638 Prof. Townsend and Mr. Kirkby on Conductivity produced 



X 



mines the value of — , which gives the pressure for which 



ais a maximum when the force is constant. This result is 

 easily obtained by differentiating a with respect to p in 

 equation 1. The points corresponding to experiments 8 with 

 hydrogen and 6 and 7 with carbonic acid gas are not repre- 

 sented in the diagrams, as the scale would have to be greatly 

 reduced in order to take in points corresponding to such large 



values of - . 



The determination of the constant n for experiments 6 

 tmd 7 with hydrogen and experiments 5 and 6 with carbonic 



acid were determined from the curves. The value of 



x . p 



was found from the curve for the value of — corresponding 



P 

 to the smallest current found experimentally. The value 



of n was then deduced from the equation 



and the number so found was used to obtain the values 



of — for the larger forces. Instead of having recourse 



to this method, we might have made a series of experiments 

 with forces smaller than 80 volts per centimetre, and thus 

 have determined w . This would have involved a good deal 

 of experimental work, as the electrometer-deflexions were 

 very small. We therefore considered it best to calculate n 

 from the determinations of current with 80 volts per centi- 

 metre. 



The last experiments serve to determine the maximum 



values of -. The value of - obtained from experiment 8 

 p p 



with hydrogen was 1T5. In carbonic acid gas, the largest 

 value of - obtained from experiment 6 was 23, corresponding 



to — = 3600 : from experiment 7 it was found that - reached 



P X P 



the value 29 when — was 9120. 



V 



4. The results at which we have thus arrived enable us to 

 compare the mean free paths of ions with those of molecules. 

 We have found that a negative ion makes 11*5 collisions per 

 centimetre in hydrogen at 1 mm. pressure, and 29 collisions 



