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



can be accounted for by experimental error. We have 

 seen (sec. 4 C) that in the pure vapour of petroleum ether 

 the negative carriers are practically all electrons, and that 

 the negative ions come into evidence only when the vapour is 

 allowed to remain for some time in a closed vessel. We are 

 therefore constrained to associate the negative ions in this 

 vapour with impurities ; and it is of course not improbable 

 that there are other vapours in which the existence of 

 negative ions is conditioned by the presence of some impurity. 

 Refined experiments on ionic mobilities in vapours 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 signi- 

 ficance of the results of the present experiments in connexion 

 with the theory of electric conduction in gases. Several of 

 the points brought forward have already received attention 

 in the previous paper ; reference 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 mm.). 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 pk — 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 io?is. It was 

 found that the apparently anomalous increase at reduced 

 pressures 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 from electrons all the anomalies disappeared, 



the velocity being expressible in the form v = k — . 



It is instructive in this connexion to consider the difference 

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

 been known that in air at very low pressures the current of 

 negative electricity is due practically entirely to free electrons; 

 at the higher pressures, however, the current is due to the 

 motion of negative ions. What is the nature of the negative 

 carrier at intermediate pressures ? The answer hitherto given 



