608 Mr. S. Russ on Distribution in Electric Fields of 
small compared with that o£ the a particle, is high compared 
with the velocity o£ an ordinary gaseous molecule. 
On the view above cited a certain number of collisions 
with the gas molecules is necessary to sufficiently reduce the 
velocity o£ the active deposit particles in order that moderate 
electric fields may direct them to cathode or anode. 
Hence we might expect the effectiveness of a molecular 
encounter to depend on the nature of the gaseous molecules 
with which the active matter is associated. Experiments 
to test this point were therefore made with three gases 
differing considerably in molecular constitution, namely, air, 
hydrogen, and sulphur dioxide, and a comparison between 
the activities of cathode and anode over a range of pressure 
extending from *1 mm. to 1 mm. was made with the low 
pressure system already described. The silvered vessel was, 
Fie-. 4. 
however, replaced by another glass vessel containing two 
concentric cylinders as seen in fio-. 4. 
J o 
