jVIcClklland and Noian — Ions Produced through Mercurij. 25 



and close to the surface of the mercury, so that the air sprayed the mercury 

 against the iron plate. In some of the experiments the iron plate was not 

 used, and a straight nozzle pointing downwards was employed. 



A. part of the air which had passed through the mercury was drawn off 

 by means of a gasometer attached to iV, through a length of tubing between 

 C and B, and through the measuring tube .1/. The surplus air escaped through 

 the orifice 0, which could be opened or closed to suit the current drawn off 

 by the gasometer. The quantity of air drawn through the measuring tube M 

 could be deduced from the rate of motion of the gasometer, and this rate 

 could be varied as desired. 



The measuring-tube M consisted of a cylindrical brass tube with a metal 

 rod 55 cms. long stretched along its axis. The tube was connected to a 

 battery of small storage cells, and the inner terminal to an electrometer, so 



ElectforneVer 

 Emth. 



A 



C D_ 



M 



^EartK 



\attery 



Fial. 



that the current corresponding to different voltages could be measured in 

 the usual way. A sensitive electrometer was used giving about 1700 scale 

 divisions per volt; but in many of the observations extra capacity was 

 employed to reduce the motion on the scale. 



Any desired length of tubing could be interposed between G and D so as 

 to vary the time between the formation of the ions and the measurement of 

 their mobility. The rate of flow ot the air-current through the tubing could 

 also be varied. 



When dealing with the more Uiobile ions, a different measuring-tube was 

 used, with greater cross-section, and with a shorter inner terminal. 



In some of the experiments the air was dried before being passed through 

 the mercury. The apparatus used in drying the air consisted of two towers 



R.l.A. EKOC, VOL. XXXIII., SECT. A, [4] 



