Stages of Ionization by Collision. 965 



planes of the electrodes. B is 'a quartz plate o£ diameter 

 4' 5 cm. The bell-jar, J, was so affixed as to render the 

 apparatus air-tight. J was attached to drying-tubes, a 

 Toepler pump, a MacLeod gauge, a pressure adjustor, and a 

 reservoir containing a drying agent. 



Fig. 2. 



The apparatus was made in the laboratory by Mr. Bush, 

 to whom I should like to take this opportunity of expressing 

 my indebtedness for the accuracy shown in the construction. 



The ultra-violet light was obtained from an aluminium 

 spark-gap of length 3*5 mm. placed 6 cm. vertically below 

 the centre of Gr, the length of the spark-gap being at right 

 angles to the length of electrode 2. The Ruhmkorff coil 

 and spark-gap were enclosed in an earthed lead box with a 

 suitable aperture above the spark-gap. 



Let q be the positive charge left on electrode 2 due to the 

 liberation of negative ions by the ultra-violet light, and let 

 <?ij <?2? an( J <?3 be the positive charges which arrive at elec- 

 trodes 1, 2, and 3 respectively, as a result of the ionization 



Phil. Mag. S. 6. Vol. 27. No. 162. June 1914. 3 kS 



