40 Proceedings of the Royal Irish Academy. 



In examiniug curves in this connexion it must be remembered that the 

 correction for pressure has to be applied, and also that a variation of 1 volt 

 in the critical voltage corresponds to a variation of less than 2 per cent, in 

 the mobility deduced. One curve is given which shows no current until a 

 voltage of 60 is reached. The ion of mobility about 2'5 shown by the other 

 curves and not found in the air-stream method is here absent. This one 

 curve was plotted with saturated air, a vessel containing water having been 

 introduced into the box. The values found from the best curves are tabulated 

 below. 



lONiSATiON IN Undried Aie. Alternating Field Method. 

 (Corrected to 76 cm. pressure.) 







Positive. 











Negi 



itive. 





2-56 



2-13 



1-81 



1-59 



1-46 





2-lo 



1-71 



1-52 



1-43 



2-50 



2-10 



1-79 



1-58 



1-46 







1-60 



1-49 



1-40 



2-41 



2-07 



1-75 



1-58 



1'46 







l-o7 



1-50 



1-41 





1-93 



1-74 

 1-72 

 1-70 



1-57 

 1-52 

 1-50 

 1-50 

 1-49 



1-44 

 1-44 

 1-44 

 1-43 

 1-40 









1-49 



1-39 





"Weighted means : 



2-5, 204, 



1-73, 



1-54, 



and 



1-42. 





In addition to the values given above, a number of positive curves have 

 been found showing an ion of mobility about 1'30 ; in some cases this ion has 

 shown up on curves which also showed the ion of mobility a little greater 

 than 1'40. We hope to re-examine this point later. These observations 

 serve to demonstrate the existence of the four principal groups which we 

 have already found by the other method. They indicate the existence of an 

 ion which we had not previously found, and they show no trace of the ions 

 of higher mobility clearly shown by the other method. We decided to 

 proceed at once to the examination of dry air. 



Uxamination of Dry Air hy the Alternating Field Method. 



There is no doubt that the best way to dry a gas is to leave the drying 

 agent in continuous contact with it. With this in view, we fitted a rim to 

 the upper plate of our apparatus (as shown in fig. 3), and spread a layer of 

 phosphorus pentoxide over it. The apparatus was immediately closed up, all 

 joints being carefully sealed. The effect of drying was at once noticeable. 

 A progressive increase in the ionisation immediately set in. We were 

 examining the negative ionisation, and we found that the curves were increas- 



