IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 279 



weights ; the smaller mobilities invariably belonged to gases possessing high critical 

 temperatures (the vapours) ; the larger mobilities to gases with low critical tempe- 

 ratures. 



6. From the kinetic theory of gases an expression has been deduced for the 

 mobility of an ion moving through a gaseous medium under the influence of an 

 electric field. This expression takes into account the effect of the charge carried by 

 the ion on its mean free path and involves only known physical constants of the gas. 



7. As a result of the theoretical considerations it appears that the experimental 

 values of the mobilities in the different gases at various pressures, as well as certain 

 observed deviations from the law connecting the mobility and gaseous pressure, can 

 be explained approximately on the supposition that the ion consists of a single 

 molecule with which is associated a charge equal to that carried by the monovaleut 

 ion in electrolysis. 



I take this opportunity of expressing my indebtedness to Mr. H. C. WILSON for 

 setting up part of the apparatus ; in conclusion, my thanks are due to Prof. THOMSON 

 for the generous interest he has manifested in the research and to Prof. LARMOR for 

 helpful advice. 



