in Hydrogen and Nitrogen. 509 



are two other kinds of ion shown at B and by curves 2 

 and 3 respectively. The results are grouped together in the 

 following table : — 



Hydrogen. r/=6*45 cm. rc=50. Mean temp. 19° C. 

 Gas not specially purified. 



Negative ion u= 7*4 cm./sec. 



Positive ion ii— G'O 



Gas purified. 



Negative ions A u= 9'4 (mean nine values). 



]> U — 1S9 (mean two values). 



u = 2S'l (mean five values). 



D u = 170 (electrons). 



Positive ion u= 60 (unaltered). 



It will be seen that the ions appear in different propor- 

 tions in different cases. What the conditions are that deter- 

 mine this variation remain obscure, for the degree of purity 

 required is one beyond adequate control or definition. It 

 can be said generally that the higher the mobility of the ion, 

 the more sensitive it is to impurities in the gas. The ion B 

 is the exception, as it appears in some cases but is quite 

 absent in others seemingly as favourable. It is hoped that 

 some further work will throw light on this point, and provide 

 data for a theoretical discussion of the results. 



Summary, 



The mobilities of free electrons in pure nitrogen at atmo- 

 spheric pressure have been measured, and the effect on them 

 of various imparities has been tested. In some cases only a 

 minute trace of the foreign gas is required to prevent com- 

 pletely the appearance of the free electrons. In nearly all 

 cases their life in the free condition, as measured by the 

 mobility, is much reduced by small proportions of the 

 impurity. 



Hydrogen has been found to belong to the list of gases in 

 which free electrons can exist at atmospheric pressure. The 

 hydrogen must be purified with great care to obtain these 

 results. Under these conditions other negative ions of high 

 mobility appear, for which measurements have been made. 



It is interesting to note that, according to results recently 

 published by Wellisch*, the electrons appear in dry air at 

 pressures below 10 cm. 



It is a pleasure to record my thanks for constant encourage- 

 ment and advice to Professor K. J. Strutt, of the Imperial 

 College of Science and Technology, where the work was 

 carried out. 



* Wellisch, Amer. Journal of Science, May 1915. 



