26 Proceedings of the Royal Irish Academy. 



packed with calcium chloride, and a large bulb half filled with phosphorus 

 pentoxide. The air first passed through the calcium chloride,, and then 

 slowly over the pentoxide. The drying produced in this way was certainly 

 not perfect, but it was sufficient to show the general effect produced by the 

 partial removal of the water- vapour. 



Mobility of the lois. 



The method used to measure the mobility consisted in plotting the current- 

 voltage curve for a steady rate of flow of the air through the measuring-tube 

 M. To obtain the saturation current corresponding to ions of mobility ii 

 a voltage V is required given by the expression 



eiog^ 



27rF7 



where Q is the volume of gas passing thi-ough the tube per second, i and a 

 the radii of the tube and inner terminal, and / the length of the terminal. 

 If there are a number of groups of ions of different mobilities present in the 

 air, it is clear that the current-voltage curve will consist of a number of 

 straight portions the last of which will be parallel to the axis of voltage, and 

 the voltage for each bend of the curve will be that required to saturate a 

 group of ions. If the bends on the eiu-ve are not clearly defined, it will indicate 

 that we are not dealing with distinct groups of ions, but that the range of 

 molalities is continuous, and the maximum current is reached for the voltage 

 sufficient to saturate the slowest ions present.- 



This method is, no doubt, not a very accurate one, but it is suitable 

 for giving approximate values of the mobilities when the ions occur in a 

 considerable number of quite distinct groups. 



Frelimi'iuiry Observations. 



During the earlier observations a straight nozzle was used, the air simply 

 bubbling through the mercury which was sprayed to some extent against the 

 sides of the glass cylinder. 



It was observed that the mobilities varied with the speed of the air-current 

 and with the length of tubing inserted between the mercury vessel and the 

 measuring-tube M. The longer the time that elapsed between the bubbling 

 through the mercury and the measurements, the slower were the mobilities. 

 When the length of tubing was sufficiently great, the mobilities reached steady 

 values. The current was fully saturated at a voltage corresponding to a 

 mobility of -00034 cms. per second for a gradient of one volt per em. 



