220 Prof. Rutherford on Dejiendence of the Current through 



The ratio observed for air saturated with alcohol- vapour 

 at 18° C. was 1*04. 



We thus see that, since the currents are proportional to the 

 velocity of the ions, the negative ion at first moves more 

 slowly with the addition of alcohol-vapour, while the velocity 

 of the positive ion is unaffected. This is probably due to the 

 condensation of alcohol round the negative ion. The final 

 slow decrease of the currents in both directions is probably 

 due to the disturbance of the current of ionization at various 

 points, and alteration of the velocity of the ions in conse- 

 quence of moving through air mixed with alcohol-vapour, 

 rather than to a condensation of alcohol on both positive and 

 negative ions. 



If the air was only partially dry, the effect of alcohol-vapour 

 was not nearly so marked. The ratio of the currents for air 

 with some moisture present was observed to be 1*19. On 

 addition of alcohol-vapour the ratio fell to 1*12 ; while if the 

 air had been completely dried, it would have fallen to 1 04, or 

 even less. It thus appears that if the two vapours are present, 

 the water condenses on the negative ion more readily than 

 alcohol. 



Since alcohol has a smaller value of surface-tension than 

 water, it was thought that possibly surface-tension determined 

 the amount of condensation in the negative ion. For this 

 reason the vapour of ether was tried, which has a smaller 

 surface-tension than alcohol. Ether-vapour, however, was 

 found to occupy an intermediate position between alcohol- 

 and water-vapour. The reduction observed was from 1*35 

 for fairly dry air, to 1*23 for air saturated with ether-vapour 

 at 18° C. 



The introduction of the vapour of methyl-iodide reduced 

 the ratio of velocities from 1*37 to 1*11. 



These results can only be considered as approximate ; but 

 they serve to show in a simple manner the effect of these sub- 

 stances in altering the velocity of the negative ion. 



It is intended to continue the investigation of the effect of 

 vapours and other agents in affecting the velocity of the ions 

 by this method, which is very simple and convenient. 



To account for the slow velocity of the ions through the gas, 

 I have shown that, on the principle of the kinetic theory of 

 gases *, the ion behaved as if a cluster of molecules travelled 

 with it. This result has been confirmed by later investigations f . 

 It has also been suggested that the slowness of motion through 

 the gas might not be due to the size of the ions, but possibly 



* Phil. Mag. [5] xliv. p. 422. 



f Townsend, Trans. Roy. Soc. 1900 ; Lenard, Drude, Ann. iii. p. 298' 



