

Hydrogen and Nitrogen by Electron Impacts. 785 



each time there had been a thorough outgassing. If the oat- 

 gassing were omitted, other conditions being the same, the 

 rate of clean up was diminished considerably. For the two 

 lover pressures (the columns C and D, Table I.) the log 

 curve corresponding to the lower pressure of the two has less 

 curvature, as might be expected from the fact that the 

 amount of gas which has disappeared and which retards the 

 clean up is always less in case D than in case C. (0 and G 

 are identical.) 



To calculate u b" for each run, it is clear that we must 

 take the value of dp\dt when t = 0, for this is the time when 

 " b " is least affected by the supposed inability of the 

 surface to take up all the dissociated gas reaching it. 

 It was found convenient to deduce dp/dt at t = from the 

 initial slope of the corresponding log curve by means of the 



relation -£- = 2*30- — ~t Jd - : ^ • ^ ne pressure unit was 10" 5 mm., 



the time unit was 1 minute. N the number of electrons 

 emitted in one minute was found to be 2*44 x 10 17 (from the 

 electron current of 650 microamps.). 



There remain to be calculated the free path \ for the elec- 

 tron, and n 2 \n x £° r t ne ratio of the densities of the gas in the 

 parts of the apparatus at liquid air temperature and at room 

 temperature respectively. When the mean free path of the 

 molecule is considerably less than the distance apart of the 

 walls of the apparatus the ordinary gas laws hold, and n 2 \n v 

 = /o 2 /p 1 = T 1 /T 2 where T 1 and T 2 are the absolute temperatures. 

 However, as Knudsen * has found, when the mean free 

 path of the molecule is considerably less than the diameter 

 of the tubes, another set of gas laws is applicable, from 

 which we get njn 1 = p 2 /p 1 = , V /T 1 /T 2 . We shall refer to 



these two sets of laws as the high pressure, " H.P.", and 

 the low pressure, " L.P.", laws respectively. The critical 

 distance in this apparatus is about 3 mm., this being the 

 distance between the inner tube and outer tube at the level 

 of the surface of the liquid air, for this is the place where 

 the temperature transition occurs. (The hydrogen molecule 

 has a mean free path of 3 mm. at 290 x 10~ 5 mm. pressure 

 at 0° C.) 



There would have been less ambiguity if the experiments 

 could have been carried out at pressures much below the 

 critical values, where the "low pressure'"' laws apply accu- 

 rately. This, unfortunately, would have meant restricting 

 the range of observations to within a very few mm. at the 

 * Knudsen, Ann. der Phys. xxxi. p. 205 (1010). 



