28 



Proceedings of the Royal Irish Academy. 



measuring-tube is greater for fig. 2 than for fig. 3. The calculated mobilities 

 corresponding to the well-marked bends are given. The agreement is fairly 

 good, considering the nature of the observations. The time which has elapsed 

 from the production of the ions to the measurement of their mobilities is 

 about 60 seconds in the case of fig. 3 and 30 seconds for fig. 2. In neither of 

 these curves is the bend shown corresponding to the mobility "00034 cms. 

 per second. This was carefully determined by observations with slower air- 

 currents. 



200 

 20 



'too 



Voids 



6oo 



So 



Fig. 2. 



2oo 

 90 



Fig. 3. 



VUts 



3oo 

 6o 



The curves do not pass through the origin, which indicates that there is at 

 least one further group of ions of mobility greater than "0013 cms. per 

 second. In later work it will be found that five sets of ions were shown to 



