78 Drs. Mohler, Foote, and Stimson on Ionization and 



measurement, so the resonance potentials have been com- 

 puted from the points a, c, and d. The mean value of c—a 

 gives for the first resonance potential 1"90 volts. The 

 second resonance potential, d — a is 2*85 volts. The observed 

 ionization potential is 6*01 volts. 



The occurrence of two resonance potentials has been 

 suspected in the case of other metals, notably zinc and 

 magnesium, but this is the first instance in which the 

 phenomena were unmistakable. It suggests the possibility 

 that in all metals there may be many potentials of inelastic 

 impact, of which the observed resonance potential is the 

 most probable. 



For the metals of the second column of the periodic table 

 previously studied, it has been found that the resonance 

 potential is determined by the frequency of the combination 

 series line 1*5 S - 2 p 2 , and the ionization potential by the 

 limiting frequency 1*5 S. The first line of the principal 

 series of single lines 1*5 S — 2P is, however, predominant 

 in spectra of the alkali earths. 



The frequency 1*5 S for calcium is computed by F. A. 

 Saunders * to be r= 49304:$, \ = 2027*56. This corresponds 

 to V = 6'081 volts in close agreement with the observed 

 value V = 6*01 volts. The line 1*5 S — 2p 2 has not been 

 correctly identified in any published work, but is un- 

 doubtedly \= 6572*78, as this agrees with the best values 

 of series constants, and the physical properties of this line 

 are characteristic. Accordingly we obtain for the theoretical 

 value V = 1*877 volts, while the observed value is V = l*90 

 volts. The line 1-5S-2P, \ = 4226*73 corresponds to 



V = 2*918 volts in good agreement with the observed value 



V = 2* 85 volts. 



The spectral relations of the second resonance potential 

 are not surprising. McLennan f has found that the line 

 1*5 S — 2P alone appears in the magnesium arc below the 

 ionization potential, and that in zinc and cadmium arcs both 

 1*5 S — P and 1*5 S — 2p 2 appear. Davis and Groucherf by 

 their photoelectric method of detecting different types of 

 radiation found evidence that 1*5 S — 2P, as well as 

 1*5 S — 2p 2 , appears in mercury at the voltage corresponding 

 to its frequency. Interesting light on these fundamental 

 frequencies in the spectra of the alkali earths is given by 



* Data furnished by Dr. Saunders. 



t McLennan, Proc. Roy. Soc. xcii. p. 574 (1916). McLennan & 

 Iretou,Pliil. Mag. xxxvi. p. 461 (1918). 

 X Phys. Rev. x. p. 101 (1917). 



