Hydrogen. 743 



If these observations are combined with those of J. Koch* 

 and H. C. Reutschlerf given for the visible spectrum and 

 carried as far as \ = 3342 /u, we must suppose the existence 

 of another band of absorption in the ultra-violet, as the values 

 of n increase with the diminution of \. 



In fine, the study of the dispersion of carbon dioxide 

 between 0'8/a and 13 /jl gives a marked band of absorption 

 at 4*270//. and the tendency towards another one at 14'7//,. 



The values obtained for n may be used to a first approxi- 

 mation to find the formula of dispersion for carbon dioxide, 

 which formula I propose to try to find. Its discovery would 

 be an interesting one as it would be an application of the 

 theory of dispersion for gases. 



This work was done in the Physics Laboratory at Tubingen. 

 I am much obliged to Prof. F. Paschen for his kind advice, 

 as well as for the excellent apparatus which he put at my 

 disposal, and to Miss Buckley of Girton College for her 

 assistance in preparing this paper for the press. 



LXXIX. Ionic Mobilities in Hydrogen, 

 lo the Editors of the Philosophical Magazine. 



Gentlemen, — 

 ^I^HE experiments on the mobility of ions in hydrogen, 

 J_ described by Haines in the October issue of the 

 Philosophical Magazine, confirm and amplify some measure- 

 ments published in 1910 \ by Dr. Chattock and myself. 



It was found that in very pure hydrogen the pressure of 

 the electric wind from a discharging point was very minure. 

 The mean mobility of the negative ions, deduced from this, 

 on the assumption that the discharge was unilateral through 

 a portion of its path, was about 230 cm./sec. volt/cm. The 

 influence of the addition of known percentages of oxygen in 

 reducing the mobility of the negative ions was studied very 

 completely, the percentages added ranging from 0*0002 per 

 cent, to 3'7 per cent. The effect of these additions on the 

 mobility of the positive ions was shown to be nil. 



At the time there was no evidence from the work of other 

 observers of the existence of electronic negative ions in any 

 gas at atmospheric pressure, and another explanation of the 

 results was sought. It was suggested that the high values 

 of mobility obtained were spurious, and the results due to the 

 setting in of a back-discharge from the plate as the purity of 

 the gas increased. 



* J. Koch, " Dispersionmessungen an Gasen . . ." Nova Acta Reg. 

 Soc. Scient. Upsaliensis, ser. 4, vol. ii. No. 5. 

 t H. C. Reutschler, Aatrophys. Journ. xxviii. p. 435 (1908). 

 % Phil. Mag. [6] xix. p. 453 '(1910;. 



