50 Mr. E. M. Wellisch on the Motion of 



ions and of free electrons ; the ions enter at 80 volts and 

 possess a normal mobility, viz. K = *430. The curve marked 1 

 was obtained only a few minutes before that marked 2, and it 

 will be observed that the electrons have decayed appreciably 

 during this short interval of time. The third curve was 

 obtained 2\ hours after the introduction of the vapour ; there 

 is now only the slightest indication of free electrons, while 

 the negative ion has still a normal mobility (K = *428). 



Subsequent experiments were made with freshly intro- 

 duced vapour at a pressure of 20 mm., and gave evidence of 

 a very large percentage of electrons ; however, the ageing 

 effect was very pronounced, the free electrons decreasing in 

 number so rapidly that no regular curve was obtained. 



The general indications seem to be that in the pure vapour 

 of petroleum ether a large fraction of the negative carriers 

 are free electrons, the negative ions if present at all appearing 

 only in small numbers; the free electrons are, however, 

 extremely sensitive to the presence of some constituent which 

 arises gradually in the vapour, with the result that at the 

 expiration of a few hours the electrons have disappeared and 

 the current of negative electricity is due entirely to ions. 



The nature of the constituent which occasions the ageing- 

 effect in the vapour can at present only be conjectured : 

 systematic experiments are necessary before a definite con- 

 clusion can be reached. There is distinct evidence, however, 

 that we are dealing here with a true electron sink ; in other 

 words, this constituent, whatever be its nature, is capable of 

 absorbing an electron during its drift motion through the 

 vapour, and in this respect must be carefully distinguished 

 from impurities such as oxygen, which seem to require for 

 the absorption of electrons velocities considerably higher 

 than those which are afforded by thermal agitation at ordinary 

 temperatures. The effect of the latter type of impurity is to 

 reduce the number of free electrons in a gas and at the same 

 time to diminish appreciably the velocity of the electron 

 through the gas ; this diminution in velocity has been ascribed 

 (v. sec. 4 B) to the comparatively inelastic impact between 

 the electron and the molecule of the impurity. 



In the experiments with the vapour of petroleum ether the 

 effect of the impurity is to Cause likewise a reduction in the 

 number of free electrons and a diminution in the electron 

 velocity; the electron, however, appears now to be capable 

 of acquiring all velocities intermediate between that of a 

 free electron in the pure vapour and that of a negative ion. 

 We seem, therefore, to be dealing with a carrier which 

 changes continuously and progressively from a free electron 



