Ions and Electrons through. Gases. 43 



extremely sensitive to the presence of impurities in the gas 

 under consideration, the slightest trace of oxygen, for 

 example, causing a considerable reduction in the value. 

 Recently Haines * has investigated the motion of free elec- 

 trons in pure nitrogen at atmospheric pressure, and has 

 obtained a mean value of 367 for the mobility. 



Carbon dioxide appeared especially suitable for expe- 

 riments in this connexion because the electrons were rela- 

 tively numerous in it, and at the same time the density of the 

 gas was sufficiently great to justify the belief that the 

 velocities would not be inordinately large and thus incapable 

 of measurement w r ith the apparatus at disposal. Even with 

 the high frequency of 800 alternations per second and at the 

 highest practicable pressures of the C0 2 , it was found that 

 the values of the critical potential (V ) were considerably less 

 than 10 volts, so that the observation error in the deter- 

 mination of the electron mobility was of necessity considerable. 

 Moreover, there was also the difficulty connected with the 

 presence of the ageing effect which, as mentioned above, 

 occurs at the higher pressures ; it was of course not feasible 

 to attempt determinations at the lower pressures where this 

 effect is absent, because the electron velocities become 

 excessively large. 



It was in every instance found that the effect of age (i. e. of 

 allowing the C0 2 to remain for any length of time in the 

 apparatus) was to reduce considerably the velocity of the 

 electrons. On this account great care was taken to exclude 

 impurities, the gas being in all cases swept several times 

 through the measuring vessel, and the observations quickly 

 made after the final introduction. 



In figs. 5 and 6 there are given a few typical E curves 

 which w r ere obtained in the determination of Y for the free 

 electrons ; the ions do not make their appearance until much 

 higher potentials are employed. Reference will be made 

 later to the fact that the experimental results rendered 

 doubtful the assumption that the velocity of the electron is 

 proportional to the applied field, so that the use of the term 

 u mobility''' is not certainly justified ; however, it was 

 thought useful to make the calculations on the assumption 

 that there exists a distinct mobility for the electron just as 

 for the ion. In the following table there are given the 

 results of the mobility determinations for freshly prepared 

 C0 2 together with some of the results for C0 2 in various 

 degrees of impurity ; the symbol K denotes the mobility 



* Haines, Phil. Mag. vol. xxx. p. 503 (1915). 



