

Hydrogen and Nitrogen by Electron Impacts. 789 



the pressure was again about 550 x 10~ 5 mm. On repeating 

 the experiment the yield of non-recondensible gas was 

 54 per cent. The procedure was carried through twice 

 again, the yields rising to 67 per cent, and 81 per cent. 

 Thus, taking the presence of non-recondensible gas as a 

 criterion of dissociation of hydrogen, there is strong evidence 

 in favour of the view that hydrogen can be dissociated by 

 electron impacts. The progressive rise in the yield corre- 

 sponds to the decreasing amount of surface available for the 

 atomic hydrogen. 



Let us consider some alternative explanations. There 

 may be chemical action between some gaseous impurity 

 (which must, obviously be gaseous at 90° K) and the 

 hydrogen when ionized by electrons. It is extremely 

 unlikely that the impurity would be present in sufficient 

 quantity to account for a clean up of as much as four-fifths 

 of the hydrogen time after time. It may be urged that 

 hydrogen when ionized combines with the nickel, or some- 

 thing on it or on the glass. But in neither of these 

 alternatives is there any reason to suppose that the com- 

 pound would show the exceedingly characteristic non- 

 recondensible effect. We may now take up the behaviour 

 of ionized hydrogen, which in some way may show the 

 non-recondensible effect. . Take first the positively charged 

 hydrogen molecule. Such a charged molecule would be 

 driven to the filament, and to account for the clean up found 

 in many runs, a layer of hydrogen 100 molecules deep 

 would be formed on the filament. Again, a clean up of this 

 type would be unaffected by removing the liquid air if the 

 filament were kept hot. Such was not the case. Take now 

 the case of negatively charged hydrogen molecules. These 

 would be driven to the gauze. If they stuck there by virtue 

 of their charge, a very small clean up would lead to surface 

 charge sufficient in amount to annul the field and reduce 

 the electron current almost to zero, which is not the case. 

 If, on the other hand, they gave up their charge, there is no 

 reason to suppose that they would stay on the surface any 

 more than any other uncharged molecule. Another objection 

 is that electrons with energies less than 10 volts would be as 

 likely to form a negative ion by uniting with a molecule 

 as electrons with greater energies. The clean up, however, 

 below 13 volts is very small. Still another reason against 

 this view is that various investigations have shown that the 

 negative ion in hydrogen is an electron, proving that there is 

 no tendency for a hydrogen molecule to unite with an 

 electron. 



