1894.] 



Mixtures of Nitrogen and Hydrogen. 



269 



FIG. 7. 



400 



30/717^. 



These curves are not traced for pressures lower than about 2 mm. 

 The two curves are nearly identical in the case of pure nitrogen ; 

 in the other cases, with one exception, that of N:H = 2:1, the 

 discharge seems to pass more easily when grease is present. 



It will be noticed that at fairly high pressures the E.M.F. required 

 to produce a spark diminishes nearly uniformly as the pressure 

 diminishes, but that there exists for each mixture a critical pressure 

 at which the E.M.F. is a minimum, and that as the pressure dimin- 

 ishes from the critical value the E.M.F. continually increases. When 

 the pressure is slightly less than the critical pressure the E.M.F. 

 increases with remarkable rapidity as the pressure diminishes. In 

 fact, as the pressure falls by about ^ mm. the E.M.F. may increase 

 by several hundred volts. 



The critical pressure diminishes as the proportion of nitrogen to 



