TATE AND FOOTE : RESONANCE POTENTIALS 523 



sudden increase had taken place. It would then drop back, 

 again discontinuously, to its original low value. Similar effects 

 have been observed in mercury vapor. Continuous curves were 

 obtained by decreasing the vapor pressure of the sodium. 



The points of higher order c, d, e of curves 1 to 8 are of 

 interest in that they are a combination of two overlapping effects. 

 After ionization has set in at an observed potential of 4.33 volts it 

 is to be expected that the electrons which start at this point will 

 collide inelastically at potentials of 6.45, 8.57 volts, etc., whereas 

 the electrons which had not been involved in an ionization would 

 collide inelastically at 5.56, 7.68, etc. What is actually ob- 

 served is a combination of the point 5.56 with 6.45, etc. (see 

 especially curve 5 which shows distinct double maxima in these 

 regions). On account of this overlapping no use was made of 

 these points of higher order in the determination of critical 

 potentials. 



If, on the basis of Bohr's theory, we calculate the minimum 

 wave length to be expected assuming a critical potential of 2.12 

 volts, we get X = 5830 A and it seems obvious that we are dealing 

 here with the potential necessary to bring out the D lines. 

 Using X = 5893 A we find for the critical potential necessary to 

 excite the D lines 2.10 volts. 



The minimum wave length corresponding to the observed 

 ionizing potential is 2410 A. The limiting wave length of the 

 principle series of the sodium spectrum is 2412.63 A. 13 



Summary. — (1) Electrons having a velocity corresponding to 

 2.12 ± 0.06 volts collide inelastically, without ionization, with 

 the atoms of sodium vapor. The energy lost by the colliding 

 electrons is probably radiated in light of wave lengths corre- 

 sponding to the D lines. Assuming this to be the case the theo- 

 retical value of the resonance potential is 2.10 volts. 



(2) Electrons having a velocity corresponding to 5.13 ± 0.10 

 volts are able to ionize sodium vapor and cause it to emit a 

 brilliant yellow light. The theoretical value of the ionizing 

 potential using the limiting wave length of the principal series 

 in the sodium spectrum is 5.13 volts. 



13 Wood and Fortrat. Astrophys. Journ., 43: 73. 1916. 



