Welllsch — Motion of Ions and Electrons through Gases. ^3 



The agreement in the case of the positive ion is as good as could 

 reasonably be expected in view of tlie difficulties attendant upon 

 experimenting with vapors ; we can say with a high degree of 

 certainty that to each vapor there corresponds a definite value 

 of the mobility of the positive ion. The mobilities of the neg- 

 ative ions in alcohol and petroleum ether are, however, in 

 greater disagreement than can be accounted for by experi- 

 mental error. We have seen (sec. 4C) that in the pure vapor 

 of petroleum ether the negative carriers are practically all 

 electrons and that the negative ions come into evidence only 

 when the vapor is allowed to remain for some time in a closed 

 vessel. We are therefore constrained to associate the negative 

 ions in this vapor with impurities; and it is of course not 

 improbable that there are other vapors in which the existence 

 of negative ions is conditioned by the presence of some impurity. 

 Refined experiments on ionic mobilities in vapors are necessary 

 before the nature of the negative carriers can be determined. 



5. Discussion of Results. 



It is proposed to discuss briefly in this section the significance 

 of the results of the present experiments in connection with the 

 theory of electric conduction in gases. Several of the points 

 brought forward have already received attention in the previous 

 paper ; referenc3 is made here to these only for the sake of 

 continuity. 



The experiments with air showed that the mobility {k) of the 

 positive ion varied inversely as the pressure {p) of the gas 

 down to the lowest pressure which it was convenient to employ 

 ('05"""^). It was not thought necessary to proceed to very low 

 pressures in the case of carbon dioxide and hydrogen but all 

 the indications were that the law jf?^=const. would continue to 

 be valid. The validity of this law over a wide pressure range 

 signifies that the nature of the positive ion remains unchanged 

 throughout this range. 



The same law was found to be valid for the negative gas ion 

 but only after care had been taken to separate the negative 

 carriers into the two components, electrons and ions. It was 

 found that the apparently anomalous increase at reduced pres- 

 sures of the mobility of the negative ion to which many 

 observers had previously drawn attention was occasioned by 

 this dual nature of the negative carrier; when the ions were 

 considered apart frotn electrons all the anomalies disappeared, 



the velocity being expressible in the form v=-\ .. 



It is instructive in this connection to consider the difference 

 between the present and the older point of view. It has long 



