E. M. Wellisch — Mobilities of Ions in Air. 599 



the lowest pressures employed in these experiments ; for the 

 positive ion this pressure was ^V'" 1 and for the negative -15 mm . 



The experimental result of Todd that at pressures below 

 l mm there is a simplification in the nature of the positive ion is 

 not confirmed. 



The difference in the mobilities of the negative and positive 

 ion is in all probability the result of a difference in the 

 potential energy of inductive attraction between the ion and 

 the uncharged molecule. Such a difference of course requires 

 explanation. 



Finally, the actual mobilities obtained for the negative and 

 positive ions, viz., 1*93 and 1*23 in thoroughly dried air, are 

 worthy of notice ; it would appear that the effect of thoroughly 

 drying the air is to increase slightly the value of the negative 

 mobility and decrease that of the positive. 



Summary. 



1. An investigation has been made of the velocities of the 

 positive and negative carriers in thoroughly dried air under 

 a variety of experimental conditions. 



2. From 1 atmosphere down to the lowest pressure employed 

 (viz. -^Q mm ) it was found that the mobility of the positive ion 

 varied inversely as the pressure, indicating that the nature of 

 the positive ion remains unchanged over this wide range of 

 pressures. 



3. The negative carriers were found in general to consist of 

 two kinds, electrons and ions, the former increasing in number 

 relatively to the latter with diminishing pressure. Each kind 

 possesses its characteristic velocity in an electric field and 

 there was no evidence of any intermediate stage in the nature 

 of the negative carrier. The electrons appeared in measurable 

 amount in the present experiments at pressures below 8 cms . 



4. The negative ion remains unaltered in character over a 

 range of pressures from one atmosphere down to the lowest 

 pressure employed (viz. '15 mm ). 



5. A brief discussion of the results is given in which the 

 view is expressed that all previous experiments on the velocity 

 of the negative ion at relatively low pressures are based on the 

 untenable assumption of the presence of an " average " ion and 

 are thereby invalidated. 



Sloane Laboratory. Yale University, 

 New Haven, Conn., April 14, 1915. 



Am. Jour. Sct.— Fourth Series, Vol. XXXIX, No. 233.— May, 1915. 



