Resonance Potentials for Electrons in Lend and Calcium. 79 



the work of A. S. King * with the tube furnace at different 

 temperatures. In the calcium spectrum at high temperature 

 the line A = 1227 is predominant and X = 6'573 quite taint, 

 while at low temperature \ = 4227, though still the brightest 

 line, has lost considerably in relative intensity and X=6573 

 has increased until it is second only to A- = 4227. Mag- 

 nesium shows the same phenomena, but in a more striking 

 manner, for the line 1*5 8 — 2p 2 , \ = 4571 is the brightest 

 line in the spectrum at low temperature though faint at hio-h 

 temperature, and in ordinary arc spectra. 



That these two lines in calcium are related to the two 

 observed resonance potentials is to be expected. However, 

 the relative prominence of the 1st resonance potential due 

 to 1*5 S — 2p 2 in calcium and magnesium is difficult to 

 reconcile with the researches of McLennan on low-voltage 

 arcs in magnesium where only the line 1*5 S — 2P was 

 observed. The problem is evidently complicated. The tube 

 furnace spectra show at least that the two emission centres 

 are affected differently by a change in physical condition. 



Summary. 



The resonance and ionization potentials of lead are 1*26 

 and 7'93 volts respectively. The line X = 10291 gives the 

 probable theoretical value for the former as 1*198 volts. 



Calcium has two resonance potentials, 1*90 volts and 

 2*85 volts, of which the first is more prominent. Ionization 

 was observed at 6*01 volts. 



The following spectral frequencies determine these po- 

 tentials : — 



First resonance 1-5 S— 2j> 2 , X = 6572*78 A., V = 1-877 volts. 



Second resonance . . 1-5 S-2P, \=4226-7;3 A., V = 2-918 volts. 

 Ionization 1*5 -S, A = 2027-56 A., V = 0-08 L volts. 



Bureau of Standards, 



Washington, D.C., 



October 20, 1919. 



* Astrophys. Journ. xlviii. p. 13 (1918). 



