186 Prof. E. M. Wellisch on 



be odd the second and fourth. If p and q are both odd, the 

 terms of the third order in C disappear altogether. 



As appears at once from (34), (36), the last statement may 

 be generalized. However numerous the components maybe, 

 if only odd suffixes occur, the terms of the third order 

 disappear and (36) reduces to (26), 



January 20, 1916. 



XXVII. Free Electrons in Gases. 



To tlie Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN the May number of the 'American Journal of Science ' 

 I published a brief account of the results of certain 

 experiments dealing with the motion of ions and electrons 

 through dry air : it was shown that the electrons remained 

 free during their motion through the gas and that their 

 relative number increased with diminution of pressure, the 

 electrons coming into evidence in my experiments at about 

 8 cm. pressure. These results were found to contain the 

 explanation of the apparently anomalous increase of the 

 mobility of the gas ion at reduced pressures ; when the ions 

 were considered apart from electrons, the mobility remained 

 normal throughout. 



The existence of free electrons in gases at relatively high 

 pressures had previously been demonstrated by Franck for 

 argon, helium, and nitrogen at atmospheric pressure; it was 

 found by him that when these gases were carefully freed 

 from impurities (especially oxygen) the negative carriers 

 were practically all electrons ; the slightest trace of oxygen, 

 however, was sufficient to convert the carriers into ions. 

 This ability to contain electrons in the free state was regarded 

 as being a peculiarity exhibited by the inert gases ; the 

 demonstration of their existence in air, which was indeed so 

 a priori improbable on account of the presence of the electro- 

 negative constituent oxygen, rendered it at once evident that 

 Franck's result was a particular case of a more general 

 principle, and that the free electrons along with ions would 

 be found for all the ordinary gases, the inert gases being 

 conspicuous by reason of their reluctance to form negative 

 ions. 



Immediately after the preliminary publication I resumed 

 the experimental work ; this was completed last July, but, 



