﻿764 Dr. E. B. Ludlam on the Action of 



was not the case. The Becker condenser is admirably suited 

 to the measurement of the mobility of large slow-moving 

 ions, but suffers from the disadvantage that a very per- 

 ceptible loss of the smaller ions takes place by recombination 

 and diffusion to the walls of the apparatus before the con- 

 denser is reached. To avoid this the radial field was sacrificed 

 by adopting an unusual system of electrical connexions 

 whereby all the ions of the required sign were drawn towards 

 the axial rod of the condenser which was connected to the 

 electrometer. 



Influence of the rate of flow of the gas on the number of ions 

 carried into the condenser. — Compressed air from a cylinder 

 was driven at various speeds through the apparatus and the 

 deflexion due to the negative ions was measured. The 

 number of positive ions was so small in comparison with the 

 negative ions that the latter could be regarded as all produced 

 by the action of the light on the walls, and therefore constant, 

 thus the complication arising from the fact that with increased 

 speed more gas is exposed to the action of the light was 

 avoided. With a speed of 1 litre per minute the deflexion 

 was 1"5 cm., with 3 litres per minute this had increased to 

 4'2 cm., and to 8 cm. at 5 litres per minute. In consequence 

 of this result a rapid stream of gas was employed in all the 

 later experiments. 



Experiments on Chlorine. 



A current of air at a speed of four litres per minute was 

 sent through the apparatus, and chlorine in gradually 

 increasing amounts was mixed with it. 



When no chlorine was present very few positive ions were 

 formed, nor was there any marked increase when 0"5, 1*5, 

 15, and 90 per cent, chlorine were added, the velocity of the 

 mixed gases being always 4 litres per minute. 



With the pure chlorine the deflexion was hardly measur- 

 able at all (velocity 3 litres per minute, as the meter would 

 not measure more of the heavy gas). 



The addition of very small quantities of chlorine (less than 

 •5 per cent.) to pure air was found in some early experiments 

 to diminish the number of negative ions and increase the 

 number of positive ones. The decrease in the negative ions 

 was to be expected, for their number is swelled by the 

 presence of ions from the walls of the vessel, and these are 

 fewer when the light has to traverse a powerfully absorbing 

 gas before striking the walls. 



