Hydrogen and Nitrogen by Electron Impacts. 787 



In Table IV. the values of " b " are given for approxi- 

 mately the same initial pressure (^ 950 x 10 -5 mm.), but 

 for different accelerating potentials. Runs E, ¥, ... . K 

 (Table II.) were used. (The initial pressure in run L, which 

 is included, was only 430 x 10" 5 mm.). 



Table IV. 

 Values of " &." 









! 291 









; VoltS. 



1 



■« h.p; 



' law. 



! -ioo 



"b»\ 











«l.p.' 



' law. 



! 141 



F 



G 



H 



I 



J 



K 



L 



141 



volts. 



71 



volts. 



43 



volts. 



28-3 

 volts. 



23-7 

 volts. 



17-9 

 volts. 



13-3 

 volts. 



•133 



•122 



•088 



•064 



•056 



•029 



0031 



•180 



•168 



•122 



•088 



•077 



011 



•0045 



For 8'9 volts, "6" could not be measured, it was certainly 

 less than *0005. The values for '• b " are shown in lig. 3. 

 It will be seen that " b " rises rapidly between 13 and about 

 40 volts, and does not change much after about 60 volts. 

 (An earlier experiment showed that " b " was much the same 

 for 600 volts as for 70 volts ; these later and more accurate 

 experiments indicate a drop between 140 and 290 volts. It 

 is proposed to investigate this further.) 



Fisr. 3. 



•20, 



200 250 



Accelerating volts. 



Control Experiments with Helium. — Helium was admitted 

 to a pressure of 950 x 10~° mm., and a run was taken with 

 71 volts accelerating potential. The clean up was Lss than 

 one two-hundredth part of that in the corresponding hydro- 

 gen run (G). The helium was purified for a short time 

 over charcoal in liquid air. The slight clean up was no 

 doubt due to impurities, probably hydrogen, as the apparatus 

 was not outgassed for more than half an hour, nor were 

 excessive precautions taken to secure pure helium. 



